Glossary | World Library of Science

Glossary

Index

A-DNA

Right-handed helical structure of DNA that exists when little water is present.

abiotic

Non-living; describes a property of an environment that is purely physical or chemical, such as light, air, water, or nutrients.

absolute risk

In epidemiological studies, the probability that an individual will develop a particular condition, such as a disease or some other outcome, based on genetic profile, behavioral patterns, and/or test results.

acceptor arm

The arm in tRNA to which an amino acid attaches.

accessory chromosome

Any extra chromosome in the karyotype of an organism.

acentric chromatid

Lacks a centromere; produced when crossing over takes place within a paracentric inversion. The acentric chromatid does not attach to a spindle fiber and does not segregate in meiosis or mitosis, so it is usually lost after one or more rounds of cell division.

acetylation

An enzymatic reaction that results in the addition of an acetyl group to a biochemical.

acidic activation domain

Commonly found in some transcriptional activator proteins, a domain that contains multiple amino acids with negative charges and stimulates the transcription of certain genes.

acrocentric chromosome

Chromosome with a centromere located off-center, resulting in longer arms on one side than another.

action potential

An electric signal that travels along neurons; a continuously regenerated impulse that transfers information throughout the nervous system of organisms.

activation domain

Part of a transcription factor that is modular and independent from the DNA-binding activity. An activation domain stimulates PolII activity at the locus.

activator

Protein in eukaryotic cells that binds to consensus sequences in regulatory promoters or enhancers and affects transcription initiation by stimulating or inhibiting the assembly of the basal transcription apparatus.

adaptation

A process of change in a population that occurs under selective forces over time and through multiple generations, to produce genetic differences in future generations; the change in a population that increases fitness.

adaptive immunity

Immunity acquired after exposure to a pathogen, or some foreign antigen. Distinct from native immunity.

adaptive landscape

A three-dimensional depiction of population mean fitness as a function of genotype or phenotype, in which the horizontal axes are allele frequencies at two loci or two phenotypic traits, and the vertical axis is population mean fitness.

adaptive mutation

Process by which a specific environment induces mutations that enable organisms to adapt to the environment.

adaptive peak

The maxiumum mean fitness of a population, calculated with allele frequencies or traits.

adaptive radiation

Evolutionary divergence of members of a single phylogenetic lineage into a variety of different adaptive forms; usually the taxa differ in the use of resources or habitats, and have diverged over a relatively short interval of geological time. The term "evolutionary radiation" describes a pattern of rapid diversification without assuming that the differences are adaptive.

adaptive topography

adaptive trait

A trait that contributes either directly or indirectly to an individual's fitness.

adaptive valley

A set of allele frequencies at which mean fitness has a minimum.

adaptive zone

A set of similar ecological niches occupied by a group of (usually) related species, often constituting a higher taxon.

addition rule

States that the probability of any of two or more mutually exclusive events occurring is calculated by adding the probabilities of the individual events.

additive effect

The magnitude of the effect of an allele on a character, measured as half the phenotypic difference between homozygotes for that allele compared with homozygotes for a different allele.

additive genetic correlation

A measure of the degree to which two traits are affected by the same genes (pleiotropy) or pairs of genes (linkage disequilibrium). Selection on one trait produces an evolutionary change in all traits that have an additive genetic correlation with the selected trait.

additive genetic variance

The genetic variance in a character that is attributable to additive effects of alleles, and that determines how different progeny are from their parents.

additive genetic variance-covariance matrix

A square matrix with additive genetic variances for the traits on the diagonal and additive genetic covariances on the off-diagonal.

additivity

The type of gene action in which the alleles at a locus do not affect each other's expression or the expression of alleles at other loci; in other words, gene action with no dominance or epistasis.

adenine

A purine base in DNA and RNA.

adenosine-3',5'-cyclic monophosphate

Modified nucleotide that functions in catabolite repression. Low levels of glucose stimulate high levels of cAMP; cAMP then attaches to CAP, which binds to the promoter of certain operons and stimulates transcription.

adjacent-1 segregation

Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then adjacent-1 segregation takes place when N1 and T2 move toward one pole and T1 and N2 move toward the opposite pole.

adjacent-2 segregation

Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then adjacent-2 segregation takes place when N1 and T1 move toward one pole and T2 and N2 move toward the opposite pole.

adult-onset

Describes a condition in which the phenotype does not manifest itself until later in life. The physical manifestation of such a condition is not present until after puberty.

affinity chromatography

A method of separating biochemical components based on specific interactions between the components of the biochemical mixture and other molecules (e.g., antigen-antibody or receptor-ligand).

AFLP

Genetic markers detected by cleaving DNA with one or more restriction enzymes and then amplifying some of these fragments by PCR using primers with random nucleotide sequences.

African sleeping sickness

A vector-borne disease caused by parasites from the genus Trypanosoma.

aging

The decline in population or organismal fitness or tissue/cell health over time.

alkaptonuria

A single-gene disorder identified by Archibald Garrod that is characterized by dark urine. Garrod first coined the term "inborn error in metabolism" to describe this and other congenital, inherited disorders that affect metabolic pathways.

allele

An alternate form of a gene; a single gene can have multiple alleles, or versions.

allele frequency

A number that represents the incidence of a gene variant in a population.

alleles

Alternative versions of genes; located at specific positions on a specific chromosome; interactions between allelic expression lead to trait phenotype.

allometric growth

The growth of some feature or body part during development at a rate different from that of another feature.

allopatric

Of a population or species, occupying a geographic region different from that of another population or species.

allopatric speciation

A differentation process whereby one species becomes two usually due physical isolation of the populations involved.

allopolyploid

A polyploid in which the several chromosome sets are derived from more than one species.

allosteric protein

Protein that changes its conformation on binding with another molecule.

alternate segregation

Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then alternate segregation takes place when N1 and N2 move toward one pole and T1 and T2 move toward the opposite pole.

alternation of generations

Complex life cycle in plants that alternates between the diploid sporophyte stage and the haploid gametophyte stage.

alternative processing pathway

One of several pathways by which a single pre-mRNA can be processed in different ways to produce alternative types of mRNA.

alternative splicing

The varieties of ways that pre-mRNA is cut and reassembled to make different mRNA sequences, and therefore alternate expressions of a gene.

altruism

Conferral of a benefit on other individuals at an apparent cost to the donor.

Alu

Part of a family of short, interspersed repeats, these are the most abundant sequence repeats in the human genome (making up 5%.10% of the total). Alu sequences can be propagated by retrotransposition, although most are sterile, or DNA "fossils."

amanitin

A highly poisonous polypeptide that selectively inhibits the activity of mammalian RNA polymerase.

Ames test

Test in which special strains of bacteria are used to evaluate the potential of chemicals to cause cancer.

amino acid

Amino acids are small molecules that serve as building blocks of proteins

aminoacyl (A) site

One of three sites in a ribosome occupied by a tRNA in translation. All charged tRNAs (with the exception of the initiator tRNA) first enter the A site in translation.

aminoacyl-tRNA synthetase

Enzyme that attaches an amino acid to a tRNA. Each aminoacyl-tRNA synthetase is specific for a particular amino acid.

amniocentesis

Procedure used for prenatal genetic testing to obtain a sample of amniotic fluid from a pregnant woman. A long sterile needle is inserted through the abdominal wall into the amniotic sac to obtain the fluid.

anagenesis

Evolution of a feature within a lineage over an arbitrary period of time.

analysis of variance

Statistical technique for testing for differences among the means of several groups with respect to a continuous variable.

analytical model

A model in which the relationships among variables are defined using equations.

anaphase

Anaphase is the fourth phase of mitosis, which is a process that separates the duplicated genetic material carried in the nucleus of a parent cell into two, identical daughter cells

anaphase I

Stage of meiosis I. In anaphase I, homologous chromosomes separate and move toward the spindle poles.

anaphase II

Stage of meiosis II. In anaphase II, chromatids separate and move toward the spindle poles.

ancestral species

A species at the root of a clade of related organisms.

aneuploidy

Change from the wild type in the number of chromosomes; most often an increase or decrease of one or two chromosomes.

angiosperms

A group of seed plants that produce flowers and fruits for reproduction; the fruit encloses the seed and typically increases chances of seed distribution.

animal development

The process whereby a fertilized egg undergoes cell division and cellular differentiation to generate the different tissue types of a fully functional complex organism.

antagonistic selection

A source of natural selection that opposes another source of selection on a trait.

Antennapedia complex

Cluster of five homeotic genes in fruit flies that affects development of the adult fly's head and anterior thoracic segments.

Antennapedia homeodomain

A sequence-specific transcription factor from Drosophila melanogaster. The wild-type Antennapedia homeodomain complex works to initiate a series of transcription events that results in anterior-posterior polarity in the organism.

anthropogenic

A condition, process, or result stemming from human activity, such as air pollution or agriculture.

antibiotic resistance

Heritable changes in bacteria that allow them to withstand antibiotic treatments that would normally kill them.

antibody

Produced by a B cell, a protein that circulates in the blood and other body fluids. An antibody binds to a specific antigen and marks it for destruction by making it easier for a phagocytic cell to ingest the antigen.

anticipation

Increasing severity or earlier age of onset of a genetic trait in succeeding generations. For example, symptoms of a genetic disease may become more severe as the trait is passed from generation to generation.

anticodon

Sequence of three nucleotides in tRNA that pairs with the corresponding codon in mRNA in translation.

antigen

Substance that is recognized by the immune system and elicits an immune response.

antigenic drift

A process by which circulating influenza viruses are constantly changing, which allows the viruses to cause annual epidemics of illness. Antigenic drift occurs when mutations accumulate in the hemagglutinin and neuraminidase genes that alter the antigenicity of these proteins such that the "drifted" strains are no longer neutralized by antibodies that were specific for previously circulating strains.

antigenic shift

A process by which a new influenza A virus hemagglutinin subtype (with or without an accompanying new neuraminidase subtype) is introduced into the human population, which lacks prior experience of and immunity to the subtype. Antigenic shift can occur as a result of the direct introduction of an influenza virus from an animal or avian host into humans, or by the exchange or reassortment of gene segments between human and non-human influenza viruses when they co-infect animals or humans.

antiparallel

Refers to a characteristic of the DNA double helix in which the two polynucleotide strands run in opposite directions.

antisense RNA

Small RNA molecule that base pairs with a complementary DNA or RNA sequence and affects its functioning.

antiterminator

Protein or DNA sequence that inhibits the termination of transcription.

apomixis

Parthenogenetic reproduction in which an individual develops from one or more mitotically produced cells that have not experienced recombination or syngamy.

apomorphic

Having a derived character or state, with reference to another character or state. See also synapomorphy.

apoptosis

Programmed cell death, in which a cell degrades its own DNA, the nucleus and cytoplasm shrink, and the cell undergoes phagocytosis by other cells without leakage of its contents.

aposematic

Coloration or other features that advertise noxious properties; warning coloration.

aptamer

Nucleic acid that binds to a specific target molecule.

Archaea

One of the three primary divisions of life. Archaea consist of unicellular organisms with prokaryotic cells.

array comparative genomic hybridization

Similar to conventional comparative genomic hybridization (CGH), but during hybridization, cloned chromosomal DNA fragments (about 200 kb in size) replace the metaphase chromosomes. This method offers greater sensitivity and resolution than conventional CGH in detecting copy number changes.

artificial selection

Selective breeding of organisms to produce domesticated animals with more desirable traits; selective breeding to test for genetic variation and covariation in a population. Compare to natural selection.

asexual

A term describing reproduction that does not involve gene transfer between parent cells.

aspect

A characteristic among many that characterize a person or object; more specifically, the angle of view on and object or face.

assisted reproductive technologies

Procedures or methods that promote succesful fertilization and pregnancy in humans.

association study

In genetics, a case-control study in genotype frequencies are compared between healthy and diseased individuals; SNPs that form haplotypes are a typical focus for comparison.

assortative mating

Nonrandom mating on the basis of phenotype; usually refers to positive assortative mating, the propensity to mate with others of like phenotype.

asymmetrical response

A common result in artificial selection experiments in which there is a greater response to selection in one direction than there is in the opposite direction for the same trait.

atavism

The reappearance in an organism of characteristics that are present in the organism's remote ancestors.

ATP

Adenosine 5-triphosphate, or ATP, is the principal molecule for storing and transferring energy in cells.

attached-X

A pair of X chromosomes in Drosophila melanogaster that are connected together at one end and inherited jointly.

attachment site

Special site on a bacterial chromosome where a prophage may insert itself.

attenuation

Type of gene regulation in some bacterial operons, in which transcription is initiated but terminates prematurely before transcription of the structural genes.

attenuator

Secondary structure that forms in the 5' untranslated region of some operons and causes the premature termination of transcription.

autoantibodies

Antibodies that form in response to antigens in one's own tissue; antibodies that attack cells native to the organism.

autoimmune disease

Characterized by an abnormal immune response to a person's own (self) antigen.

autonomous element

Transposable element that is fully functional and able to transpose on its own. DNA sequence that confers the ability to replicate; contains an origin of replication.

autopolyploidy

Condition in which all the sets of chromosomes of a polyploid individual possessing more than two haploid sets are derived from a single species.

autoradiography

Method for visualizing DNA or RNA molecules labeled with radioactive substances. A piece of X-ray film is placed on top of a slide, gel, or other substance that contains DNA labeled with radioactive chemicals. Radiation from the labeled DNA exposes the film, providing a picture of the labeled molecules.

autosomal

A trait resulting from a chromosome that is not a sex chromosome.

autosomal dominant

An inheritance pattern through generations of a family that indicates a trait is passed via an autosome, and is dominant to other traits. The inherited trait can affect males and females equally.

autosomal inheritance

Genetic transmission of genes on autosomes (nonsex chromosomes).

autosomal recessive

An inheritance pattern through generations of a family that indicates a trait is passed via an autosome, and is recessive to other traits. The inherited trait can affect males and females equally.

autosome

Any chromosome in the set of chromosomes that is not a sex chromosome.

autotrophs

Organisms that obtain energy from the sun or from the oxidation of inorganic substances; organisms that convert energy into nutrients through a series of chemical reactions.

auxillary chromosome

Former name for an unpaired sex chromosome. Used today to indicate an artificial "extra" (i.e., engineered) chromosome.

auxotroph

Bacterium or fungus that possesses a nutritional mutation that disrupts its ability to synthesize an essential biological molecule; cannot grow on minimal medium but can grow on minimal medium to which has been added the biological molecule that it cannot synthesize.

TOP

B-cell

Particular type of lymphocyte that produces humoral immunity; matures in the bone marrow and produces antibodies.

B-DNA

Right-handed helical structure of DNA that exists when water is abundant; the secondary structure described by Watson and Crick and probably the most common DNA structure in cells.

A cross in which an F1 or F1' individual is mated to one of its parents (P1) or to another individual that is genetically identical to one of its parents.

A cross in which an F1 or F1' individual is mated to one of its parents (P2) or to another individual that is genetically identical to one of its parents.

backcross

A mating between an F1 or F1' individual to one of its parents (P1 or P2) or to another individual that is genetically identical to one of its parents. The term "backcross" may be used as a verb (describing the process of setting up the required mating) or as a noun to describe the mating or to describe the progeny that result from such a hybrid cross.

backcross 1

Cross of an F1 or F1' individual to a P1 individual.

backcross 2

Cross of an F1 or F1' individual to a P2 individual.

background extinction

A long-prevailing rate at which taxa become extinct, in contrast to the highly elevated rates that characterize mass extinction.

background selection

Elimination of deleterious mutations in a region of the genome; may explain low levels of neutral sequence variation.

bacteria

Single-celled prokaryotes that can be free-living or live as parasites.

bacterial artificial chromosome

Cloning vector used in bacteria that is capable of carrying DNA fragments as large as 500 kb.

bacterial colony

Clump of genetically identical bacteria derived from a single bacterial cell that undergoes repeated rounds of division.

bacteriophage

Bacteriophage; a type of virus that infects bacteria.

balance hypothesis

Proposes that much of the molecular variation seen in natural populations is maintained by balancing selection that favors genetic variation.

balanced lethals

The arrangement of two recessive lethal alleles such that the alleles lie in repulsion; here, it is essentially as if the organism was a heterozygote for the lethal allele. Homozygosity would result in death of the organism.

balanced polymorphism

The persistence of an multiple alleles in a population due to reproductive advantage of the heterozygote over any homozygote.

balanced translocation

When pieces of chromosomes are rearranged but no genetic material is gained or lost in the cell.

balancing selection

A form of natural selection that maintains polymorphism at a locus within a population.

Barr body

Condensed, darkly staining structure that is found in most cells of female placental mammals and is an inactivated X chromosome.

basal transcription apparatus

Complex of transcription factors, RNA polymerase, and other proteins that assemble on the promoter and are capable of initiating minimal levels of transcription.

base analog

Chemical substance that has a structure similar to that of one of the four standard bases of DNA and may be incorporated into newly synthesized DNA molecules in replication.

base substitution

A type of mutation that changes the identity of a single base in a DNA sequence, so that the number of bases remains the same but the codon and downstream amino acid are altered.

base-excision repair

DNA repair that first excises modified bases and then replaces the entire nucleotide.

base-pairing

The interaction of complementary nitrogen-containing bases, such as cytosine and guanine, in the formation of double stranded nucleic acids; occurs in DNA replication.

Beckwith-Wiedemann Syndrome

Syndrome of unknown etiology characterized by the presence of macroglossia (large tongue), visceromegaly (large organs), macrosomia (large body size), and hypoglycemia. Patients show an increased susceptibility to tumor development.

benthic

Inhabiting the bottom, or substrate, of a body of water.

Bicoid homeodomain

The maternally transcribed gene bicoid organizes anterior development in Drosophila. Bicoid encodes a homeodomain-containing transcriptional factor, its gradient acting to position the transcription of gap and pair rule genes along the anterior-posterior axis.

bidirectional replication

Replication at both ends of a replication bubble.

binomial expansion

Describes a statistical method of examining characteristics in a population. Hardy-Weinberg used this technique to provide a mathematical model of population genetics.

biodiversity

Genetic and phenotypic variation both within and among species, plus the variety of ecosystems created by these species.

bioethics

The study of the ethical, moral, and societal implications of biological research and discovery.

biogeography

The study of the geographic distribution of organisms.

bioinformatics

Synthesis of molecular biology and computer science that develops databases and computational tools to store, retrieve, and analyze nucleic acid and protein sequence data.

biological observations

Data collected during the course of a biological experiment or survey; can be quantitative or qualitative.

biological species

A population or group of populations within which genes are actually or potentially exchanged by interbreeding, and which are reproductively isolated from other such groups. See also species, phylogenetic species concept.

biomass

The total mass of all organisms, plant, animal or otherwise, in a given area.

biotechnology

Use of biological processes, particularly molecular genetics and recombinant DNA technology, to produce products of commercial value.

birth defects

Any malformations or defects of development found at birth.

bithorax complex

Cluster of three homeotic genes in fruit flies that influences the adult fly's posterior thoracic and abdominal segments.

bivalent

Refers to a synapsed pair of homologous chromosomes.

BLAST

Basic Local Alignment Search Tool; a sequence comparison algorithm, optimized for speed, used to search sequence databases for regions of local similarity between sequences. The program compares nucleotide or protein sequences to sequence databases and calculates the statistical significance of matches.

blastocyst

A mass of cells formed around a central cavity; an early stage of development after multiple cleavages in a zygote.

blastomere

An individual cell of a blastocyst, formed after multiple cleavages.

blending inheritance

Early concept of heredity proposing that offspring possess a mixture of the traits from both parents.

bottleneck

A severe, temporary reduction in population size.

branch

Evolutionary connections between organisms in a phylogenetic tree.

branch migration

Movement of a cross bridge along two DNA molecules.

branch point

Adenine nucleotide in nuclear pre-mRNA introns that lies from 18 to 40 nucleotides upstream of the 3' splice site.

breeding value

The effect of an individual's genes on the value of a given trait in its offspring; sometimes called the additive genotype. It is equal to two times the deviation of the mean of the individual's offspring from the overall population mean.

broad-sense heritability

Proportion of the phenotypic variance that can be attributed to genetic variance.

TOP

C value

Amount of DNA contained in a haploid nucleus; usually a picogram measurement.

c-Fos

One of the Fos family of transcription factors, expressed downstream from signal transduction; identified as proto-oncogene.

C-value paradox

The lack of correlation between the DNA content of eukaryotic genomes and a given organism's phenotypic complexity (i.e., the genome of a less complex eukaryotic organism, such as a plant, may contain far more DNA than that of a more complex organism, such as a human being). The paradox is explained by the amount of noncoding repetitive DNA sequences in a genome.

Caenorhabditis elegans

A model eukaryotic, multicellular organism. C. elegans is a nematode that serves as a valuable model to study basic developmental processes, neurological function, and cell communication.

calcitonin

An example of multiple different polypeptides being generated by alternative splicing from the same gene. Different tissues express the different transcripts. For example, calcitonin is more prevalent in the thyroid, while CGRP is highly expressed in brain tissue.

canalization

The evolution of internal factors during development that reduce the effect of perturbing environmental and genetic influences, thereby constraining variation and consistently producing a particular (usually wild-type) phenotype.

cancer

A type of disease caused by uncontrolled cell division; can exist in many forms.

candidate gene

A gene thought to be involved in the evolution of a particular trait based on its mutant phenotype or the function of the protein it encodes.

candidate gene approach

A technique which attempts to determine if genes of known function affect complex phenotypic traits.

carcinogen

Any substance that causes cancer or aggravates tumors; a substance that damages the genome or metabolism of a cell resulting in uncontrolled cell division.

caretaker gene

A tumor suppressor gene that protects the genome from damage or mutations; usually encodes proteins that recognize or repair DNA damage.

carrier organism

An individual whose genotype includes a deleterious allele that is not expressed or evident in the phenotype, yet can pass this allele onto offspring ; the individual is "carrying" that allele to a subsequent generation.

carrying capacity

The population density that can be sustained by limiting resources.

case-control study

An epidemiological method that involves pairing observations of diseased or affected individuals, cases, with matched controls that are as much like the cases but without the disease.

caspase

Enzyme that cleaves other proteins and regulates apoptosis. Each caspase is synthesized as a large, inactive precursor (a procaspase) that is activated by cleavage, often by another caspase.

caste

A group of individuals that are socially distinct from others in the population, and engage in specialized behavior within a specific social group.

catabolite activator protein

Protein that functions in catabolite repression. When bound with cAMP, CAP binds to the promoter of certain operons and stimulates transcription.

catabolite repression

System of gene control in some bacterial operons in which glucose is used preferentially and the metabolism of other sugars is repressed in the presence of glucose.

causal variance components

In a sibling analysis, the portions of phenotypic variance that are due to the underlying genetic and environmental sources of variance (e.g., additive genetic variance, dominance variance, environmental variance).

CCCTC binding factor

A transcriptional regulator that plays important roles in epigenetic control of gene expression. CTCF is a zinc finger protein that is known for transcriptional insulation.

cDNA

Any DNA molecule that is the complementary sequence of an mRNA, typically created synthetically using reverse transcriptase; when labeled and collected in sets, usually reflects the expression profile of a cell or tissue.

cDNA library

Collection of bacterial colonies or phage colonies containing DNA fragments that have been produced by reverse transcription of cellular mRNA.

cell

A single fundamental unit of organisms, the most basic unit of tissues.

cell culture

The treatment of cells removed from an organism, and sustained in an artificial environment that simulates the condition of the tissue the cells came from; the proces of maintaining or multiplying cells in a nutrient solution, and at an optimal temperature, under incubation.

cell cycle

Stages through which a cell passes from one cell division to the next.

cell determination

Process during cell maturation or development wherein it becomes committed to a particular cell type; the overall process toward the endpoint of cell differentiation.

cell line

Genetically identical cells that divide indefinitely and can be cultured in the laboratory.

cell theory

States that all life is composed of cells, that cells arise only from other cells, and that the cell is the fundamental unit of structure and function in living organisms.

cellular immunity

Type of immunity resulting from T cells, which recognize antigens found on the surfaces of self cells.

centimorgan

cM; A unit for measuring distance on a genetic map. One cM is reflects a 1% recombination rate (i.e., c = 0.01). Named after Thomas Hunt Morgan.

central dogma

Concept that genetic information passes from DNA to RNA to protein in a one-way information pathway.

centriole

Cytoplasmic organelle consisting of microtubules; present at each pole of the spindle apparatus in animal cells.

centromere

Constricted region on a chromosome that stains less strongly than the rest of the chromosome; region where spindle microtubules attach to a chromosome.

centromeric sequence

DNA sequence found in functional centromeres.

centrosome

Structure from which the spindle apparatus develops; contains the centriole.

cephalization

A process over the evolutionary history or development of an organism characterized by a centralization of nervous tissue and nervous system control in the anterior part of the body.

chaperone protein

A protein that helps another protein fold and keeps it stable when not in use, yet is not involved in normal function of that protein.

character displacement

Usually refers to a pattern of geographic variation in which a character differs more greatly between sympatric than between allopatric populations of two species; sometimes used for the evolutionary process of accentuation of differences between sympatric populations of two species as a result of the reproductive or ecological interactions between them.

character state

One of the variant conditions of a character (e.g., yellow versus brown as state of the character .color of snail shell.).

characteristic

A feature or trait.

Chargaff's rules

Rules developed by Erwin Chargaff and his colleagues concerning the ratios of bases in DNA.

checkpoint

A key transition point at which progression to the next stage in the cell cycle is regulated.

chi square test

A statistical test that allows one to determine whether observed quantities of a specific characteristic differed from the expected value purely by chance.

chiasma

Point of contact between paired (homologous) chromosomes at which crossing over takes place, during the first metaphase of meiosis.

chimeric oligonucleotide

An oligonucleotide with a backbone that is composed of subunits with different backbone structures.

ChIP on ChIP assay

A method that combines chromatin immunoprecipitation and DNA microarray analysis to analyze protein-DNA interactions across the genome.

chloramphenicol acetyl transferase

An enzyme in bacteria that acetylates chloramphenicol, rendering it inactive in bacterial cells. In molecular biology, it is often used as a reporter to measure gene expression.

chloroplast

An organelle inside a cell that is responsible for photosynthesis.

chloroplast DNA

DNA in chloroplasts; has many characteristics in common with eubacterial DNA and typically consists of a circular molecule that lacks histone proteins and encodes some of the rRNAs, tRNAs, and proteins found in chloroplasts.

chorionic villus sampling

Procedure used for prenatal genetic testing in which a small piece of the chorion (the outer layer of the placenta) is removed from a pregnant woman. A catheter is inserted through the vagina and cervix into the uterus. Suction is then applied to remove the sample.

chromatid

A single, long DNA molecule and its associated proteins, forming half of a replicated chromosome.

chromatin

Chromatin is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells.

chromatin immunoprecipitation

An assay used in biology to identify proteins, such as transcription factors, that bind to a specific piece of chromatin in vivo.

chromatin remodeling

Adding or removing chemical groups to or from histones, which can alter gene expression.

chromatin-remodeling complex

Complex of proteins that alters chromatin structure without acetylating histone proteins.

chromatin-remodeling protein

Binds to a DNA sequence and disrupts chromatin structure, causing the DNA to become more accessible to RNA polymerase and other proteins.

chromosomal abnormality

Any change in the total number of chromosomes or the physical structure of a chromosome.

chromosomal puff

Localized swelling of a polytene chromosome; a region of chromatin in which DNA has unwound and is undergoing transcription.

chromosomal rearrangements

A class of mutations in which whole segments of chromosomes are involved, including inversions and translocations.

chromosomal scaffold protein

Protein that plays a role in the folding and packing of the chromosome, revealed when chromatin is treated with a concentrated salt solution, which removes histones and some other chromosomal proteins.

chromosome

A chromosome is a single, long molecule of DNA. Chromosomes are highly organized structures that store genetic information in living organisms

chromosome deletion

Loss of a chromosome segment.

chromosome duplication

Mutation that doubles a segment of a chromosome.

chromosome inversion

A 180-degree reversal of the orientation of a part of a chromosome, relative to some standard chromosome.

chromosome map

Representation of the physical location of genes on a chromosome, generally derived from studies looking at recombination rates between known loci.

chromosome mutation

Difference from the wild type in the number or structure of one or more chromosomes; often affects many genes and has large phenotypic effects.

chromosome painting

Visualization of individual, whole chromosomes by fluorescence in situ hybridization (FISH).

chromosome rearrangement

Change from the wild type in the structure of one or more chromosomes.

chromosome territories

The areas of the nucleus in which particular chromosomes reside.

chromosome theory of inheritance

An accepted theory that chromosomes are the material of inheritance, and that they are associated in paternal-maternal pairings; dervied from the collective work of Boveri, Sutton, and Morgan.

chromosome walking

Method of locating a gene by using partly overlapping genomic clones to move in steps from a previously cloned, linked gene to the gene of interest.

chronospecies

A segment of an evolving lineage preserved in the fossil record that differs enough from earlier or later members of the lineage to be given a different binomial (name). Not equivalent to biological species.

cis

cis-acting elements affect only loci on the same strand of DNA

cis configuration

Arrangement in which two or more wild-type genes are on one chromosome and their mutant alleles are on the homologous chromosome; also called coupling configuration.

cis-regulatory element

A noncoding DNA sequence in or near a gene required for proper spatiotemporal expression of that gene, often containing binding sites for transcription factors. Often used interchangeably with enhancer.

clade

The set of species descended from a particular ancestral species.

cladistic

Pertaining to branching patterns; a cladistic classification classifies organisms on the basis of the historical sequences by which they have diverged from common ancestors.

cladogenesis

Branching of lineages during phylogeny.

cladogram

A cladogram is an evolutionary tree that diagrams evolutionary relationships among organisms; branch points show different relationships in time; distinct from a phylogenetic tree branch points which show departure from common traits (derived traits), and common ancestors.

cleavage

The series of mitotic cell divisions that produce a blastocyst from a zygote.

climate change

Changes in global climate patterns attributed to increased levels of carbon dioxide in the atmosphere as well as changes in average global temperature.

cline

A gradual change in an allele frequency or in the mean of a character over a geographic transect.

clinical trial

The process by which new drugs are tested in humans; involve multiple stages and rounds (stage I-III); results are used for seeking approval from government agencies that permit drugs to be sold to the public.

clonal evolution

Process by which mutations that enhance the ability of cells to proliferate predominate in a clone of cells, allowing the clone to become increasingly rapid in growth and increasingly aggressive in proliferation properties.

clone

A single organism, or a lineage of individual organisms that have reproduced asexually, by mitotic division. Can occur naturally or by synthetic manipulation of reproductive cells.

cloning strategy

Particular set of methods used to clone a gene or DNA fragment.

cloning vector

Stable, replicating DNA molecule to which a foreign DNA fragment can be attached and transferred to a host cell.

cloverleaf structure

Secondary structure common to all tRNAs.

co-option

The evolution of a function for a gene, tissue, or structure other than the one it was originally adapted for. At the gene level, used interchangeably with recruitment and, occasionally, exaptation.

coactivator

Protein that cooperates with an activator of transcription. In eukaryotic transcriptional control, coactivators often physically interact with transcriptional activators and the basal transcription apparatus.

coadapted gene pool

A population or set of populations in which prevalent genotypes are composed of alleles at two or more loci that confer high fitness in combination with each other, but not with alleles that are prevalent in other such populations.

coalescence

Derivation of the gene copies in one or more populations from a single ancestral copy, viewed retrospectively (from the present back into the past).

coding strand

The side of the double helix for a particular gene from which RNA is not transcribed.

codominance

Type of allelic interaction in which the heterozygote simultaneously expresses traits of both homozygotes.

codominant marker

A genetic marker in which the heterozygotes can be distinguished from both homozygotes.

codon

A triplet sequence of DNA or RNA nucleotides corresponding to a specific amino acid or a start/stop signal in translation.

coefficient of coincidence

Ratio of observed double crossovers to expected double crossovers.

coefficient of relatedness

The proportion of genes identical by descent (IBD) among two individuals I and J.

coevolution

The influence that two ecologically-interacting species have on each other, wherein each species responds to selection imposed by the other; a history of joint divergence of ecologically associated species.

cofactor

A term for any substance that necessary for optimal activity of an enzyme.

cognition

Brain function involving memory, decision making, and temporal awareness; problem solving capacity of the brain.

cohesin

Molecule that holds the two sister chromatids of a chromosome together. The breakdown of cohesin at the centromeres enables the chromatids to separate in anaphase of mitosis and anaphase II of meiosis.

cohesive end

Short, single-stranded overhanging end on a DNA molecule produced when the DNA is cut by certain restriction enzymes. Cohesive ends are complementary and can spontaneously pair to rejoin DNA fragments that have been cut with the same restriction enzyme.

cointegrate structure

Produced in replicative transposition, an intermediate structure in which two DNA molecules with two copies of the transposable element are fused.

coisogenic

Two strains that are genetically identical (i.e., isogenic), except for a single locus. This occurs most often by a spontaneous mutation by many generations of backcrossing. Coisogenic strains are also becoming available due to target mutagenesis (knockouts) in embryonic stem (ES) cells.

colinearity

In molecular biology, the concept that there is a direct correspondence between the linear order of a nucleotide sequence of a gene and the continuous sequence of amino acids in a protein.

Combined DNA Index System (CODIS)

A DNA database used in forensic analysis in the United States; a distributed database organized into three hierarchical levels, local, state, and national, and stores indexed and searchable digitized representations of DNA samples.

commensalism

An ecological relationship between species in which one is benefited but the other is little affected.

commitment to differentiation

The notion that a cell has irreversibly initiated a series of transcriptional events that results in a change in various aspects of cell physiology, such as size, shape, polarity, metabolism, signal transduction, and gene expression profiles.

common garden

A place in which (usually conspecific) organisms, perhaps from different geographic populations, are reared together, enabling the investigator to ascribe variation among them to genetic rather than environmental differences. Originally applied to plants, but now more generally used to describe any experiment of this design.

common garden experiment

An experimental design in which individuals from multiple populations are raised together in the same environment in order to test for genetic differentiation in phenotypic traits.

comparative genomic hybridization

A molecular cytogenetic method of screening cells for DNA gains and losses at a chromosomal level. Differentially labelled test and reference DNA are hybridized simultaneously to metaphase chromosomes to generate a map of DNA copy number changes.

comparative genomics

The evaluation of similarities and differences between genomes of different organisms; can reveal differences between individuals and species as well as evolutionary relationships.

comparative method

A procedure for inferring the adaptive function of a character by correlating its states in various taxa with one or more variables, such as ecological factors hypothesized to affect its evolution.

compartment

A contiguous group of cells, descended from the same progenitor cell, that form a spatially discrete part of a developing organ or structure and often act as a discrete developmental unit. Cells from one compartment typically do not intermix with cells from other compartments.

compensatory mutations

A mutation that ameliorates the deleterious fitness effects of another mutation.

competent cell

Capable of taking up DNA from its environment (capable of being transformed).

competition

An interaction between individuals of the same species or different species whereby resources used by one are made unavailable to others.

competitive exclusion

Extinction of a population due to competition with another species.

complementary

Refers to the relation between the two nucleotide strands of DNA in which each purine on one strand pairs with a specific pyrimidine on the opposite strand (A pairs with T, and G pairs with C).

complementation

Two different mutations in the heterozygous condition are exhibited as the wild-type phenotype; indicates that the mutations are at different loci.

complementation test

Test designed to determine whether two different mutations are at the same locus (are allelic) or at different loci (are nonallelic). Two individuals that are homozygous for two independently derived mutations are crossed, producing F1 progeny that are heterozygous for the mutations. If the mutations are at the same locus, the F1 will have a mutant phenotype. If the mutations are at different loci, the F1 will have a wild-type phenotype.

complete dominance

Exists when the phenotype of a heterozygote is identical to that of a homozygous dominant individual.

complete linkage

Linkage between genes that are located close together on the same chromosome with no crossing over between them.

complex disease

A pathological condition of the body that is the result of defects in a number of genetic and environmental factors. These conditions do not follow Mendelian inheritance patterns.

complex life cycle

A term coined by HM Wilbur in 1980 referring to a life cycle wherein an organism changes form, physiology and behavior, as well as habitat; also called CLC.

complex trait

A trait that does not follow Mendelian Inheritance patterns, is likely dervied from multiple genes, and exhibits a large variety of phenotypes.

composite transposon

Type of transposable element in bacteria that consists of two insertion sequences flanking a segment of DNA.

compound chromosome

Fusion of two separate chromosomes.

compound heterozygote

An individual organism that possesses two different mutant alleles at a locus.

concept of dominance

The idea proposed by Mendel to explain an observed phenomenon of heredity, wherein two different alleles are present in a genotype, and only one allele is expressed in the phenotype. The expression of that allele is considered to dominate the unexpressed allele.

concerted evolution

Maintenance of a homogeneous nucleotide sequence among the members of a gene family, which evolves over time.

concordance

Percentage of twin pairs in which both twins have a particular trait.

concordant

Refers to a pair of twins both of whom have the trait under consideration.

conditional mutation

Expressed only under certain conditions.

confidentiality

The idea that private information about an individual will not be shared with others.

conformation

The spatial arrangement, folding or shape of a macromolecule, such as a protein or nucleic acid.

congenic

Organisms that are almost genetically identical; ideally, they differ at only one locus.

congenic strain

An inbred strain of animals that are continually interbred. Generally, it takes 10 generations of crossing two inbred lines to create a congenic line that differs in only one locus.

congenital

A condition that is present at birth.

conjugation

Mechanism by which genetic material may be exchanged between bacterial cells. During conjugation, two bacteria lie close together and a cytoplasmic connection forms between them. A plasmid or sometimes a part of the bacterial chromosome passes through this connection from one cell to the other.

conjugation (prokaryotes)

Conjugation is a process by which one bacterium transfers genetic material to another through direct contact

connective tissue

Connective tissue is the material inside your body that supports many of its parts. It is the .cellular glue. that gives your tissues their shape and helps keep them strong. It also helps some of your tissues do their work. Cartilage and fat are examples of connective tissue.

connectives

Strains of organisms developed by backcrossing the nuclear genome from one strain into the cytoplasm of another; the mitochondrial parent is always the female parent during the backcrossing program.

consanguinity

Relation by descent from a common ancestor.

consensus sequence

Comprises the most commonly encountered nucleotides found at a specific location in DNA or RNA.

consomic strain

Organisms that are produced by repeated backcrossing of a whole chromosome such as the X or Y chromosome onto an inbred strain. As with congenic strains, a minimum of 10 backcross generations is required.

conspecific

Belonging to the same species.

constitutive mutation

Causes the continuous transcription of one or more structural genes.

contig

Set of overlapping DNA fragments that have been assembled in the correct order to form a continuous stretch of DNA sequence.

continuous characteristic

Displays a large number of possible phenotypes that are not easily distinguished, such as human height.

continuous replication

Replication of the leading strand in the same direction as that of unwinding, allowing new nucleotides to be added continuously to the 3' end of the new strand as the template is exposed.

convergent evolution

Evolution of similar features independently in different evolutionary lineages, usually from different antecedent features or by different developmental pathways.

cooperation

An association between individuals or groups that benefits all involved.

coordinate induction

Simultaneous synthesis of several enzymes that is stimulated by a single environmental factor.

copy number variation

When the number of copies of a particular genetic sequence is different between individuals.

core element

Consensus sequence in eukaryotic RNA polymerase I promoters that extends from -45 to -20 and is needed to initiate transcription; rich in guanine and cytosine nucleotides.

core enzyme

Part of bacterial RNA polymerase that, during transcription, catalyzes the elongation of the RNA molecule by the addition of RNA nucleotides; consists of four subunits: two copies of alpha (.), a single copy of beta (.), and a single copy of beta prime (.').

core promoter

Located immediately upstream of eukaryotic promoter, DNA sequences to which the basal transcription apparatus binds.

corepressor

Substance that inhibits transcription in a repressible system of gene regulation; usually a small molecule that binds to a repressor protein and alters it so that the repressor is able to bind to DNA and inhibit transcription.

correlated response to selection

An evolutionary change in an unselected trait caused by an additive genetic correlation between the unselected trait and a trait under selection.

correlation

Degree of association between two or more variables.

correlation coefficient

Statistic that measures the degree of association between two or more variables. A correlation coefficient can range from -1 to +1. A positive value indicates a direct relation between the variables; a negative correlation indicates an inverse relation. The absolute value of the correlation coefficient provides information about the strength of association between the variables.

correlational selection

A type of selection in which two traits interact nonadditively to determine fitness, characterized by the finding that certain combinations of trait values have higher fitness than other combinations.

cosmid

Cloning vector that combines the properties of plasmids and phage vectors and is used to clone large pieces of DNA in bacteria. Cosmids are small plasmids that carry . cos sites, allowing the plasmid to be packaged into viral coats.

cost of resistance

The fitness effects of an allele that confers resistance (often denoted by R) to a pesticide or antibiotic in the absence of the pesticide or antibiotic.

cotransduction

Process in which two or more genes are transferred together from one bacterial cell to another. Only genes located close together on a bacterial chromosome will be cotransduced.

cotransformation

Process in which two or more genes are transferred together during cell transformation.

CpG island

DNA region that contains many copies of a cytosine base followed by a guanine base; often found near transcription start sites in eukaryotic DNA. The cytosine bases in CpG islands are commonly methylated when genes are inactive but are demethylated before the initiation of transcription.

cri du chat

A human genetically-based defect resulting from a partial deletion on chromosome 5; affected infants exhibit a cry that is cat-like, as well as cognitive and physical limitations.

critical period

A window of time during an individual's development when a function or tissue structure is most vulnerable to external variables and intervention that has permanent effects.

cross bridge

In a heteroduplex DNA molecule, the point at which each nucleotide strand passes from one DNA molecule to the other.

cross resistance

The condition in which resistance to one pesticide or antibiotic confers increased resistance to a second pesticide or antibiotic.

cross-fostering

An experimental technique in which offspring are reared (fostered) by animals other than their genetic parents; cross-fostering is designed to reduce parental effects.

crossing over

During meiosis, the exchange of genetic material between chromatids.

cruciform

Structure formed by the pairing of inverted repeats on both strands of double-stranded DNA.

cyanobacteria

A kind of bacteria that are aquatic and photosynthetic; exist today, and also identified in fossils more than 3.5 billion years old.

cyanobacterial

Relating to or caused by the the photosynthetic organism cyanobacteria.

cyclic AMP

A second messenger formed from ATP that is involved in signal transduction, generally translating hormonal signals to the nucleus.

cyclic AMP response element

An element, or sequence, in DNA found in genes whose transcription is induced by cAMP.

cyclic AMP response element binding transcription factor

A protein that is activated by cAMP and subsequently phosphorylated by protein kinase A. It then binds to cAMP response elements in certain genes to initiate their transcription.

cyclin

A key protein in the control of the cell cycle; combines with a cyclin-dependent kinase (CDK). The levels of cyclin rise and fall in the course of the cell cycle.

cyclin-dependent kinase

A key protein in the control of the cell cycle; combines with cyclin.

cytogenetics

The study of chromosomal structure and function and how they are inherited.

cytokine

A potent immunomodulator that is secreted by an immune cell and affects host response to infection or tissue trauma; can be a protein or glyocoprotein; examples are interleukins or interferons.

cytokinesis

Cytokinesis is the physical process of cell division that divides the cytoplasm of a parent cell into two daughter cells

cytoplasm

The thick solution of the cell body where organelles and other cellular components are housed; a collection of aqueous molecules and macromolecules that fills each cell and is enclosed by the cell membrane, but not present in the nucleus; mainly composed of water, salts and proteins.

cytoplasmic inheritance

Inheritance of characteristics encoded by genes located in the cytoplasm. Because the cytoplasm is usually contributed entirely by only one parent, cytoplasmically inherited characteristics are usually inherited from a single parent.

cytoplasmic trait

A trait encoded by a gene found in a cytoplasmic organelle.

cytosine

Pyrimidine in DNA and RNA.

cytosine methylation

The addition of methyl groups to cytosine nucleotides in a DNA molecule.

cytoskeleton

The internal structural framework of a cell that contributes to cell shape and movement; composed of microflaments, intermediate filaments, and microtubulues.

cytosol

The aqueous component of the cell cytoplasm.

TOP

D loop

Region of mitochondrial DNA that contains an origin of replication and promoters; is displaced during initiation of replication, leading to the name displacement, or D, loop.

dam methylase

An enzyme that adds methyl groups to specific sites in DNA, inhibiting transcription from these sites.

dams

The female parents in a quantitative genetic breeding experiment.

Danio rerio

The Latin name for zebrafish, a commonly used model organism.

deamination

Loss of an amino group (NH2) from a base.

dedifferentiated

A cell less specialized than the cell it descended from, such as a cancer cell.

degenerate code

Refers to the fact that the genetic code contains more information than is needed to specify all 20 common amino acids.

deletion mapping

Technique for determining the chromosomal location of a gene by studying the association of its phenotype or product with particular chromosome deletions.

deletion mutation

A type of mutation in a DNA sequence involving the removal of one or more nucleotides.

deletion stock

A line of Drosophila that has a piece of a chromosome missing from its genome.

delta sequence

Long terminal repeat in Ty elements of yeast.

deme

A local interbreeding unit within a metapopulation.

demographic

Pertaining to processes that change the size of a population (i.e., birth, death, dispersal).

denaturation

Process that separates the strands of doublestranded DNA when DNA is heated.

density dependent selection

Selection that differs according to population density.

density-dependent

Affected by population density.

deoxyribocleotide

Basic building block of DNA, consisting of a deoxyribose sugar, a phosphate, and a nitrogenous base.

deoxyribonuclease I

An enzyme that makes single-stranded nicks in DNA.

deoxyribonucleic acid

The primary molecule of inheritance in nearly all organisms; a double-stranded polymer of nucleotides that contains the sugar deoxyribose; abbreviated as DNA.

deoxyribose sugar

Five-carbon sugar in DNA; lacks a hydroxyl group on the 2'-carbon atom.

depurination

Break in the covalent bond connecting a purine base to the 1'-carbon atom of the deoxyribose sugar, resulting in the loss of the purine base. The resulting apurinic site cannot provide a template in replication, and a nucleotide with another base may be incorporated into the newly synthesized DNA strand opposite the apurinic site.

derived character

A character (or character state) that has evolved from an antecedent (ancestral) character or state.

deterministic

Causing a fixed outcome, given initial conditions.

diakinesis

Fifth substage of prophase I in meiosis. In diakinesis, chromosomes contract, the nuclear membrane breaks down, and the spindle forms.

dicentric bridge

Structure produced when the two centromeres of a dicentric chromatid are pulled toward opposite poles, stretching the dicentric chromosome across the center of the nucleus. Eventually, the dicentric bridge breaks as the two centromeres are pulled apart.

dicentric chromatid

Chromatid that has two centromeres; produced when crossing over takes place within a paracentric inversion. The two centromeres of the dicentric chromatid are frequently pulled toward opposite poles in mitosis or meiosis, breaking the chromosome.

Dicer

A ribonuclease that cleaves double-stranded RNA molecules into fragments approximately 20 to 25 nucleotides long that ultimately interfere with the expression of the corresponding gene.

dideoxyribonucleoside triphosphate

Special substrate for DNA synthesis used in the Sanger dideoxy sequencing method; identical with dNTP (the usual substrate for DNA synthesis) except that it lacks a 3'-OH group. The incorporation of a ddNTP into DNA terminates DNA synthesis.

diethylstilbestrol

A synthetic nonsteroidal estrogen that was found to cause birth defects in pregnant women.

differential gene expression

Differences in the time, location, and/or quantitative level at which a gene expresses the protein it encodes. Differential gene expression involves differences between species, developmental stages, or physiological states in the specific cells, tissues, structures, or body segments that express a given gene; it is believed to be a significant agent of morphological change over evolutionary time.

differentiation

The acquisition of cell-specific differences during a multicellular organism's embryonic development or adult life; reflects gene expression and activation of transcription factors.

dihybrid cross

A dihybrid cross describes a mating experiment between two organisms that are identically hybrid for two traits

dioecious

Describes a species with male and female reproductive structures in separate individuals.

diploid

Describes a cell that contains two copies of each chromosome.

diplotene

Fourth substage of prophase I in meiosis. In diplotene, centromeres of homologous chromosomes move apart, but the homologs remain attached at chiasmata.

direct development

A life history in which the intermediate larval stage is omitted and development proceeds directly from an embryonic form to an adult-like form.

direct repair

DNA repair in which modified bases are changed back to their original structures.

direct selection

The type of selection in which there is a causal relationship between a phenotypic trait and fitness, which can result in adaptation.

directional selection

Over time, the change in the mean value of a character in a population that is either higher or lower than its current mean value, resulting in a shift in the plot of trait frequency.

discontinuous characteristic

Exhibits only a few, easily distinguished phenotypes. An example is seed shape in which seeds are either round or wrinkled.

discontinuous replication

Replication of the lagging strand in the direction opposite that of unwinding, which means that DNA must be synthesized in short stretches (Okazaki fragments).

discordant

Refers to a pair of twins of whom one twin has the trait under consideration and the other does not.

discrete generations

A life history, like that of an annual plant, in which the parental generation has died by the time the offspring generation reproduces.

discrete polymorphism

A phenotypic trait that exhibits only a few (usually two or three) distinct types or morphs.

disease

A pathological condition of the body that results in abnormal functioning of an organ or organ system.

disomy

Describes the state of cell that has two members of a pair of homologous chromosomes.

dispersal

In population biology, movement of individual organisms to different localities; in biogeography, extension of the geographic range of a species by movement of individuals.

displaced chromosome duplication

Duplication of a chromosome segment in which the duplicated segment is some distance from the original segment.

disruptive selection

In a population over time, a type of selection that favors phenotypic extremes.

divergence

The evolution of increasing difference between lineages in one or more characters.

diversification

An evolutionary increase in the number of species in a clade, usually accompanied by divergence in phenotypic characters.

diversifying selection

See disruptive selection.

dizygotic twins

Nonidentical twins that arise when two different eggs are fertilized by two different sperm; also called fraternal twins.

DNA

Deoxyribonucleic acid, the primary molecule of inheritance in nearly all organisms; a double-stranded polymer of nucleotides that contains the sugar deoxyribose.

DNA backbone

The component of DNA strands composed of deoxyribose sugar and phosphate; main structure to which nucleotides attach to create a sequence template.

DNA binding domain

A general term for a single- or double-stranded region of DNA for which a specific protein has an affinity to bind.

DNA fingerprinting

Technique used to identify individuals by examining their DNA sequences.

DNA footprinting

Technique used to determine which DNA sequences are bound by a protein.

DNA forensics

A field of science that uses DNA evidence to assist in the solving of crimes.

DNA gyrase

E. coli topoisomerase enzyme that relieves torsional strain that builds up ahead of the replication fork.

DNA helicase

Protein that unwinds double-stranded DNA by breaking hydrogen bonds.

DNA library

Collection of bacterial colonies containing all the DNA fragments from one source.

DNA ligase

Enzyme that catalyzes the formation of a phosphodiester bond between adjacent 3_-OH and 5_-phosphate groups in a DNA molecule.

DNA methylation

A heritable chemical change to DNA involving a methyl group attaching to DNA; the accumulation is a form of epigenetic control of gene expression.

DNA polymerase

An enzyme that catalyzes the synthesis of new DNA.

DNA polymerase I

Bacterial DNA polymerase that removes and replaces RNA primers with DNA nucleotides.

DNA polymerase II

Bacterial DNA polymerase that takes part in DNA repair; restarts replication after synthesis has halted because of DNA damage.

DNA polymerase III

Bacterial DNA polymerase that synthesizes new nucleotide strands off the primers.

DNA polymerase IV

Bacterial DNA polymerase; probably takes part in DNA repair.

DNA polymerase V

Bacterial DNA polymerase; probably takes part in DNA repair.

DNA probe

A general term for a short sequence of DNA or RNA that has a label attached, so that when applied to a biological sample it will reveal the location of a complementary sequence in that sample.

DNA repair

Any one of many cellular processes that attempts to correct errors in cellular DNA introduced via the environment or during cell division.

DNA sequencing

Process of determining the sequence of bases along a DNA molecule.

DNase I hypersensitive site

Chromatin region that becomes sensitive to digestion by the enzyme DNase I.

DNAse I sensitivity

A method that detects DNA sites in chromosomes that show increased sensitivity to digestion by DNAse I. These sites probably represent regions of the chromosome that are nucleosome-free, and often correspond to gene-control regions.

domain

In protein chemistry, a protein segment,100 amino acids or less, that can fold into a specific three-dimensional structure independent of other domains on the same protein; in general biology, an organizing principle used to define separate categories of life.

dominance

The extent to which a trait appears in a population or an individual, resulting from allelic interactions.

dominance genetic variance

Component of the genetic variance that can be attributed to dominance (interaction between genes at the same locus).

dominance variance

The magnitude of the phenotypic (and genotypic) variance that is due to dominance, that is, the interaction between alleles at the same locus.

dominant

Refers to a trait that appears more frequently than another trait, resulting from interactions between gene alleles.

dominant marker

A known DNA sequence that generally characterizes an example of variation in a genome, like a mutation, single nucleotide polymorphism, or variable number of tandem repeats. Dominant markers allow for analyzing the DNA in multiple parts across an entire genome.

dosage compensation

Equalization in males and females of the amount of protein produced by X-linked genes. In placental mammals, dosage compensation is accomplished by the random inactivation of one X chromosome in the cells of females.

double fertilization

Fertilization in plants; includes the fusion of a sperm cell with an egg cell to form a zygote and the fusion of a second sperm cell with the polar nuclei to form an endosperm.

double helix

The double helix is a description of the molecular shape of a double-stranded DNA molecule.

double-strand-break model

Model of homologous recombination in which a DNA molecule undergoes doublestrand breaks.

down mutation

Decreases the rate of transcription.

Down syndrome

Down syndrome; Characterized by variable degrees of mental retardation, characteristic facial features, some retardation of growth and development, and an increased incidence of heart defects, leukemia, and other abnormalities; caused by the duplication of all or part of chromosome 21 (trisomy 21).

downstream

Toward the 3' end of a chain of nucleotides.

downstream core promoter element

Consensus sequence [RG(A or T)CGTG] found in some eukaryotic RNA polymerase II core promoters; usually located approximately 30 bp downstream of the transcription start site.

Drosophila melanogaster

The fruit fly, a commonly used model organism for genetic and development studies.

drug

A substance that is used in the treatment of disease.

dyad

A term used to describe a pair; in genetics, a pair of homologous chromosomes after separation from a tetrad (foursome).

TOP

ecological climax

A state for communities which represents the final, or permanent end-stage of succession.

ecological community

A group of interacting or potentially interacting species that inhabit a particular location at a particular time, with shared environmental influences.

ecological composition

The relative proportion of habitat types in the landscape, regardless of spatial distribution.

ecological facilitation

When one species, or a group of species, colonizes a disturbed area, and subsequently alters the environment of that area, by altering soil nutrients, light accessibility, or water availability. The effect is making the area more habitable for later successional species.

ecological niche

The range of combinations of all relevant environmental variables under which a species or population can persist; often more loosely used to describe the .role. of a species, or the resources it utilizes.

ecological release

The expansion of a population's niche (e.g., range of habitats or resources used) where competition with other species is alleviated.

ecological remediation

The process or plan of improving an existing ecosystem or creating a new one by replacing what has deteriorated or been destroyed.

ecological tolerance

The specific range of environmental factors, such as light, temperature, and availability of water, within which an organism can survive.

ecotype

A genetically determined phenotype of a species that is found as a local variant associated with certain ecological conditions.

ecotypes

Locally adapted populations that are phenotypically and genetically differentiated for adaptive traits.

ectoderm

Outer germ layer of an embryo, formed by gastrulation; distinct from endoderm and mesoderm.

Edward syndrome

Characterized by severe retardation, low-set ears, a short neck, deformed feet, clenched fingers, heart problems, and other disabilities; results from the presence of three copies of chromosome 18 (trisomy 18).

effective population size

The effective size of a real population is equal to the number of individuals in an ideal population (i.e., a population in which all individuals reproduce equally) that produces the rate of genetic drift seen in the real population.

egg

Female gamete.

egg-polarity gene

Determines the major axes of development in an early fruit fly embryo. One set of egg-polarity genes determines the anterior-posterior axis and another determines the dorsalventral axis.

electrophoresis

A technique for separating macromolecules (proteins, RNA, DNA) on a gel using an electric field.

electrophoretic mobility shift assay

A method that examines whether a specific protein binds to a specific piece of DNA. After allowing the DNA and protein to interact in vitro, the complex is electrophoresed, analyzed, and compared to DNA alone versus DNA plus protein. If the protein has bound, the complex will not move as far into the gel due to its larger size.

elimination sample

A DNA sample collected from an individual not thought to be a suspect in a crime (such as the partner of a rape victim) to help investigators to analyze the evidence.

elongation factor for translation

Protein that facilitates the extension of a growing polypeptide.

elongation factor G (EF-G)

Protein that combines with GTP and is required for movement of the ribosome along the mRNA during translation.

elongation factor Ts

Protein that regenerates elongation factor Tu in the elongation stage of protein synthesis.

elongation factor Tu

Protein taking part in the elongation stage of protein synthesis; forms a complex with GTP and a charged amino acid and then delivers the charged tRNA to the ribosome.

embryo

Stage of human development between zygote and fetus, typically the first eight weeks of human development.

embryonic stem cell

A totipotent cell from the inner layer of the blastocyst, can be cultured from a preimplantation embryo.

empiric risk

Probability that something will recur based on empirical evidence, rather than theory or guess; used to predict traits in a population based on past observations of that population.

end labeling

Method for adding a radioactive or chemical label to the ends of DNA molecules.

endangered species

A species that is likely to become extinct in all or a major portion of its range.

endemic

Of a species, restricted to a specified region or locality.

endocrine system

A system of glands, organs and tissues, along with the hormones they secrete into the bloodstream, which together regulate animal physiology, reproduction, and behavior.

endoderm

The inner germ layer of an embryo; formed by gastrulation.Distinct from mesoderm and ectoderm.

endonuclease

An enzyme that cleaves a nucleic acid.

endoplasmic reticulum

A labyrinth of membranous intracellular tubules on which proteins, lipids, and sugars are synthesized.

endosymbiotic theory

States that some membrane-bounded organelles, such as mitochondria and chloroplasts, in eukaryotic cells originated as free-living eubacterial cells that entered into an endosymbiotic relation with a eukaryotic host cell and evolved into the present-day organelles; supported by a number of similarities in structure and sequence between organelle and eubacterial DNAs.

engrailed

A homeodomain containing protein that is expressed in the anterior region of 14 evenly spaced "stripes" during early Drosophila embryonic development.

enhancer

A regulatory DNA sequence that, when bound by specific proteins called transcription factors, enhances the transcription of an associated gene.

enhancer trap

A system that allows investigators to find endogenous enhancers in a genome. The reporter gene in an enhancer trap is often based on a transposable element that can easily insert into the genome at random locations. This reporter gene has minimal promoter sequence so it is only expressed when it inserts near an endogeous sequence that results in gene transcription.

environment

Usually, the complex of external physical, chemical, and biotic factors that may affect a population, an organism, or the expression of an organism's genes; more generally, anything external to the object of interest (e.g., a gene, an organism, a population) that may influence its function or activity. Thus, other genes within an organism may be part of a gene's environment, or other individuals in a population may be part of an organism's environment.

environmental correlation

A measure of the degree to which two traits respond to variation in the same environmental factors.

environmental deviation

The difference between the phenotypic and genotypic values caused by the environment.

environmental variance

Any phenotypic variance resulting from differing environmental pressures on portions of a population.

enzymatic

Describes a process mediated by an enzyme.

enzyme

A macromolecule, usually a protein, that catalyzes biochemical reactions, lowering the activation energy and increasing the rate of reaction.

epidemiology

The study of the frequency and determinants of disease across populations.

epigenetic process

A reversible process that affects the expression of genes; often occurring on top of ("epi") genetic material, and accumulating throughout an organisms's life. Example) the methylation of DNA.

epigenomic

The pattern of epigenetic silencing across an entire genome of an organism.

episome

Plasmid capable of integrating into a bacterial chromosome.

epistasis

The interaction of nonallelic genes to control a phenotype in a different gene; contrast to same-gene allelic interactions to control a single-gene phenotype.

epistatic gene

Masks or suppresses the effect of a gene at a different locus.

epistatic selection

The type of selection in which fitness depends upon nonadditive interactions between alleles at different loci.

epistatic variance

The portion of the phenotypic (and genotypic) variance that is due to epistasis, that is, interactions among gene loci.

epitope

Part of a protein that is recognized by an antibody.

equational division

The second meiotic division; any cell division that does not reduce chromosome number, and creates equal halves, having divided along the equatorial axis of the cell.

equilibrium

An unchanging condition, as of population size or genetic composition. Also, the value (e.g., of population size, allele frequency) at which this condition occurs. An equilibrium need not be stable. See also stability, unstable equilibrium.

equilibrium density gradient centrifugation

Method used to separate molecules or organelles of different density by centrifugation.

erythroblast

A type of red blood cell that has a nucleus.

erythrocyte

Red blood cell, shaped like a disc with a concave depression on both sides, carrier of hemoglobin.

erythroid cell

The progenitor cell of erythrocytes.

essentialism

The philosophical view that all members of a class of objects (such as a species) share certain invariant, unchanging properties that distinguish them from other classes.

estrogen

A steroid hormone produced by the ovaries.

eubacteria

One of the three primary divisions of life. Eubacteria consist of unicellular organisms with prokaryotic cells and include most of the common bacteria.

euchromatin

Chromatin that undergoes condensation and decondensation in the course of the cell cycle.

eugenics

The practice of controlled breeding in humans with the goal of achieving specific traits among offpsring; usually characterized by a social goal.

eukaryote

Eukaryotes are organisms whose cells contain a nucleus and other organelles. There is a wide range eukaryotic organisms, including all animals, plants, fungi, protists and most algae, and eukaryotes may be either single-celled or multicellular

eukaryotic

Describing a characteristic of a eukaryote.

Euler-Mascheroni constant

A mathematical constant denoted by the greek letter gamma.

euploid

A term describing a somatic cell containing the normal number of chromosomes for that species; typcially the number is a multiple of the hapoloid number.

eutherian

A taxon that includes placental mammals (including humans).

evolution

Evolution is a process that results in changes in the genetic material of a population over time

evolutionarily significant units

Populations of threatened or endangered organisms that need to be saved from extinction.

evolutionarily stable strategy

A phenotype such that, if almost all individuals in a population have that phenotype, no alternative phenotype can invade the population or replace it.

evolutionary constraint

Any biological factor that slows the rate of adaptive evolution.

evolutionary fitness

The probability that the genetic contribution of an individual's specific trait or traits will not die out in future generations; calculated as the number of offspring contributed by an individual relative to the number of offspring produced by other members of the population.

evolutionary grade

A group of species that have evolved the same state in one or more characters; typically constitute a paraphyletic group.

evolutionary reversal

The evolution of a character from a derived state back toward a condition that resembles an earlier state.

evolutionary synthesis

The reconciliation of Darwin's theory with the findings of modern genetics, which gave rise to a theory that emphasized the coaction of random mutation, selection, genetic drift, and gene flow; also called the modern synthesis.

evolutionary trajectories

Pathways that populations traverse across adaptive landscapes during evolution, tracing the ways that the joint allele frequencies or mean phenotypes might evolve.

ex vivo gene therapy

A clinical therapy that applies genetic alteration to cells removed from a patient, followed by re-implanation of the same cells.

exaptation

The evolution of a function of a gene, tissue, or structure other than the one it was originally adapted for; can also refer to the adaptive use of a previously nonadaptive trait.

excision repair

A DNA repair mechanism, usually enzyme-catalyzed, that invovles the removal of damaged nucleotides, and replacement of the correct sequence, guided by the intact complementary DNA strand.

exit site

One of three sites in a ribosome occupied by a tRNA. In the elongation stage of translation, the tRNA moves from the peptidyl (P) site to the E site from which it then exits the ribosome.

exon

The coding sections of an RNA transcript, or the DNA encoding it, which are translated into a protein.

exon shuffling

The formation of new genes by assembly of exons from two or more preexisting genes. The classical model of exon shuffling generates new combinations of exons mediated via recombination of intervening introns; however, exon shuffling can also come about by retrotransposition of exons into preexisting genes.

exonic splicing enhancer

Sequences located in exons adjacent to 5' and 3' splice sites that are required for proper recognition of the splice sites by the spliceosome. These enhancers are position dependent.

exonuclease

An enzyme that cleaves one nucleotide at a time in the 3' --> 5' direction.

expanding trinucleotide repeat

Mutation in which the number of copies of a trinucleotide (or some multiple of three nucleotides) increases in succeeding generations.

expected heterozygosity

Proportion of individuals that are expected to be heterozygous at a locus when the Hardy-Weinberg assumptions are met.

expected outcome

The predicted outcome of observable events in an experiment.

expressed-sequence tag

Unique fragment of DNA from the coding region of a gene, produced by the reverse transcription of cellular RNA. Parts of the fragments are sequenced so that they can be identified.

expression vector

Cloning vector containing DNA sequences such as a promoter, a ribosome-binding site, and transcription initiation and termination sites that allow DNA fragments inserted into the vector to be transcribed and translated.

expressivity

Degree to which a trait is expressed.

extent

The range over which a physical or temporal event occurs.

extrinsic hypothesis

Prediction of the number of observed individuals with specific characteristics based on calculations performed before the experiment is completed.

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F factor

Episome of E. coli that controls conjugation and gene exchange between E. coli cells. The F factor contains an origin of replication and genes that enable the bacterium to undergo conjugation.

F1 cross

A cross between two pure-breeding or homozygous lines. Such homozygous lines are also called parental lines. Parental lines differ in a pair of contrasting traits (e.g, red eyes or white eyes in Drosophila, or tall or short pea plants). Females from one of the parental lines (Parental Line 1 or P1) are mated to males from the other parental line (Parental Line 2 or P2). The mating is called the F1 cross, and the progeny resulting from this mating are the F1 progeny or F1 generation.

F1 generation

Offspring of the initial parents (P) in a genetic cross.

F1 reciprocal

The reverse of an F1 cross. If females from one line (Parental Line 1) are crossed with males from Parental Line 2 for an F1 cross, the reciprocal cross would use males from Parental Line 1 and females from Parental Line 2. P2 females can be mated with P1 males. Progeny from this mating are the reciprocal F1 progeny or the reciprocal F1 generation.

The F2 cross is the quintessential segregating generation. For a single autosomal gene, the F2 generation is the first generation in which both parental phenotypes occur. For two or more genes, it is the first generation in which all new combinations of phenotypes can occur at the same time. These new phenotypes reflect independent assortment among the allele genes that formed the gametes of the F1 generation.

F2 generation

Offspring of the F1 generation in a genetic cross; the third generation of a genetic cross.

facultative

Optional, discretionary. The opposite of obligate.

familial Down syndrome

Caused by a Robertsonian translocation in which the long arm of chromosome 21 is translocated to another chromosome; tends to run in families.

family history

A diagram representing the genealogical history of an individual. In medicine, a family history also includes any health information known about an individual's relatives.

fecundity

The quantity of gametes (usually eggs) produced by an individual.

fertilization

Fusion of gametes, or sex cells, to form a zygote.

fetal cell sorting

Separation of fetal cells from maternal blood. Genetic testing on the fetal cells can provide information about genetic diseases and disorders in the fetus.

fetus

A term for the prenatal mammal; a prenatal human after the eighth week of development.

first filial cross

Crossing of two parental lines.

first polar body

One of the products of meiosis I in oogenesis; contains half the chromosomes but little of the cytoplasm.

fitness

The success of an entity in reproducing; hence, the average contribution of an allele or genotype to the next generation or to succeeding generations.

fitness cost

A reduction in fitness of a trait occurring in correlation with a benefit or increase in fitness of another trait.

fitness function

The curve that describes the relationship between fitness and a phenotypic trait.

fitness surface

A three-dimensional representation of the relationship between two phenotypic traits and individual fitness.

fixation index

Wright's measure of population differentiation.

fixed

A population in which all members are homozygous for the same allele at a given locus (antonyms: segregating, polymorphic).

flanking direct repeat

Short, directly repeated sequence produced on either side of a transposable element when the element inserts into DNA.

fluorescence assay

A method that tags a biological sample with a fluorophore in order to monitor that sample.

fluorescence in situ hybridization

A technique that binds fluorescently labeled DNA or RNA probes to complementary sequences occuring in cells and tissues; used to label the location of specific sequences in a biological sample.Abbreviated as FISH.

footprinting

A technique in biology that attempts to map protein binding sites on DNA by examining the areas on a DNA molecule that are protected from nuclease activity due to protein binding.

forward mutation

Alters a wild-type phenotype.

founder effect

The principle that the founders of a new population carry only a fraction of the total genetic variation in the source population.

founder event

A type of bottleneck, defined as the creation of a new population by a small number of colonists.

fragile site

Constriction or gap that appears at a particular location on a chromosome when cells are cultured under special conditions. One fragile site on the human X chromosome is associated with mental retardation (fragile-X syndrome) and results from an expanding trinucleotide repeat.

frameshift mutation

A genetic mutation caused by a deletion or insertion in a DNA sequence that shifts the way the sequence is read.

fraternal twins

Nonidentical twins that arise when two different eggs are fertilized by two different sperm.

free radical

A highly reactive molecule that has at least one unpaired, or free, electron.

frequency

Usually used to mean proportion (e.g., the frequency of an allele is the proportion of gene copies having that allelic state).

frequency dependent selection

A type of natural selection in which the fitness of each genotype or phenotype depends on its frequency in the population. In positive frequency dependence, fitness increases as the genotype or phenotype becomes more common, and in negative frequency dependence, fitness increases as the genotype or phenotype becomes rarer.

frequency distribution

Graphical way of representing values. In genetics, usually the phenotypes found in a group of individuals are displayed as a frequency distribution. Typically, the phenotypes are plotted on the horizontal (x) axis and the numbers (or proportions) of individuals with each phenotype are plotted on the vertical (y) axis.

function

The way in which a character contributes to the fitness of an organism.

functional genomics

Area of genomics that studies the functions of genetic information contained within genomes.

fusion pattern

Method of using protein fusion to infer gene function. If two proteins that are separate in one species exist as a fused protein in another species, the two separate proteins in the first species may be functionally related.

fusion protein

A protein formed from transcription of two or more genes in sequence, with no stop codon in between them. They can occur naturally in cancer cells, and create complex proteins involved in cancerous mechanisms.

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G-banding

A staining protocol for chromosomes. Chromosomes are generally pretreated with an enzyme that facilitates staining with Giemsa dye. Each chromosome has a unique staining pattern that allows it to be distinguished from other chromosomes.

G-matrix

A square matrix with additive genetic variances for the traits on the diagonal and additive genetic covariances on the off-diagonal.

Nondividing stage of the cell cycle.

Stage in interphase of the cell cycle in which the cell grows and develops.

G1/S checkpoint

Important point in the cell cycle. After the G1/S checkpoint has been passed, DNA replicates and the cell is committed to dividing.

Stage of interphase in the cell cycle that follows DNA replication. In G2, the cell prepares for division.

G2/M checkpoint

Important point in the cell cycle near the end of G2. After this checkpoint has been passed, the cell undergoes mitosis.

gain-of-function mutation

Produces a new trait or causes a trait to appear in inappropriate tissues or at inappropriate times in development.

gamete

The reproductive cell of an organism; typically contains half or a reduced number of chromosomes compared to a somatic cell. In mammals, gametes are haploid cells that fuse to form a diploid zygote.

gamete intrafallopian transfer

A method of transferring sperm and ooocytes to a woman's uterine tube, to assist in human reproduction.

gametes

The plural form of gamete.

gametic array

A method of describing in mathematical terms that the sum of the proportion of gametes that carries a specific allele is equal to 1. For example, a heterozygote embryo of genotype Aa has the gametic array of 1/2A + 1/2a = 1.

gametic phase disequilibrium

A nonrandom relationship between the alleles present at two or more loci, which can cause a genetic correlation.

gametophyte

Haploid phase of the life cycle in plants.

gap genes

In fruit flies, set of segmentation genes that define large sections of the embryo. Mutations in these genes usually eliminate whole groups of adjacent segments.

gastrula

The term for a three-layered embryo that forms after the gastrulation event.

gatekeeper

Tumor suppressor genes or proteins that regulate cellular responses that prevent the survival or proliferation of potential cancer cells.

gel electrophoresis

Gel electrophoresis is a laboratory method used to separate mixtures of DNA, RNA, or proteins according to their sizes

gene

A region of DNA (deoxyribonucleic acid) coding either for the messenger RNA encoding the amino acid sequence in a polypeptide chain or for a functional RNA molecule.

gene action

The manner in which genotype affects phenotype, including additivity, dominance, pleiotropy, and epistasis.

gene array

A technology that allows investigators to measure the relative abundance of transcripts of specific genes by binding labeled cDNA probes from cells to a microchip with covalently attached microscopic spots of DNA, with each spot representing a single gene.

gene cloning

Inserting DNA fragments into bacteria in such a way that the fragments will be stable and copied by the bacteria.

gene complex

A group of two or more genes that are members of the same family and in most cases are located in close proximity to one another in the genome, often in tandem separated by various amounts of intergenic, noncoding DNA.

gene conversion

A process involving the unidirectional transfer of DNA information from one gene to another. In a typical conversion event, a gene or part of a gene acquires the same sequence as the other allele at that locus (intralocus or intraallelic conversion), or the same sequences as a different, usually paralogous, locus (interlocus conversion). One consequence of gene conversion may be the homogenization of sequences among members of a gene family.

gene doping

Modifying the genetic makeup of an individual with the specific purpose of improving athletic performance.

gene duplication

When new genes arise as copies of preexisting gene sequences. The result can be a gene family.

gene expression

The process by which the genetic information in DNA is transcribed into mRNA.

gene expression profiling

A method for defining the particular set of gene expression patterns in a particular source of tissue or cells; the use of DNA microarrays to detect mRNA expression sets with reverse transcription to labeled cDNAs.

gene family

Two or more loci with similar nucleotide sequences that have been derived from a common ancestral sequence.

gene flow

The movement of alleles between previously separate populations caused by migration and subsequent mating.

gene frequency

See allele frequency.

gene interaction

Interactions between genes at different loci that affect the same characteristic.

gene knockdown

Temporary decrease in gene expression caused by an experimental technique, often an antisense oligo.

gene knockout

Permanent change in DNA leading to the loss of function of a gene, caused by a manipulation of the organism's DNA in a laboratory followed by breeding to produce a population of organisms that are homozygotes for the changed gene.

gene mutation

Affects a single gene or locus.

gene neighbor analysis

Analysis of the locations of genes in different species to infer gene function. If two genes are consistently linked in different species, they may be functionally related.

gene ontology

A hierarchical organization of concepts (ontology) with three organizing principles: molecular function, the tasks done by individual gene products, an example of which is 'transcription factor'; biological process, broad biological goals, such as mitosis, that are accomplished by ordered assemblies of molecular functions; cellular component, subcellular structures, locations and macromolecular complexes (examples include the nucleus and the telomere).

gene pool

The totality of the genes of a given sexual population.

gene regulation

Mechanisms and processes that control the phenotypic expression of genes.

gene regulatory site

A section of DNA where regulatory proteins such as transcription factors bind preferentially; binding of regulatory proteins to such as site changes gene expression.

gene sequencing

Determining the complete sequence of a molecule or molecules of DNA, often through highly automated procedures.

gene silencer

A gene sequence that represses transcription, yet has similar properties to a gene enhancer.

gene targeting

A technique that modifies genes using homologous recombination, and uses a specific vector to do it; may involve the removal of exons or imposition of point mutations.

gene therapy

Use of recombinant DNA to treat a disease or disorder by altering the genetic makeup of the patient's cells.

gene-environment interaction

In epidemiological studies, the differential effect that environmental conditions can have on an organism depending on the organism's genotype; in studies of disease, the observed effect of behavior and environmental exposures on human health, through the corrleated tracking of specific genotypes and vulnerabilities to disease.

general transcription factor

Transcription factors are proteins that are involved in the process of converting, or transcribing, DNA into RNA

generalized transduction

Transduction in which any gene may be transferred from one bacterial cell to another by a virus.

generic drug

A copy of a drug that is introduced after the patent expires.

genes

A sequence of nucleotides that code for a trait; the basic unit of inheritance.

genetic bottleneck

Sampling error that arises when a population undergoes a drastic reduction in population size; leads to genetic drift.

genetic clone

A genetically identical copy of an individual, cell or fragment of DNA.

genetic code

The genetic code is a set of rules that defines how the four-letter code of DNA is translated into the 20-letter code of amino acids, which are the building blocks of proteins.

genetic conflict

Antagonistic fitness relationships between alleles at different loci in a genome.

genetic correlation

Phenotypic correlation due to the same genes affecting two or more characteristics.

genetic counseling

Educational process that attempts to help patients and family members deal with all aspects of a genetic condition.

genetic cross

A controlled mating.

genetic differentiation

Differences between populations in allele frequencies at one or more loci, or in mean phenotypes in a common environment.

genetic distance

Any of several measures of the degree of genetic difference between populations, based on differences in allele frequencies.

genetic drift

The change in frequency of alleles in a population over time, due to changes in the transmission of gametes to successive generations among individuals in a finite population.

genetic engineering

Common term for recombinant DNA technology.

genetic identity

A measure indicating the proportion of genes that are identical between two populations; on a small scale, when two (or more) genetic sequences are exactly the same.

Genetic Information Nondiscrimination Act

A federal law will protect Americans against discrimination based on their genetic information when it comes to health insurance and employment.

genetic load

Any reduction of the mean fitness of a population resulting from the existence of genotypes with a fitness lower than that of the most fit genotype.

genetic map

A linear representation of the arrangement of genes on a chromosome, typically based on the frequency of recombination between different loci.

genetic marker

Any gene or DNA sequence used to identify a location on a genetic or physical map.

genetic maternal effect

Determines the phenotype of an offspring. With genetic maternal effect, an offspring inherits genes for the characteristics from both parents, but the offspring's phenotype is determined not by its own genotype but by the nuclear genotype of its mother.

genetic screen

An experiment in which mutant organisms are generated in the laboratory and isolated based on a specific, desired phenotype.

genetic screening

Analyzing DNA to determine the presence of a genetic variation that is responsible for an inherited disease.

genetic substitution

The complete replacement of one allele by another within a population or species over evolutionary time.

genetic test

In humans, a medical test using blood or tissue that can determine if there is a specific known sequence in the patient's genome.

genetic variance

Component of the phenotypic variance that is due to genetic differences among individual members of a population.

genetic variation

Naturally occurring genetic differences among organisms in the same species.

genetic-environmental interaction variance

Component of the phenotypic variance that results from an interaction between genotype and environment. Genotypes are expressed differently in different environments.

genetically engineered organism

Any organism whose genetic makeup has been artificially modified, either by intentional directed breeding or transgenic insertion.

genetically modified foods

Foods derived from genetically modified crops.

genic balance system

Sex-determining system in which sexual phenotype is controlled by a balance between genes on the X chromosome and genes on the autosomes.

genic interaction variance

Component of the genetic variance that can be attributed to genic interaction (interaction between genes at different loci).

genic selection

A form of selection in which the single gene is the unit of selection, such that the outcome is determined by fitness values assigned to different alleles.

genic sex determination

Sex determination in which the sexual phenotype is specified by genes at one or more loci, but there are no obvious differences in the chromosomes of males and females.

genome

A genome is the complete set of genetic information in an organism. It provides all of the information required by an organism to function.

genome sequence

The order of nucleotides of a particular genome.

genome size

The total number of DNA base pairs contained within one copy of a genome.

genome wide association study

A case-control study in which genetic variation, often measured as SNPs that form haplotypes across the entire genome, is compared between people with a particular condition and unaffected individuals.

genomic imprinting

Differential expression of a gene that depends on the sex of the parent that transmitted the gene. If the gene is inherited from the father, its expression is different than if it is inherited from the mother.

genomic library

Collection of bacterial or phage colonies containing DNA fragments that consist of the entire genome of an organism.

genomics

Genomics is the large-scale study of all the genes in an organism.

genotype

A genotype is the particular the combination of alleles for a particular gene or locus.

genotypic array

A prediction of the possible genotypic combinations in an offspring based on the genotypes of two parents.

genotypic frequencies

The proportion of each of the various genotypes present in a population or sample of a population.

genotypic frequency

Proportion of a particular genotype.

genotypic value

The phenotype produced by a given genotype averaged across environments.

genotypic variance

The magnitude of the phenotypic variance that is due to all genetic causes, corresponding to the sum of the additive, dominance, and epistatic variances.

geographic variation

Differences among spatially distributed populations of a species.

germ-line mutation

Mutation in a germ-line cell (one that gives rise to gametes).

germ-plasm theory

States that cells in the reproductive organs carry a complete set of genetic information.

globin

The protein that carries the oxygen in red blood cells.

glycolipid

A complex molecule made of a carbohydrate sugar bonded to a lipid, usually embedded in cell membranes.

glycoprotein

A protein with carbohydrate modifications; can be secreted from the cell or embedded in the membrane; interactions between glycoproteins on adjacent cells permit cell-cell communication.

goodness of fit

An expression in statistics referring to the measure of how closely aligned a function derived from actual cumulative data is to a predicted model function.

gradualism

The proposition that large differences in phenotypic characters have evolved through many slightly different intermediate states.

green fluorescent protein

A protein originally isolated from the jellyfish (Aequorea victoria) that retains the property of fluorescing green when exposed to blue light and when fused to other cellular proteins.

group I intron

Belongs to a class of introns in some ribosomal RNA genes that are capable of self-splicing.

group II intron

Belongs to a class of introns in some protein-encoding genes that are capable of self-splicing and are found in mitochondria, chloroplasts, and a few eubacteria.

group selection

The differential rate of origination or extinction of whole populations (or species, if the term is used broadly) on the basis of differences among them in one or more characteristics.

guanine

Purine in DNA and RNA.

guide RNA

RNA molecule that serves as a template for an alteration made in mRNA during RNA editing.

gynandromorph

Individual organism that is a mosaic for the sex chromosomes, possessing tissues with different sex-chromosome constitutions.

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habitat fragmentation

The process of breaking a natural landscape into fewer, smaller and more disjointed areas of habitat. Loss of total area and variety of habitats is called change in composition. Change in the variety, spatial arrangement, shape and size of habitats is change in configuration.

habitat selection

The capacity of an organism (usually an animal) to choose a habitat in which to perform its activities. Habitat selection is not a form of natural selection.

hairpin

Secondary structure formed when sequences of nucleotides on the same strand are complementary and pair with each other.

hairpin loop (mRNA)

A hairpin loop is an unpaired loop of messenger RNA (mRNA) that is created when an mRNA strand folds and forms base pairs with another section of itself. Hairpins are a common type of secondary structure in RNA molecules

Hamilton's rule

A formula posited by William Donald Hamilton that specifies the conditions for reproductive altruism to evolve; attempts to explain;behavior whereby benefit is given to a relative at the expense of an individual's own reproductive success.

haplodiploid

A sex determination system in which sex is determined by the number of copies of the genome in the cells of the individual, rather than the presence or absence of a sex chromosome.

haploid

Describes cells that contain a single set of chromosomes.

haploid state

The condition or stage of a reproductive cycle or process whee a cell contains only one set of chromosomes.

haploinsufficiency

The appearance of a mutant phenotype in an individual cell or organism that is heterozygous for a normally recessive trait.

haploinsufficient gene

Must be present in two copies for normal function. If one copy of the gene is missing, a mutant phenotype is produced.

haplotype

A haplotype is a group of genes, which is inherited together by an organism from a single parent

HapMap Project

An international consortium of scientists working together to catalog the genetic variation in the human genome.

Hardy-Weinberg assumption

Allele frequencies will remain constant over time if there are no forces to change them.

Hardy-Weinberg equation

The Hardy-Weinberg equation is a mathematical expression that can be used to calculate the genetic variation of a population at equilibrium.

Hardy-Weinberg equilibrium

The Hardy-Weinberg equilibrium is a principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors.

Hardy-Weinberg law

Important principle of population genetics stating that, in a large, randomly mating population not affected by mutation, migration, or natural selection, allelic frequencies will not change and genotypic frequencies stabilize after one generation in the proportions p2 (the frequency of AA), 2pq (the frequency of Aa), and q2 (the frequency of aa), where p equals the frequency of allele A and q equals the frequency of allele a.

Heat shock element

A DNA sequence to which heat shock transcription factors bind when activated.

Heat shock transcription factor

A transcription factor that becomes activated upon cellular stress, resulting in its trimerization, binding to heat shock elements, and induction of transcription of a number of proteins.

heat-shock protein

Produced by many cells in response to extreme heat and other stresses; helps cells prevent damage from such stressing agents.

HeLa cell

An immortalized human cell line frequently used in biological research.

helicase

Helicase is an enzyme that unwinds and separates the two strands of the DNA double helix

helix-loop-helix

An area of a protein that folds in a predictable way and characterizes a group of transcription factors. Specifically, helix-loop-helix is a polypeptide that has a structure of two alpha-helices connected by a loop.

helix-turn-helix

A common protein motif characterized by two adjacent alpha helices that make contact with DNA and regulate gene transcription.

hemagglutinin

A type I integral membrane glycoprotein that binds to cell-surface receptors and facilitates fusion between the viral envelope and endosomal membrane. It is the main target antigen of the humoral immune response to influenza viruses.

hemizygote

An organism that has only one allele at a given locus.

hemizygous

Possessing a single allele at a locus. Males of organisms with XX-XY sex determination are hemizygous for X-linked loci, because their cells possess a single X chromosome.

heritability

The proportion of the variance in a trait in a population that is attributable to genetic variation (differences in genotype). Mathematically, the ratio of additive genetic variance to phenotypic variance.

hermaphroditism

Condition in which an individual organism possesses both male and female reproductive structures. True hermaphrodites produce both male and female gametes.

heterochromatin

Chromatin that remains in a highly condensed state throughout the cell cycle, found at the centromeres and telomeres of most chromosomes.

heterochrony

An evolutionary change in phenotype caused by an alteration of timing of developmental events.

heteroduplex DNA

DNA consisting of two strands, each of which is from a different chromosome.

heterogametic sex

The sex (male or female) that produces two types of gametes with respect to sex chromosomes. For example, in the XX-XY sex-determining system, the male produces both X-bearing and Y-bearing gametes.

heterokaryon

Cell possessing two nuclei derived from different cells through cell fusion.

heterokaryotype

A genome or individual that is heterozygous for a chromosomal rearrangement such as an inversion.

heteromorphic chromosomes

A pair of chromosomes that share some genetic homology but differ in physical characteristics like size, shape, or staining patterns.

heteroplasmy

Presence of two or more distinct variants of DNA within the cytoplasm of a single cell.

heterosis

Phenomenon in which the F1 generation has higher fitness than the parental strains or subpopulations that were crossed (mated) to produce them.

heterotroph

Organism that cannot synthesize its own food and must consume other food sources to receive complete nutrition requirements.

heterozygosity

In a population, the proportion of loci at which a randomly chosen individual is heterozygous, on average.

heterozygote

An organism with a genotype that includes different alleles at a gene locus.

heterozygote advantage

The phenomenon of the heterozygous genotype having a higher fitness (or other phenotypic value) than any homozygous genotype at the same gene locus.

heterozygote screening

Testing members of a population to identify heterozygous carriers of a disease-causing allele who are healthy but have the potential to produce children with the disease.

heterozygous

Refers to an individual organism that possesses two different alleles at a locus.

heterozygous genotype

A genotype that has more than one type of allele at a gene locus.

high-mobility group

Small, highly-charged proteins that vary in amount and composition in different tissues and different stages of the cell cycle; may play an important role in chromatin structure.

highly repetitive DNA

DNA that consists of short sequences that are present in hundreds of thousands to millions of copies; clustered in certain regions of chromosomes.

HIPAA

A U.S. federal law that requires improved efficiency in health care delivery by standardized electronic data interchange and protection of confidentiality and security of health data.

histone

A protein that is part of the histone family of basic proteins which associate with DNA in the nucleus and help to condense the DNA into a smaller volume.

histone acetyl transferase

A family of proteins that is responsible for catalyzing the bonding of an acetyl group to specific lysines on histones, generally resulting in increased transcription at that locus.

histone code

The combination of all the different modifications that can occur on histones.

histone deacetylase

A protein that catalyzes the removal of an acetyl group from histones.

histone demethylase

A protein that catalyzes the removal of a methyl group from histones.

histone methyltransferase

Proteins that catalyze the addition of methyl groups to lysines or arginines in histone proteins, causing the DNA to be less available for transcriptional machinery.

hitchhiking

Change in the frequency of an allele due to linkage with a selected allele at another locus.

HMG nuclear protein

A protein component of chromatin that was named based on its mobility in polyacrilimide gels.

holandric

Refers to a trait that is encoded by a gene on the Y chromosome.

Holliday intermediate

Structure that forms in homologous recombination; consists of two duplex molecules connected by a cross bridge.

Holliday junction

Model of homologous recombination that is initiated by single-strand breaks in a DNA molecule.

holoblastic cleavage

Describes a cell division event during development that results in complete cleavage between the two daughter cells. Often this type of cleavage is evident in animals in which the zygotic stage does not have a large amount of yolk.

holoenzyme

Complex of enzyme and other protein factors necessary for complete function.

homeobox

Conserved subset of nucleotides in homeotic genes. In Drosophila, it consists of 180 nucleotides that encode 60 amino acids of a DNA-binding domain related to the helix-turn-helix motif.

homeobox genes

A large family of eukaryotic genes that contains a DNA sequence known as the homeobox. The homeobox sequence encodes a protein homeodomain about 60 amino acids in length that binds DNA. Most homeobox genes are transcriptional regulators.

homeodomain

The region of a homeobox protein that can bind DNA.

homeostasis

The ongoing maintenance of equilibrium state in a cell or organism; maintained by biochemical processes that balance each other.

homeotic complex

Major cluster of homeotic genes in fruit flies; consists of the Antennapedia complex, which affects development of the adult fly's head and anterior segments, and the bithorax complex, which affects the adult fly's posterior thoracic and abdominal segments.

homeotic genes

Genes that determine the developmental fate of entire segments of an animal.

homeotic mutation

A mutation that causes a transformation of one structure into another of the organism's structures.

homogametic sex

The sex (male or female) that produces gametes that are all alike with regard to sex chromosomes. For example, in the XX-XY sex-determining system, the female produces only X-bearing gametes.

homokaryotype

A genome or individual that is homozygous for a chromosomal rearrangement such as an inversion.

homologous

Similar in position, structure, function, or characteristics.

homologous dna

Portions of DNA from different organisms that have similar sequences because they are derived from a common evolutionary ancestor.

homologous genes

Evolutionarily related genes, having descended from a gene in a common ancestor.

homologous pair of chromosomes

Two chromosomes that are alike in structure and size and that carry genetic information for the same set of hereditary characteristics. One chromosome of a homologous pair is inherited from the male parent and the other is inherited from the female parent.

homologous recombination

Exchange of genetic information between homologous DNA molecules.

homologous recombination repair

A relatively error-free pathway that repairs DNA double-strand breaks using an undamaged sister chromatid or homologous chromosome as a template.

homology

Possession by two or more species of a character state derived, with or without modification, from their common ancestor. Homologous chromosomes are those members of a chromosome complement that bear the same genes.

homomorphic chromosomes

Homologous chromosomes that are morphologically identical.

homonymous

Pertaining to biological structures that occur repeatedly within one segment of the organism, such as teeth or bristles.

homoplasmy

Presence of only one version of DNA within the cytoplasm of a single cell.

homoplasy

Possession by two or more species of a similar or identical character state that has not been derived by both species from their common ancestor; embraces convergence, parallel evolution, and evolutionary reversal.

homozygosity

The frequency of homozygous genotypes, often symbolized as P or Q.

homozygote

An individual that has copies of the same allele at paired genetic loci.

homozygotic

Describes twins that derive from the same zygote.

homozygous

A diploid genotype or individual with two indistinguishable alleles at a given locus.

homozygous line

Another name for a "pure-breeding" line; a strain of organisms that are homozygous for the alleles associated with a particular phenotype.

horizontal gene exchange

Transfer of genes from one organism to another by a mechanism other than reproduction.

horizontal gene transfer

Transfer of genetic information from one species to another in ways other than common descent.

horizontal transfer

Movement of genes between species.

horizontal transmission

Movement of genes or symbionts (such as parasites) between individual organisms other than by transmission from parents to their offspring (which is vertical transmission). Horizontal transmission of genes is also called lateral gene transfer.

Hox genes

Genes that control gross development in most plant and animal organisms along the major anterior-posterior axis; also called homeotic genes.

Hpa II endonuclease

A methyltransferase that recognizes the sequence CCGG and is responsible for methylating the second cysteine.

Human Genome Project

The Human Genome Project was an international research project that sequenced all of the genes in humans

humoral immunity

Type of immunity resulting from antibodies produced by B cells.

hybrid

An individual formed by mating between unlike forms, usually genetically differentiated populations or species.

hybrid dysgenesis

Sudden appearance of numerous mutations, chromosome aberrations, and sterility in the offspring of a cross between a male fly that possesses P elements and a female fly that lacks them.

hybrid vigor

Phenomenon in which the F1 generation has higher fitness than the parental strains or subpopulations that were crossed (mated) to produce them.

hybrid zone

A region in which genetically distinct populations come into contact and produce at least some offspring of mixed ancestry.

hybridization

Pairing of two partly or fully complementary single-stranded nucleotide chains. The nucleotide chains may come from the same species or different species.

hydrogen bond

A weak electrostatic bond which arises from the attraction between the slight positive charge on a hydrogen atom and a slight negative charge on a nearby oxygen or nitrogen atom.

hypermorphosis

An evolutionary increase in the duration of ontogenetic development, resulting in features that are exaggerated compared to those of the ancestor.

hypostatic gene

Gene that is masked or suppressed by the action of a gene at a different locus.

hypoxia

An environmental condition in which the concentration of oxygen is lower than normally found in the environment.

TOP

I kappa B transcription inhibitor

A protein that interacts with NF-kappaB in the cytoplasm, ensuring its partner does not homodimerize. When phosphorylated, I-kappaB releases NF-kappaB, which can then homodimerize and initiate downstream transcriptional events.

Id protein

A helix-loop-helix containing protein that can inhibit differentiation of muscle; protein that lacks the DNA binding domain common to this helix-loop-helix of proteins, consequently they heterodimerize to other transcription factors, and inhibit subsequent transcription.

identical by descent

Of two or more gene copies, being derived from a single gene copy in a specified common ancestor of the organisms that carry the copies.

identical twins

Twins that arise when a single egg fertilized by a single sperm splits into two separate embryos.

idiogram

A diagram of chromosomal morphology, particularly depicting the banding patterns of specific chromosomes.

immunogenicity

The ability of a substance to elicit an immune response, such as the production of specific antibodies in an organism.

immunoglobulin gene

A gene that encodes the basic functional unit of an antibody, an immunoglobulin.

immunoglobulins

A class of proteins, with a characteristic structure, active as receptors and effectors in the immune system.

immunohistochemistry

A set of methods for using an antibody to detect the presence and distribution of a protein in a tissue.

imprinting

In genetics, the differential modification of a gene depending on whether it is present in a male or a female. In animal behavior, a rapid form of learning in which an animal comes to make a particular response, which is maintained for life, to some object or other organism.

in situ hybridization

Method used to determine the chromosomal location of a gene or other specific DNA fragment or the tissue distribution of an mRNA by using a labeled probe that is complementary to the sequence of interest.

in vitro fertilization

The process in which an oocyte is fertilized by sperm in a laboratory setting.

in-frame deletion

Deletion of some multiple of three nucleotides, which does not alter the reading frame of the gene.

inborn error of metabolism

Heritable disorder of an organism's biochemistry.

inbred strain

A group of organisms that are homozygous at every locus.

inbreeding

The mating of closely related individuals.

inbreeding coefficient

Measure of inbreeding; the probability (ranging from 0 to 1) that two alleles are identical by descent.

inbreeding depression

Decreased fitness arising from inbreeding; often due to the increased expression of lethal and deleterious recessive traits.

inclusive fitness

The fitness of a gene or genotype as measured by its effect on the survival or reproduction of both the organism bearing it and the genes, identical by descent, borne by the organism's relatives.

incomplete dominance

Refers to the phenotype of a heterozygote that is intermediate between the phenotypes of the two homozygotes.

incomplete linkage

Linkage between genes that exhibit some crossing over; intermediate in its effects between independent assortment and complete linkage.

incomplete penetrance

Refers to a genotype that does not always express the expected phenotype. Some individuals possess the genotype for a trait but do not express the phenotype.

incorporated error

Incorporation of a damaged nucleotide or mismatched base pair into a DNA molecule.

independent assortment

Independent separation of chromosome pairs in anaphase I of meiosis; contributes to genetic variation.

indirect development

A life history consisting of a larval stage between embryo and adult stages.

indirect selection

A covariance between a trait and fitness within a generation that is caused by a phenotypic correlation between that trait and another trait that experiences direct selection.

individual selection

A form of natural selection consisting of nonrandom differences among different genotypes (or phenotypes) within a population in their contribution to subsequent generations.

induced mutation

Results from environmental agents, such as chemicals or radiation.

inducer

Substance that stimulates transcription in an inducible system of gene regulation; usually a small molecule that binds to a repressor protein and alters that repressor so that it can no longer bind to DNA and inhibit transcription.

inducible operon

Operon or other system of gene regulation in which transcription is normally off. Something must happen for transcription to be induced, or turned on.

induction

Stimulation of the synthesis of an enzyme by an environmental factor, often the presence of a particular substrate.

industrial melanism

The increase in frequency of dark (melanic) pigmentation in insects as an adaptation to remain inconspicuous on surfaces darkened by soot from air pollution.

infectious disease

An illness that can be passed from one individual to another.

inherit

The process by which offspring acquire genetic material from their parents.

inheritance of acquired characteristics

Early notion of gene transmission proposing that acquired traits are passed to descendants.

inhibition

The phenomenon of reducing or silencing neural activity by the action of an inhibitory neurotransmitter or conductance shunts within a cell.

initiation codon

The codon in mRNA that specifies the first amino acid (fMet in bacterial cells; Met in eukaryotic cells) of a protein; most commonly AUG.

initiation factor 1

Protein required for the initiation of translation in bacterial cells; enhances the dissociation of the large and small subunits of the ribosome.

initiation factor 2

Protein required for the initiation of translation in bacterial cells; forms a complex with GTP and the charged initiator protein and then delivers the charged tRNA to the initiation complex.

initiation factor 3

Protein required for the initiation of translation in bacterial cells; binds to the small subunit of the ribosome and prevents the large subunit from binding during initiation.

initiator protein

Binds to an origin of replication and unwinds a short stretch of DNA, allowing helicase and other single-strand binding proteins to bind and initiate replication.

insertion

A mutation that occurs when one or more base pairs is added to a DNA sequence.

insertion sequence

Simple type of transposable element found in bacteria and their plasmids that contains only the information necessary for its own movement.

instinctive

A behavior that is not learned, but rather stemming from impulse or unconscious behavior; can be based on physiological processes determined by genetic inheritance, and therefore common among individuals in a population.

institutional review board

A specially constituted review body established or designated by an entity to protect the welfare of human subjects recruited to participate in biomedical or behavioral research.

insulator

DNA sequence that blocks or insulates the effect of an enhancer; must be located between the enhancer and the promoter to have blocking activity; also may limit the spread of changes in chromatin structure.

insulin

A hormone synthesized in islet cells of the pancreas that lowers glusoce levels in the blood, by promoting the conversion of glucose into the storage material, glycogen.

integrase

Enzyme that inserts prophage, or proviral, DNA into a chromosome.

integrative levels of organization

The idea that matter is arrayed in orders of increasing complexity, and that at each level, there are emergent properties such that the higher level cannot be reduced to the lower.

inter-, intra-

Prefixes meaning, respectively, .between. and .within.. For example, .interspecific. differences are differences between species and .intraspecific. differences are differences among individuals within a species.

interaction

On any scale, the collision or interplay between entities that affects one or more of them, such as the merging of molecules into a compound, or the competition between species.

intercalated

Describes something that is inserted between two other things.

intercalating agent

Chemical substance that is about the same size as a nucleotide and may become sandwiched between adjacent bases in DNA, distorting the three-dimensional structure of the helix and causing single-nucleotide insertions and deletions in replication.

interchromosomal recombination

Recombination among genes on different chromosomes.

interdemic selection

The third phase of the shifting balance theory, in which subpopulations (demes) at higher adaptive peaks export migrants to subpopulations at lower adaptive peaks, causing the lower-fitness subpopulations to evolve toward the higher peak.

interference

Degree to which one crossover interferes with additional crossovers.

intergenic suppressor mutation

Occurs in a gene (locus) that is different from the gene containing the original mutation.

interkinesis

Period between meiosis I and meiosis II.

internal promoter

Located within the sequences of DNA that are transcribed into RNA.

interphase

Period in the cell cycle between the cell divisions. In interphase, the cell grows, develops, and prepares for cell division.

interspersed repeat sequences

Repeated sequences at multiple locations throughout the genome.

intrachromosomal recombination

Recombination among genes located on the same chromosome.

intragenic mapping

Mapping the locations of mutations within a single locus.

intragenic suppressor mutation

Occurs in the same gene (locus) as the mutation that it suppresses.

intrinsic hypothesis

When expected proportions of individuals with the observed characteristics are calculated after the experiment is done using a specific piece of required data.

intrinsic rate of natural increase

The potential per capita rate of increase of a population with a stable age distribution whose growth is not depressed by the negative effects of density.

introduced species

A species that originated in a different region that becomes established in a new region, often due to deliberate or accidental release by humans.

introgression

The permanent incorporation of genes from one species to another by repeated breeding of an interspecific hybrid with one of its parent species.

intron

Introns are non-coding sections of an RNA transcript, or the DNA encoding it, which are spliced out, or removed, before the RNA molecule is translated into a protein.

invasive species

Non-native species that increase rapidly in numbers and that have negative impacts on native species.

inversion stock

A line of organisms, usually Drosophila, that maintain a balancer chromosome. The presence of a specific balancer chromosome inhibits crossing over, allowing investigators to retain lines of animals that are heterozygous for a specific mutation without screening each generation for the phenotype.

inverted repeats

Sequences on the same strand that are inverted and complementary.

iron-response element

Known as IRE, a short region found in mRNAs that forms a stem-loop structure and is bound by iron-response proteins; responds to presence of iron (high or low levels) that change mRNA stability and therefore increase (high iron) or decrease (low iron) translation.

island model

The simplest model of gene flow, in which a proportion of migrants are exchanged between discrete subpopulations in each generation.

isoaccepting tRNAs

Different tRNAs with different anticodons that specify the same amino acid.

isolating barrier

A genetically determined difference between populations that restricts or prevents gene flow between them. The term does not include spatial segregation by extrinsic geographic or topographic barriers.

isotopes

Different forms of an element that have the same number of protons and electrons but differ in the number of neutrons in the nucleus.

isotopic labeling

Use of radioisotopes to label biomolecules, enabling scientists to investigate them in cells or tissue.

isozymes

Different forms of the same enzyme. Commonly used as genetic markers, especially between 1970 and 1990. The term isozyme is often used interchangeably with allozyme.

iteroparous

Pertaining to a life history in which individuals reproduce more than once.

TOP

karyotype

Picture of an individual organism's complete set of metaphase chromosomes.

key adaptation

An adaptation that provides the basis for using a new, substantially different habitat or resource.

kin selection

A form of selection whereby alleles differ in their rate of propagation by influencing the impact of their bearers on the reproductive success of individuals (kin) who carry the same alleles by common descent.

kinetochore

On a chromosome centromere, a set of proteins that provide the point of attachment of spindle fibers.

Klinefelter syndrome

Human condition in which cells contain one or more Y chromosomes along with multiple X chromosomes (most commonly XXY but may also be XXXY, XXXXY, or XXYY). Persons with Klinefelter syndrome are male in appearance but frequently possess small testes, some breast enlargement, and reduced facial and pubic hair; often taller than normal and sterile, most have normal intelligence.

knockout

An organism in which the normal gene expression at a given locus (or sometimes multiple loci) has been disrupted. Therefore, this organism has little or no expression of the RNA or protein encoded by this gene.

knockout mouse

A knockout mouse is a laboratory mouse in which a gene or genes have been turned off or "knocked out."

Kruppel transcription factor

A gap protein that has four tandemly repeated zinc finger domains.

TOP

One of three hybrid crosses used to detect linkage. In the absence of linkage, the progeny distribution in the F3 (F2 x F2) or L1 generation is identical to that in the F2 (F1 x F1) generation.

One of three hybrid crosses used to detect linkage. In the absence of linkage the progeny distribution in the L2 (F2 x P1) generation is identical to that in the B1 (F1 x P1) generation.

One of three hybrid crosses used to detect linkage. In the absence of linkage, the progeny distribution in the L3 (F2 x P2) generation is identical to that in the B2 (F1 x P2) generation.

lagging strand

DNA strand that is replicated discontinuously.

Lamarckism

The theory that evolution is caused by inheritance of character changes acquired during the life of an individual due to its behavior or to environmental influences.

large ribosomal subunit

The larger of the two subunits of a functional ribosome.

lariat

A looped structure formed during pre-mRNA splicing, named for its resemblance to ropes used to catch livestock.

lateral gene transfer

See horizontal transmission.

leader of mRNA

The 5' untranslated region (UTR) of an mRNA molecule.

leading strand

In unwound DNA, the single strand that is replicated continuously, in contrast to its partner the lagging strand.

leaky mutation

When a mutation does not cause a complete loss of function in the wild-type phenotype.

leptotene

First substage of prophase I in meiosis. In leptotene, chromosomes contract and become visible.

lethal allele

An allele (usually recessive) that causes virtually complete mortality, usually early in development.

leucine zipper

A common name for a secondary structural motif in proteins that occurs when two alpha helices interact through leucine amino acids that are located at a specific position in the repeated sequence of amino acids that forms the two helices. The leucine residues interact along the hydrophobic core of the zipper.

lifestyle

A complex concept that includes an individual's environment, eating habits, and social behavior.

line

A pure-breeding group of organisms.

lineage

A series of ancestral and descendant populations through time; usually refers to a single evolving species, but may include several species descended from a common ancestor.

lineage sorting

The process by which each of several descendant species, carrying several gene lineages inherited from a common ancestral species, acquires a single gene lineage; hence, the derivation of a monophyletic gene tree, in each species, from the paraphyletic gene tree inherited from their common ancestor.

linear

Describes a physical arrangement along a single line, such as a series of bases in a DNA sequence; in mathematics, a direct proportional relationship between two variables that does not change, regardless of scale.

linear regression

A statistical technique of finding the best fitting straight line through a set of points representing joint values for two variables.

linkage

In genetics, refers to how two genes that are nearby to one another on the same chromosome are often inherited together

linkage disequilibrium

Describes the state of two genotypes at different loci being dependent, showing a correlation; does not require gene linkage.

linkage equilibrium

The association of two alleles at two or more loci at the frequency predicted by their individual frequencies.

linkage group

Multiple genes located nearby on the same chromosome that are often inherited together.

linkage map

A map of gene loci for a specific chromosome based on linkage studies.

linked genes

Genes that are often inherited together.

linked loci

Loci that are on the same chromosome and that show a recombination frequency less than 0.5.

linker DNA

Stretch of DNA separating two nucleosomes.

litterfall

Undecomposed plant and animal material that accumulates on the ground.

livestock

Animals kept or raised for a specific purpose.

local variation

Variation in secondary structure within a single molecule.

locus

The specific location of a gene or gene sequence along a chromosome; can be used to refer to the gene itself; plural is loci.

locus control region

A cis-acting DNA element that is able to regulate gene expression from a specific region of DNA. The human beta-globin gene cluster is a leading example of genes regulated by a locus control region.

lod score

Logarithm of the ratio of the probability of obtaining a set of observations, assuming a specified degree of linkage, to the probability of obtaining the same set of observations with independent assortment; used to assess the likelihood of linkage between genes from pedigree data.

log-odds ratio

logistic equation

An equation describing the idealized growth of a population subject to a density-dependent limiting factor. As density increases, the rate of growth gradually declines until population growth stops.

long interspersed element

Long DNA sequence repeated many times and interspersed throughout the genome.

longevity

Average or maximum lifespan of a cohort of organisms.

loss-of-function mutation

Causes the complete or partial absence of normal function.

low copy repeat

A term with variable meaning that is sometimes used synonymously with segmental duplication. It can denote a group of juxtaposed duplicons (duplication block), individual segmental duplication events, or individual duplicons. The term emphasizes the low copy number of repeats (2.50 copies) relative to most transposable elements.

luciferin

The substrate for a basic luminescence assay.

luciferin/luciferase reporter system

A reporter gene assay system that allows investigators to measure biological activity after transfecting cells with the luciferase gene and conducting an experiment. Activity of the gene after experimental treatment is measured after adding luciferin and measuring luminescence.

luminescence assay

A method to measure biological activity through a reporter construct that uses the luciferase-luciferin interaction for quantitation.

Lyon hypothesis

Proposed by Mary Lyon in 1961, this hypothesis proposes that one X chromosome in each female cell becomes inactivated (a Barr body) and suggests that which X becomes inactivated is random and varies from cell to cell.

lysogenic cycle

Life cycle of a bacteriophage in which phage genes first integrate into the bacterial chromosome and are not immediately transcribed and translated.

lytic cycle

Life cycle of a bacteriophage in which phage genes are transcribed and translated, new phage particles are produced, and the host cell is lysed.

TOP

M phase

Period of active cell division; includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

M-phase promoting factor

Protein functioning in the control of the cell cycle; consists of a cyclin combined with cyclin-dependent kinase (CDK). Active MPF stimulates mitosis.

macroevolution

A vague term, usually meaning the evolution of substantial phenotypic changes, usually great enough to place the changed lineage and its descendants in a distinct genus or higher taxon.

major gene

A gene locus responsible for a large proportion of the phenotypic variation in a trait.

major histocompatibility complex antigen

Belongs to a large and diverse group of antigens found on the surfaces of cells that mark those cells as self; encoded by a large cluster of genes known as the major histocompatibility complex. T cells simultaneously bind to foreign and MHC antigens.

malignant tumor

Consists of cells that are capable of invading other tissues.

map density

The number of markers per centimorgan on a genetic map.

map distance

The distance between two markers on the same chromosome based on recombination frequency, usually measured in centimorgans (cM).

map-based sequencing

Method of sequencing a genome in which sequenced fragments are ordered into contigs with the use of genetic or physical maps.

mapping

DNA mapping describes a variety of different methods that can be used to describe the positions of genes.

mapping population

An experimental population constructed by crossing, designed for the production of a genetic map.

mark-recapture study

A technique in which animals are captured, marked, and released back into nature. The frequency at which they are recaptured is used to estimate survival or migration rates.

marker

An item used to label a location; a gene allele serving as a probe that predicts a certain phenotype.

mass extinction

A highly elevated rate of extinction of species, extending over an interval that is relatively short on a geological time scale (although still very long on a human time scale).

maternal

Of or relating to the female parent of an organism.

maternal blood testing

Testing for genetic conditions in a human fetus by analyzing the blood of the mother. For example, the level of alpha-fetoprotein in maternal blood provides information about the probability that a fetus has a neural-tube defect.

maternal effect

A nongenetic effect on phenotype originating from the female parent, resulting from elements in the egg cytoplasm, or transmission of symbionts to offspring during development or postnatal contact, or nutritional conditions during development or after birth.

maternal homolog

The allele of a gene inherited from the female parent.

mating type protein

A protein that designates specific microorganisms (like yeast) as a specific "type" (equivalent to sex in higher organisms), allowing that type to mate with an individual haploid cell of the opposite mating type.

maximum parsimony

See parsimony.

mean fitness

The arithmetic average fitness of all individuals in a population, usually relative to some standard.

mean square

The sums of squares divided by the degrees of freedom (n . 1).

mean statistic

Also called an average; the calculated sum of all measurements divided by the number of measurements; the statistic representing the center of a distribution of measurements in a set.

medical genetics

The study of the natural history and etiology of diseases that are at least partially genetic in origin.

medical treatment

An intervention in an individual's health supervision that is designed to lessen the symptoms associated with a disease.

megaspore

One of the four products of meiosis in plants.

megasporocyte

In the ovary of a plant, a diploid reproductive cell that undergoes meiosis to produce haploid macrospores.

meiosis

Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells.

meiosis I

First phase of meiosis. In meiosis I, chromosome number is reduced by half.

meiosis II

Second phase of meiosis. Events in meiosis II are essentially the same as those in mitosis.

meiotic drive

A process resulting in more than 50% of gametes possessing one of the alleles from a heterozygoous genotype; indicates some bias in distribution during meiosis.

melting temperature

Midpoint of the melting range of DNA.

memory cell

Long-lived lymphocyte among the clone of cells generated when a foreign antigen is encountered. If the same antigen is encountered again, the memory cells quickly divide and give rise to another clone of cells specific for that particular antigen.

Mendelian population

Group of interbreeding, sexually reproducing individuals.

Mendelian segregation

The production of equal numbers of gametes containing each allele from a heterozygous genotype.

Mendelian test cross

A tool first described by Gregor Mendel to determine an unknown genotype; a cross between an organism with a homozygous recessive genotype an organism with an unknown genotype.

Mendelian trait

Any trait controlled by a genetic locus that can be described by Mendelian principles of inheritance.

meristic characteristic

Characteristic whose phenotype varies in whole numbers, such as number of vertebrae.

meroblastic cleavage

Describes a cell division event during development that results in incomplete cleavage between daughter cells. Meroblastic cleavage is common in zygotes with large concentrations of yolk.

merozygote

Bacterial cell that has two copies of some genes.one copy on the bacterial chromosome and a second copy on an introduced F plasmid; also called partial diploid.

meta-analysis

A statistical technique for jointly analyzing the results of many studies on the same topic.

metabolism

In an organism, the total physical and chemical processes that support energy molecule production from nutrients, and the converse use of energy molecules to support cellular and organismal homeostasis.

metacentric chromosome

Chromosome in which the two chromosome arms are approximately the same length.

metallothionine promoter

A gene promoter that is activated by heavy metals and oxidative stress.

metaphase

Metaphase is the third phase of mitosis, which is a process that separates the duplicated genetic material carried in the nucleus of a parent cell into two, identical daughter cells

metaphase I

Stage of meiosis I. In metaphase I, homologous pairs of chromosomes align in the center of the cell.

metaphase II

Stage of meiosis II. In metaphase II, individual chromosomes align on the metaphase plate.

metaphase plate

Plane in a cell between two spindle poles. In metaphase, chromosomes align on the metaphase plate.

metapopulation

A set of local populations, among which there may be gene flow and patterns of extinction and recolonization.

metastasis

Refers to cells that separate from malignant tumors and travel to other sites, where they establish secondary tumors.

methylation

The addition of a methyl group (-CH3) to a molecule. Extensive methylation of cytosine in DNA is correlated with reduced transcription.

metric trait

A phenotypic character that is continuously distributed with more than just a few distinct types.

microarray

A microarray is a laboratory tool used to detect the expression of thousands of genes at the same time

microevolution

A vague term, usually referring to slight, short-term evolutionary changes within species.

microRNA

Small RNAs, typically 21 or 22 bp in length, that are produced by cleavage of double-stranded RNA arising from small hairpins within RNA that is mostly single stranded. The miRNAs combine with proteins to form a complex that binds (imperfectly) to mRNA molecules and inhibits their translation.

microsatellite

Genetic markers consisting of repeated units that are each 2 to 9 nucleotides long, arrayed in repeated sequences 10k to 100k nucleotides long. Sometimes they are also referred to as simple sequence repeats (SSR), simple sequence repeat polymorphisms (SSRP), or short tandem repeats (STR).

microspore

Haploid product of meiosis in plants.

microsporocyte

Diploid reproductive cell in the stamen of a plant; undergoes meiosis to produce four haploid microspores.

microtubule

Long fiber composed of the protein tubulin; plays an important role in the movement of chromosomes in mitosis and meiosis.

midparent

The average phenotypic value of each pair of parents in an offspring- parent regression.

mimicry

Similarity of certain characters in two or more species due to convergent evolution, when there is an advantage conferred by the resemblance. Common types include Batesian mimicry, in which a palatable mimic experiences lower predation because of its resemblance to an unpalatable model; and Mullerian mimicry, in which two or more unpalatable species enjoy reduced predation due to their similarity.

minimal medium

Used to culture bacteria or some other microorganism; contains only the nutrients required by prototrophic (wild-type) cells.typically, a carbon source, essential elements such as nitrogen and phosphorus, certain vitamins, and other required ions and nutrients.

minisatellites

A class of repetitive sequences, 7100 nucleotides each, that span 500-20,000 bp, and are located throughout the genome, towards chromosome ends; typically longer in length than microsatellites (STRs).

minor gene

A locus that determines a relatively small proportion of phenotypic variation in a trait.

mismatch repair

Process that corrects mismatched nucleotides in DNA after replication has been completed. Enzymes excise incorrectly paired nucleotides from the newly synthesized strand and use the original nucleotide strand as a template when replacing them.

missense mutation

Alters a codon in the mRNA, resulting in a different amino acid in the protein.

mitigation

The reduction of intensity of any variable. In ecology, the legally mandated remediation for loss of protected species or ecosystems.

mitochondria

Organelles in eukaryotic cells in which the citric acid cycle occurs and ATP is produced.

mitochondrial DNA

DNA in mitochondria; has some characteristics in common with eubacterial DNA and typically consists of a circular molecule that lacks histone proteins and encodes some of the rRNAs, tRNAs, and proteins found in mitochondria.

mitosis

Mitosis is a process of nuclear division in eukaryotic cells that occurs when a parent cell divides to produce two identical daughter cells

mitotic spindle

Array of microtubules that radiate from two poles; moves chromosomes in mitosis and meiosis.

model

A representation of a complex system with a simpler one (noun); to simulate a complex system by creating a stand-in representation, and using that stand-in to test hypotheses about the complex system (verb).

model organism

An organism suitable for studying a specific trait, disease, or phenomenon, due to its short generation time, characterized genome, or similarity to humans; examples are a fly, fish, rodent or pig, whose biology is well known and accessible for laboratory studies.

moderately repetitive DNA

DNA consisting of sequences that are from 150 to 300 bp in length and are repeated thousands of times.

modern synthesis

See evolutionary synthesis.

modified base

Rare base found in some RNA molecules. Such bases are modified forms of the standard bases (adenine, guanine, cytosine, and uracil).

modifier gene

A gene that alters the phenotypic expression of another gene.

modularity

The ability of individual parts of an organism, such as segments or organs, to develop or evolve independently from one another; the ability of developmental regulatory genes and pathways to be regulated independently in different tissues and developmental stages.

molecular chaperone

Molecule that assists in the proper folding of another molecule.

molecular clock

The concept of measuring the time when lineages of organisms diverged, based on the assumtion that mutations occur at a steady rate over time.

molecular genetics

The study of the chemical nature of genetic information and how it is encoded, replicated, and expressed.

molecular motor

Specialized protein that moves cellular components.

molecule

The smallest unit of a chemical element that still retains properties of that element; composed of two or more atoms joined by covalent bonds or ionic attractions.

monoallelic expression

Expression of a gene from only one of two alleles in a diploid organism.

monoecious

Refers to the presence of both male and female reproductive structures in the same individual organism.

monohybrid cross

A cross between two individuals that differ in a single characteristic.more specifically, a cross between individuals that are homozygous for different alleles at the same locus (AA _ aa); also refers to a cross between two individuals that are both heterozygous for two alleles at a single locus (Aa _ Aa).

monomorphic

Having one form; refers to a population in which virtually all individuals have the same genotype at a locus.

monophyletic

Refers to a taxon, phylogenetic tree, or gene tree whose members are all derived from a common ancestral taxon. In cladistic taxonomy, the term describes a taxon consisting of all the known species descended from a single ancestral species.

monosomy

Absence of one of the chromosomes of a homologous pair.

monozygote

A zygote derived from a single egg.

monozygotic twins

Identical twins that arise when a single egg fertilized by a single sperm splits into two separate embryos.

morgan

100 map units.

morphism

The condition of having a form, shape, or observable character.

morphogen

Molecule whose concentration gradient affects the developmental fate of surrounding cells.

mosaic evolution

Evolution of different characters within a lineage or clade at different rates, hence more or less independently of one another.

mosaicism

Condition in which regions of tissue within a single individual have different chromosome constitutions.

mRNA

Also known as mRNA, the RNA molecule exported from the nucleus that carries genetic information for the amino acid sequence of a protein.

mRNA stability

A measure of the half-life of an mRNA molecule.

mRNA surveillance

Mechanisms for the detection and elimination of mRNAs that contain errors that may create problems in the course of translation.

MspI endonuclease

A restriction endonuclease that cleaves at the recognition site C/CGG (with the / representing the cleavage site). When the 5'-C is methylated, MspI is inhibited. When the internal C is methylated, MspI can cleave normally.

multifactorial characteristic

Determined by multiple genes and environmental factors.

multigene family

Set of genes similar in sequence that arose through repeated duplication events; often encode different proteins.

multiple 3' cleavage sites

Refers to the presence of more than one 3' cleavage site on a single pre-mRNA, which allows cleavage and polyadenylation to take place at different sites, producing mRNAs of different lengths.

multiple alleles

Presence in a group of individuals of more than two alleles at a locus. Although, for the group, the locus has more than two alleles, each member of the group has only two of the possible alleles.

multiplication rule

States that the probability of two or more independent events occurring together is calculated by multiplying the probabilities of each of the individual events.

Mus musculus

The genus and species of the common mouse.

mutagen

Any environmental agent that significantly increases the rate of mutation above the spontaneous rate.

mutagenesis screen

Method for identifying genes that influence a specific phenotype. Random mutations are induced in a population of organisms, and individual organisms with mutant phenotypes are identified. These individual organisms are crossed to determine the genetic basis of the phenotype and to map the location of mutations that cause the phenotype.

mutant

A cell or organism harboring a genetic mutation.

mutant screen

An experiment used in "reverse genetics" to identify the loci responsible for specific phenotypes. Generally, the genetic material of wild-type parental organisms is altered (e.g., by treatment with a chemical mutagen) and organisms are allowed to breed to look for dominant phenotypes (F1) or recessive phenotypes (F2).

mutation

Any change in a genetic sequence, large or small.

mutation frequency

Number of mutations within a group of individual organisms.

mutation rate

Frequency with which a gene changes from the wild-type to a specific mutant; generally expressed as the number of mutations per biological unit (i.e., mutations per cell division, per gamete, or per round of replication).

mutation-selection balance

A process in which removal of variation by selection is balanced by the input of new variation into the population by mutation.

mutational meltdown

A process in which deleterious mutations are fixed due to random drift in small populations, which further decreases population size and thus increases the rate of fixation of deleterious mutations in a positive feedback loop.

mutational variance

The increment in the genetic variance of a phenotypic character caused by new mutations in each generation.

mutualism

A symbiotic relation in which each of two species benefits by their interaction.

MyoA

A myogenic transcription factor with a helix-loop-helix domain.

myoblast

An immature muscle cell.

MyoD

A transcription factor that can induce differentiation of embryonic muscle fibroblasts to myoblasts in culture. MyoD activates transcription of muscle-specific genes, including MyoA, MyoH, and itself.

MyoH

A myogenic transcription factor with a helix-loop-helix domain.

myosin

One of the two major proteins of muscle; it makes up the thick filaments.

myosin heavy chain gene

The gene that encodes the part of the myosin protein that catalyzes ATP and faciliates motor activity.

TOP

N-terminal

Also called the N-terminus, the specific end of a protein/polypeptide containing an amine group. This is distinct from the C-terminus, which is the opposite end of the polypeptide chain, and typically has a carboxyl group. The orientation of a polypeptide with two distinct ends originates in the translation process from the mRNA code, wherein synthesis of the protein begins with the N-terminus and ends with the C-terminus.

N-terminus

Also called N-terminal, the specific end of a protein/polypeptide containing an amine group. This is distinct from the C-terminus, which is the opposite end of the polypeptide chain, and typically has a carboxyl group. The orientation of a polypeptide with two distinct ends originates in the translation process from the mRNA code, wherein synthesis of the protein begins with the N-terminus and ends with the C-terminus.

Nanog

A transcription factor often associated with embryonic stem cells that is thought to be critical for maintaining the pluripotency of these cells.

narrow-sense heritability

Proportion of the phenotypic variance that can be attributed to additive genetic variance.

natural language processing

Computer understanding, analysis, manipulation and/or generation of natural (human) language.

natural selection

The differential survival and/or reproduction of classes of entities that differ in one or more characteristics. To constitute natural selection, the difference in survival and/or reproduction cannot be due to chance, and it must have the potential consequence of altering the proportions of the different entities. Thus, natural selection is also definable as a deterministic difference in the contribution of different classes of entities to subsequent generations. Usually, the differences are inherited. The entities may be alleles, genotypes or subsets of genotypes, populations, or, in the broadest sense, species.

negative assortative mating

Mating between unlike individuals that is more frequent than would be expected on the basis of chance.

negative control

Gene regulation in which the binding of a regulatory protein to DNA inhibits transcription (the regulatory protein is a repressor).

negative selection

Form of natural selection in which rare, deleterious alleles are removed from a population.

negative-strand RNA virus

RNA virus whose genomic RNA molecule carries the complement of the information for viral proteins. A negative-strand RNA virus must first make a complementary copy of its RNA genome, which is then translated into viral proteins.

neo-Darwinism

The modern belief that natural selection, acting on randomly generated genetic variation, is a major, but not the sole, cause of evolution.

neoclassical crosses

The F2, F2 B1 and B2 are "classical" hybrid crosses. Neoclassical crosses were named by Collins (1971) and are all addition hybrid crosses. The are often used in estimating the number of segregating genes for quantitative traits.

neofunctionalization

Divergence of duplicate genes whereby one acquires a new function.

neopolyploid

A polyploid that has been produced by artificially inducing chromosome doubling.

neoteny

Heterochronic evolution whereby development of some or all somatic features is retarded relative to sexual maturation, resulting in sexually mature individuals with juvenile features.

nested paternal half-sibling design

A quantitative genetic design that is well-suited for estimating additive genetic variance, additive genetic correlation, and thus the G matrix. In this design, a few unique females (dams) are mated to each of a number of males (sires), and the traits of interest are measured on a few offspring from each dam. The data are analyzed with nested ANOVA.

neuraminidase

A type II integral membrane glycoprotein that facilitates virus release from cells by removing sialic acid from sialyloligosaccharides on the cell and viral surfaces. It is also a target of the protective immune response.

neurofibromatosis

An autosomal genetic disorder that results in the growth of tumors along different types of nerves. It can also impact the development of non-nervous system tissues, such as bones and skin.

neutral alleles

Traits or loci that have a negligible effect on fitness. A trait or locus is nearly neutral or effectively neutral if the mean phenotype or allele frequencies are determined more by random genetic drift than by selection.

neutral mutation

Changes the amino acid sequence of a protein but does not alter the function of the protein.

neutral theory of evolution

Neutral theory claims that the overwhelming majority of evolutionary changes at the molecular level are not caused by selection acting on advantageous mutants, but by random fixation of selectively neutral or very nearly neutral mutants through the cumulative effect of sampling drift (due to finite population number) under continued input of new mutations.

neutral-mutation hypothesis

Proposes that much of the molecular variation seen in natural populations is adaptively neutral and unaffected by natural selection. Under this hypothesis, individuals with different molecular variants have equal fitnesses.

newborn screening

Testing newborn infants for certain genetic disorders; done most commonly for phenylketonuria and other metabolic diseases that can be prevented by early treatment or intervention.

niche

The particular set of combined conditions that are habitable for a species, and define its status within an ecological community.

nitrogenous base

Nitrogen-containing base that is one of the three parts of a nucleotide.

NMR specroscopy

A powerful method for determining the structure of organic compounds.

nonadditive genetic variance

Variance caused by dominance and interaction. Mathematically defined as dominance variance plus epistatic variance.

nonallelic homologous recombination

Homologous recombination between paralogous sequences (for example, segmental duplication and repetitive sequence); a major mechanism of recurrent rearrangements, also known as unequal crossing-over.

nonautonomous element

Transposable element that cannot transpose on its own but can transpose in the presence of an autonomous element of the same family.

noncoding RNA

RNA in a cell that does not encode a protein.

nondisjunction

Failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis.

nonhistone chromosomal protein

One of a heterogeneous assortment of nonhistone proteins in chromatin.

nonoverlapping genetic code

Refers to the fact that generally each nucleotide is a part of only one codon and codes for only one amino acid in a protein.

nonreciprocal translocation

Movement of a chromosome segment to a nonhomologous chromosome or region without any (or with unequal) reciprocal exchange of segments.

nonrecombinant gamete

Contains only original combinations of genes present in the parents.

nonrecombinant progeny

Possesses the original combinations of traits possessed by the parents.

nonreplicative transposition

Type of transposition in which a transposable element excises from an old site and moves to a new site, resulting in no net increase in the number of copies of the transposable element.

nonsegregating generations

Generation in which there is one genotype (excluding sex chromosomes). These are the P1, P2, F1, and F1R generations.

nonsense codon

Codon in mRNA that signals the end of translation; also called stop codon or termination codon. There are three common nonsense codons: UAA, UAG, and UGA.

nonsense mutation

A nonsense mutation is a genetic mutation in a DNA sequence that results in a shorter, unfinished protein product.

nonsense-mediated mRNA decay

Process that brings about the rapid elimination of mRNA that has a premature stop codon.

nonstop RNA decay

Mechanism in eukaryotic cells for dealing with ribosomes stalled at the 3' end of an mRNA that lacks a termination codon. A protein binds to the A site of the stalled ribosome and recruits other proteins that degrade the mRNA from the 3' end.

nonsynonymous substitution

A replacement substitution or a point mutation that results in a codon that is translated into a different amino acid and thus a different final protein product.

nontemplate strand

The DNA strand that is complementary to the template strand; not ordinarily used as a template during transcription.

nontransmissible disease

A disorder that cannot be passed between organisms.

norm of reaction

Range of phenotypes produced by a particular genotype in different environmental conditions.

normal distribution

A probability distribution that peaks in the center and is symmetrical, forming a bell-shape; commonly occurring among observations of natural phenomena; important in statistics and biology; the expected probability distribution when many small independent effects combine.

normoxic

The state of the environment in which oxygen concentration is at normal levels.

Northern blot

A Northern blot is a laboratory method used to detect specific RNA molecules from a mixture of RNA molecules

Northern blotting

Process by which RNA is transferred from a gel to a solid support such as a nitrocellulose or nylon filter.

nuclear envelope

Membrane that surrounds the genetic material in eukaryotic cells to form a nucleus; segregates the DNA from other cellular contents.

nuclear factor-kappa B

A family of dimeric transcription factors that regulate a variety of cellular signaling events. NF-kappaB monomers are held inactive via binding to an inhibitory protein called I-kappaB. When I-kappaB is phosphorylated, the interaction with NF-kappaB is released and allowed to dimerize, thus promoting its interaction with DNA.

nuclear lamina

The inner surface of the nuclear envelope, composed of a network of lamin filaments and associated proteins.

nuclear matrix

Network of protein fibers in the nucleus; holds the nuclear contents in place.

nuclear pore

The nuclear pore is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm

nuclear pre-mRNA introns

Class of introns in protein-encoding genes that reside in the nuclei of eukaryotic cells; removed by spliceosomal-mediated splicing.

nuclear run-off assay

A method for measuring gene transcription at a specific time that involves incorporating radioactive nucleotides into growing mRNA chains combined with Northern blotting.

nucleic acid

A nucleic acid is a long molecule made up of smaller molecules called nucleotides

nucleoid

Bacterial DNA confined to a definite region of the cytoplasm.

nucleolytic

Enzymatic activity that describes the ability to cleave nucleic acid.

nucleoside

Ribose or deoxyribose sugar bonded to a base.

nucleosome

A nucleosome is a section of DNA that is wrapped around a core of proteins

nucleotide

A nitrogenous base attached to a sugar; a component of nucleic acids.

nucleotide substitution

A point mutation where one base pair is replaced by another.

nucleotide-excision repair

DNA repair that removes bulky DNA lesions and other types of DNA damage.

nucleus

Space in eukaryotic cells that is enclosed by the nuclear envelope and contains the chromosomes.

null allele

An allele that does not yield a protein product, or whose phenotypic effect is masked.

null hypothesis

The statistical hypothesis that states that there will be no differences between observed and expected data.

nullisomy

Absence of both chromosomes of a homologous pair (2n - 1).

NusA factor

Protein subunit of bacterial RNA polymerase that facilitates the termination of transcription.

TOP

obligate

Required, or by necessity (adj.); to force (verb)

observational variance components

The sub-components of phenotypic variance due to mating design.

occipital cortex

The five outermost cortical tissue layers located in the caudal-most location of mammalian brain, the occipital lobe.

Oct4 transcription factor

A POU family homeodomain containing transcription factor.

offspring

The children resulting from a cross between two parents.

offspring-parent regression

A regression analysis wherein the slope of the regression line reflects heritability of a trait; created by collected data on organisms and their offpsring. Supports analyses of additive genetic variance, additive genetic correlation, and the G matrix.

Okazaki fragment

Short stretch of newly synthesized DNA; produced by discontinuous replication on the lagging strand, these fragments are eventually joined together.

oligogenic trait

A trait produced by only a few genes.

oligonucleotide

A short polymer of nucleotides, between 2 and 50 nucleotides long, used in the study of gene mechanism; in parlance, sometimes referred to as an "oligo."

oligonucleotide backbone

Part of an oligonucleotide that holds the nucleic acid bases together.

oligonucleotide-directed mutagenesis

Method of site-directed mutagenesis that utilizes an oligonucleotide to introduce a mutant sequence into a DNA molecule.

omnivore

An organism that relies on both animal and non-animal organisms, or even decaying matter, for food and nutrients.

oncogene

Dominant-acting gene that stimulates cell division, leading to the formation of tumors and contributing to cancer; arises from mutated copies of a normal cellular gene (protooncogene).

one gene, one enzyme hypothesis

Idea proposed by Beadle and Tatum that each gene encodes a separate enzyme.

one gene, one polypeptide hypothesis

Modification of the one gene, one enzyme hypothesis; proposes that each gene encodes a separate polypeptide chain.

ontogeny

The complete developmental history of an organism from zygote to adult.

ontology

A hierarchical organization of concepts, typically used to denote 'more-general-than' and/or 'part-of' relationships.

oogenesis

Egg production in animals.

oogonium

Diploid cell in the ovary; capable of undergoing meiosis to produce an egg cell.

open reading frame

Continuous sequence of DNA nucleotides that contains a start codon and a stop codon in the same reading frame; is assumed to be a gene that encodes a protein but, in many cases, the protein has not yet been identified.

operator sequence

The DNA sequence in the operon of a cell; binding to this sequence by a regulator protein changes gene transcription.

operon

The set of genes, together with promoter sequence and operator sequence, that control the transcription of a gene; first discovered in prokaryotes, now known to be present in eukaryotes as well.

organism

A form of life as an individual entity.

origin of replication

Site where DNA synthesis is initiated.

origin position

The location on a gel where a sample is added, marking the origin of the path of migration during electrophoresis.

orthologous

Describes homologous genes in different species that originate from the same ancestral line.

outbreeding depression

Occurs when offspring from crosses between populations have lower fitness than the offspring from crosses within each population.

outcrossing

Mating between unrelated individuals that is more frequent than would be expected on the basis of chance.

outgroup

In a group of taxa, the group that diverged first.

ovalbumin

The main protein in egg whites.

overdominance

A condition where the heterozygote is better adapted than either homozygote.

oviduct

An anatomical structure in mammals, usually tube-shaped, through which ova travel, either to a uterus or outside the body.

oviparous

Describes an animal that lays eggs.

ovum

Final product of oogenesis.

TOP

p - element

A transposable DNA element in Drosophila that is frequently used to facilitate insertional mutagenesis for genetic screens.

P generation

First set of parents in a genetic cross.

p-arm

The short arm of a chromosome.

pachytene

In meiosis, the third substage of prophase I; the synaptonemal complex forms during this substage.

paedomorphosis

The resemblance of the adult morphology to the morphology of the ancestral juvenile.

pair-rule genes

Set of segmentation genes in fruit flies that define regional sections of the embryo and affect alternate segments. Mutations in these genes often cause the deletion of every other segment.

palindrome

Sequence of nucleotides that reads the same on complementary strands; inverted repeats.

pancreas

A large gland located behind the stomach that secretes insulin and glucagon (an endocrine function) and digestive enzymes (an exocrine function).

pandemic flu

An influenza virus of a new subtype to which the general population has little or no immunity that causes disease in humans and spreads efficiently from person to person, causing community-wide outbreaks and resulting in a global outbreak of influenza.

pangenesis

Early concept of heredity proposing that particles carry genetic information from different parts of the body to the reproductive organs.

panmictic

A breeding population where mating is random.

panmixia

Random mating within a population.

paracentric inversion

Chromosome inversion that does not include the centromere in the inverted region.

parallel evolution

The independent development of similar adaptive features.

paralogous

Describes two (or more) genes located at different locations in the genome of an organism that are related, based on sequence similarity, suggesting they derived from a common ancestor gene earlier in their evolution via mutation or genetic drift.

parapatric

Relating to populations whose biogeography has a narrow contact zone.

paraphyletic

A phylogeny that only includes a portion of descendants from a certain ancestor.

parental effect

The effect of parents on traits in progeny not due to genetic effects.

parental investment

The parental devotion of resources to current progeny at the expense of potential future progeny.

parental line

The line used for the first generation of a genetic cross; usually a pure-breeding stain.

parsimony

The principle of succinctness in problem solving. A rule that states the best theoretical phylogeny is the one that proposes the least number of changes.

parthenogenesis

Development of an embryo from an unfertilized ova.

partial diploid

Bacterial cell that possesses two copies of genes, including one copy on the bacterial chromosome and the other on an extra piece of DNA (usually a plasmid); also called merozygote.

partial dominance

When one trait is not fully dominant over another; here, one can see a "mixing" of the characteristics. Incomplete dominance and codominance are two types of partial dominance.

particulate inheritance

The model described by Mendel which suggested that transmission of inherited traits occurs through distinct units (particles) in contrast to a blending of characters from each parent.

Patau syndrome

Characterized by severe mental retardation, a small head, sloping forehead, small eyes, cleft lip and palate, extra fingers and toes, and other disabilities; results from the presence of three copies of chromosome 13 (trisomy 13).

patent

A property right granted by the government of the United States of America to an inventor .to exclude others from making, using, offering for sale, or selling the invention throughout the United States or importing the invention into the United States. for a limited time in exchange for public disclosure of the invention when the patent is granted.

paternal

Of or relating to the male parent of an offspring.

paternal homolog

An allele of a locus that was inherited from the genetic contribution of the father.

paternity

Fatherhood; the state of being a father. Establishing who is the father of an offspring.

peak shift

Method by which a population navigates an adaptive landscape by moving from one peak (local maximum of fitness) through a valley (lower fitness) to another peak.

pedigree

Pictorial representation of a family history outlining the inheritance of one or more traits or diseases.

penetrance

Penetrance is a measure of the proportion of individuals in a population who carry a specific gene and express the related trait

pentaploidy

Refers to the possession of five haploid sets of chromosomes (5n).

peptide

A series of two or more amino acids connected by peptide bonds.

peptide bond

Covalent chemical bond between two amino acids; connects the carboxyl group of one amino acid to the amino group of another. A series of peptide bonds and amino acids form a protein.

peptidyl site

One of three sites in a ribosome occupied by a tRNA in translation. In the elongation stage of protein synthesis, tRNAs move from the aminoacyl (A) site into the P site.

peptidyl transferase

Activity in the ribosome that creates a peptide bond between two amino acids. Evidence suggests that this activity is carried out by one of the RNA components of the ribosome.

pericentric inversion

Chromosome inversion that includes the centromere in the inverted region.

peripatric

Referring to populations whose biogeography is adjacent but with no overlap.

peripatric speciation

The reproductive isolation of a small population that has separated from a larger group; a consequence of the sensitivity of small populations to the effects of genetic drift.

pesticide rotation

Alternating among pesticides with different modes of action to alleviate resistance.

phage

Bacteriophage; a type of virus that infects bacteria.

pharmacogenomics

The use of genetic information to guide drug prescribing.

phenetic

Referring to classification based on observable traits (phenotype).

phenocopy

Phenotype that is produced by environmental effects and is the same as the phenotype produced by a genotype.

phenotype

The observable physical properties of an organism, such as the organism's appearance, development, and behavior.

phenotypic adaptive landscape

A graphical representation of the fitness of phenotypic traits where height represents fitness.

phenotypic correlation

Degree of association of traits calculated by measuring pairs of observable characteristics in individuals within a population.

phenotypic plasticity

The capacity of an organism to express a different range of phenotypes in response to environmental changes.

phenotypic value

The combined effect of genotypic value and environmental effect resulting in the measurable observed character trait.

phenotypic variance

Measures the degree of phenotypic differences among a group of individuals; composed of genetic, environmental, and genetic-environmental interaction variances.

phenylketonuria

Genetic disease characterized by mental retardation, light skin, and eczema; caused by mutations in the gene that encodes phenylalanine hydroxylase (PAH), a liver enzyme that normally metabolizes the amino acid phenylalanine. When the enzyme is defective, phenylalanine is not metabolized and builds up to high levels in the body, eventually causing mental retardation and other characteristics of the disease. The disease is inherited as an autosomal recessive disorder and can be effectively treated by limiting phenylalanine in the diet.

phosphate backbone

The sugar-phosphate backbone forms the structural framework of nucleic acids, like DNA and RNA, and is composed of alternating sugar and phosphate groups.

phosphate group

A phosphorus atom attached to four oxygen atoms; one of the three components of a nucleotide.

phosphodiester

Molecule containing R.O.P.O.R, in which R is a carbon-containing group, O is oxygen, and P is phosphorus.

phosphodiester linkage

Phosphodiester bond connecting two nucleotides in a polynucleotide strand.

phosphorothioate

A phosphate with one oxygen replaced with a sulfur; this can also refer to an oligo with phosphorothioate linkages.

phosphorylation

The enzymatic addition of a phosphate group to a protein.

photosynthesis

A biochemical process of a cell or a tissue that captures light energy and uses it to convert carbon dioxide and water into complex organic molecules; the resulting organic molecules can be used as as nutrients or to build biomass.

phylogenetic distance

A measure of the degree of separation between two organisms or their genomes on an evolutionary scale, usually expressed as the number of accumulated DNA or RNA sequence changes, number of years, or number of generations.

phylogenetic profile

The presence-and-absence pattern of genes in different species, which may be used to infer gene function. A presence-and-absence pattern that is the same in different organisms suggests that the genes may be functionally related.

phylogenetic relationship

In evolutionary past, the relative times that two organisms or species shared a common ancestor; indicated by the relative position of a different species on a phylogenetic tree.

phylogenetic species concept

A group of organisms that share a common ancestor that is not shared by other groups of organisms.

phylogenetic tree

Graphical representation of the evolutionary connections between organisms or genes.

phylogeny

The history of descent of a group of taxa such as species from their common ancestors, including the order of branching and sometimes the absolute times of divergence; also applied to the genealogy of genes derived from a common ancestral gene.

physical distance

The number of nucleotides between two loci.

physical linkage

Phenomenon in which two genes are located on the same chromosome.

physical map

Map of physical distances between loci, genetic markers, or other chromosome segments; measured in base pairs.

pilus

Extension of the surface of some bacteria that allows conjugation to take place. When a pilus on one cell makes contact with a receptor on another cell, the pilus contracts and pulls the two cells together.

planktonic

Describes an aquatic organism that is passively-floating and forms the foundation of the aquatic food chain.

plaque

Clear patch of lysed cells on a continuous layer of bacteria on the agar surface of a petri plate. Each plaque represents a single original phage that multiplied and lysed many cells.

plasmid

A plasmid is a small, circular, double-stranded DNA molecule, which is distinct from chromosomal DNA

pleiotropy

The phenomenon in which one locus affects more than one phenotypic trait, causing a genetic correlation.

ploidy

The number of complete sets of chromosomes in an organism.

point mutation

A change in a single nucleotide in a DNA sequence. See also transition, transversion.

pollen

Granules containing the male gametophyte of seed plants.

poly(A) addition sequence

An AAUAAA sequence in the 3' end of a growing mRNA that binds to several proteins that signal the end of the templated molecule, cleavage of the mRNA molecule, and subsequent polyadenylation.

poly(A) tail

String of adenine nucleotides added to the 3' end of a eukaryotic mRNA after transcription.

poly(A)-binding protein

Binds to the poly(A) tail of eukaryotic mRNA and makes the mRNA more stable. There are several types of PABPs, one of which is PABII.

poly-A tail

The poly-A tail is a long chain of adenines nucleotides that is added to a messenger RNA (mRNA) molecule during RNA processing to increase the stability of the mRNA molecule

polycystic kidney disease

An inherited kidney disorder in which multiple cysts form on the kidneys, causing them to become enlarged; abbreviated as PKD.

polygenic

Describes a trait under the control of multiple genes.

polymerase chain reaction

An automated method for synthesizing many copies of a specific fragment of DNA by repeated rounds of DNA replication; abbreviated as PCR.

polymorphic

Pertaining to more than one common allele or trait.

polymorphism

The existence within a population of two or more genotypes, the rarest of which exceeds some arbitrarily low frequency (say, 1 percent); more rarely, the existence of phenotypic variation within a population, whether or not genetically based.

polynucleotide strand

Series of nucleotides linked together by phosphodiester bonds.

polypeptide

Chain of amino acids linked by peptide bonds; also called a protein.

polyphenism

When different environmental conditions result in differing phenotypes from the same genotype.

polyphyletic

A family or relational grouping of taxa where members do not share all ancestors in common.

polyploid

Containing more than two complete sets of chromosomes.

polyribosome

Messenger RNA molecule with several ribosomes attached to it.

polysomy

The occurrence of an extra chromosome.

polytene chromosome

Giant chromosome in the salivary glands of Drosophila melanogaster; each polytene chromosome consists of a number of DNA molecules lying side by side.

population

A group of the same species in the same area that have the potential to interbreed with one another.

population bottleneck

A population bottleneck is an event that drastically reduces the size of a population

population differentiation

The genetic differences between populations or between a population and an ancestral group.

population genetics

Study of the genetic composition of populations (groups of individuals of the same species) and how a population's collective group of genes changes through time.

position effect

Dependence of the expression of a gene on the gene's location in the genome.

positional cloning

Method that allows for the isolation and identification of a gene by examining the cosegregation of a phenotype with previously mapped genetic markers.

positive assortative mating

Mating between like individuals that is more frequent than would be expected on the basis of chance.

positive control

Gene regulation in which the binding of a regulatory protein to DNA stimulates transcription (the regulatory protein is an activator).

positive selection

Selection for an allele that increases fitness.

positive-strand RNA virus

RNA virus whose genomic RNA molecule codes directly for viral proteins.

post-transcriptional regulation

Multiple processes that regulate the efficiency of translation after mRNA is transcribed, including mRNA stability.

postnatal

Anything observed in a baby after birth.

posttranslational modification

Alteration of a protein after translation; may include cleavage from a larger precursor protein, the removal of amino acids, and the attachment of other molecules to the protein.

postzygotic

After the zygote, usually in reference to mechanisms that prevent continued development of a zygote and thus reproductive isolation.

Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a metabolic disorder caused by genetic defects; among its features are short stature, mental retardation, poor muscle tone, and hyperphagia, which leads to childhood obesity.

pre-messenger RNA

Eukaryotic RNA molecule that is modified after transcription to become mRNA.

preadaptation

A trait that can shift in function to adapt to a changed environment.

preconception

The time before a pregnancy has occurred.

preformationism

Early concept of inheritance proposing that a miniature adult (homunculus) resides in either the egg or the sperm and increases in size during development, with all traits being inherited from the parent that contributes the homunculus.

preimplantation genetic diagnosis

Used to select an embryo produced by in vitro fertilization before implantation of the embryo in the uterus.

prenatal

The period between conception and birth.

presymptomatic diagnosis

The identification of a disease before a patient exhibits physical manifestations of the disease.

presymptomatic genetic testing

Testing people to determine whether they have inherited a disease-causing gene before the symptoms of the disease have appeared.

prevention

Action taken to avoid getting a disease.

prezygotic

Before the zygote, usually refers to mechanisms that prevent sperm from reaching eggs and thus maintaining reproductive isolation.

primary immune response

Initial clone of cells specific for a particular antigen and generated when the antigen is first encountered by the immune system.

primary oocyte

Oogonium that has entered prophase I.

primary spermatocyte

Spermatogonium that has entered prophase I.

primary structure of a protein

The amino acid sequence of a protein.

primase

Primase is an enzyme that synthesizes short RNA sequences called primers, which serve as starting points for DNA synthesis

primer

A primer is a short nucleic acid sequence that provides a starting point for DNA synthesis.

primordium

The name for a tissue or organ proto-structure in its first stage of development.

principle of independent assortment

The principle of independent assortment describes how different genes independently separate from one another during the formation of reproductive cells.

principle of segregation

The principle of segregation describes how pairs of gene variants are separated into reproductive cells.

principle of uniformity

This principle describes how heterozygotes share a common phenotype.

Principles of Inheritance

The term that collectively refers to the three principles described by Gregor Mendel that together summarize his extensive experiments studying the patterns of heredity for acquired characteristics.

prion

Infectious agent that lacks nucleic acid but resembles a virus; believed to replicate by altering the shape of cellular proteins; a likely cause for diseases like scrapie and BSE.

probability

Likelihood of a particular event occurring; more formally, the number of times a particular event occurs divided by the number of all possible outcomes. Probability values range from 0 to 1.

proband

A person with a trait or disease for whom a pedigree is constructed.

probe

A chosen sequence of DNA or RNA that is complementary to a sequence of interest, and used to locate that complementary DNA sequence; alternatively, a measuring instrument.

processed pseudogene

A gene that is not expressed or does not code for any protein product and is the result of mature mRNA being subject to retrotransposition into cDNA.

processing of mRNA

The process of adding different features to a nascent mRNA strand in eukaryotes, including the addition of a 5' cap, splicing, editing, and polyadenylation.

progenesis

Development where sexual systems mature more quickly than the rest of the body.

progeny

Offspring.

prokaryote

Prokaryotes are organisms whose cells lack a nucleus and other organelles. This class of organisms includes all of the bacteria

prometaphase

Prometaphase is the second phase of mitosis, which is a process that separates the duplicated genetic material carried in the nucleus of a parent cell into two, identical daughter cells

promoter

DNA sequence to which the transcription apparatus binds so as to initiate transcription; indicates the direction of transcription, which of the two DNA strands is to be read as the template, and the starting point of transcription.

pronucleus

The nucleus in a sperm cell or oocyte prior to fertilization.

proofreading

Ability of DNA polymerases to remove and replace incorrectly paired nucleotides in the course of replication.

prophage

Phage genome that is integrated into a bacterial chromosome.

prophase

Prophase is the first phase of mitosis, which is a process that separates the duplicated genetic material carried in the nucleus of a parent cell into two, identical daughter cells

prophase I

Stage of meiosis I. In prophase I, chromosomes condense and pair, crossing over takes place, the nuclear membrane breaks down, and the spindle forms.

prophase II

Stage of meiosis after interkinesis. In prophase II, chromosomes condense, the nuclear membrane breaks down, and the spindle forms. Some cells skip this stage.

proportion of polymorphic loci

Percentage of loci in which more than one allele is present in a population.

proteasomal regulation

Protein stability mediated by the proteasome, a cellular organelle that degrades misfolded proteins that are often tagged with ubiquitin.

proteasome

A structure of multiple proteins that breaks down other proteins tagged by ubiquitin; found in eukaryotes, archaea, and some prokaryotes.

protein

A connected series of amino acids that may have up to 20 different kinds of side chains; can exist in long fibrous or globular forms; component of macromolecules; forms enzymes and macromolecules active in cellular structure and biochemical processes.

protein domain

Sections of protein sequence (<100 amino acids) that can fold into a specific three-dimensional structure; folding in each domain is typically independent of other domains in the protein.

protein kinase

An enzyme that adds phosphate groups to other cellular proteins.

protein kinase A

A signaling protein that phosphorylates other proteins in response to cAMP signals.

protein kinase B

A family of protein kinases (Akt1, Akt2 and Akt3) that mediate an array of intracellular signaling cascades. For example, Akt1 is generally thought to mediate cellular survival, and Akt2 is a primary component of insulin signaling.

protein kinase C

A family of protein kinases that can be divided ubti three subgroups can be identified depending on their mechanism of action.conventional, novel, or atypical.

protein-coding region

The part of mRNA consisting of the nucleotides that specify the amino acid sequence of a protein.

protein-protein association

When two proteins bind and have an affinity for each other.

proteome

A proteome is the complete set of proteins expressed by an organism

proto-oncogene

Normal cellular gene that controls cell division. When mutated, it may become an oncogene and contribute to cancer progression.

provinciality

The degree of differences among species in different geographic regions.

provirus

DNA copy of viral DNA or viral RNA; integrated into the host chromosome and replicated along with the host chromosome.

pseudoautosomal region

Small region of the X and Y chromosomes that contains homologous gene sequences.

pseudodominance

Expression of a normally recessive allele owing to a deletion on the homologous chromosome.

pseudogene

A nonfunctional member of a gene family that has been derived from a functional gene.

psi-blast

A variation of BLAST that uses profiles that are based on sequence multiple-alignments to improve the sensitivity of protein database searches.

pulse labeling

The addition of a labeled biochemical to cells in order to follow that specific chemical in cells of an organism or in culture.

pulse-chase assay

An experiment that requires the treatment of cells for a short time with a radioactive biochemical. The radioactive chemical is removed from the cells and the material that was incorporated into the cell is followed over time.

punctuated equilibria / punctuated equilibrium

A pattern of evolutionary change characterized by long periods of little change each followed by a short period of rapid and drastic change.

Punnett square

Shorthand method of determining the outcome of a genetic cross. On a grid, the gametes of one parent are written along the upper edge and the gametes of the other parent are written along the left-hand edge. Within the cells of the grid, the alleles in the gametes are combined to form the genotypes of the offspring.

pure-breeding

A line of organisms that breed offspring with an identical phenotype whenever intercrossed.

purifying selection

Selection that results in deleterious alleles being removed from the population.

purine

Type of nitrogenous base in DNA and RNA. Adenine and guanine are purines.

pyrimidine

Type of nitrogenous base in DNA and RNA. Cytosine, thymine, and uracil are pyrimidines

pyrimidine dimer

Structure in which a bond forms between two adjacent pyrimidine molecules on the same strand of DNA; disrupts normal hydrogen bonding between complementary bases and distorts the normal configuration of the DNA molecule.

TOP

q-arm

The long arm of a chromosome.

QTL

Quantitative trait loci; the regions of the genome underlying quantitative traits.

quantitative characteristic

Continuous characteristic; displays a large number of possible phenotypes, which must be described by a quantitative measurement.

quantitative genetics

Genetic analysis of complex characteristics or characteristics influenced by multiple genetic factors.

quantitative trait

A trait that varies along a continuum, due to polygenic effects, instead of having discrete states.

quaternary structure of a protein

Interaction of two or more polypeptides to form a functional protein.

query sequence

A genetic sequence used as a probe to search (query) a database.

TOP

R plasmid

Plasmid having genes that confer antibiotic resistance to any cell that contains the plasmid.

radiation

Energy that is transmitted as waves or particles. Some forms of radiation can cause genetic mutations.

random genetic drift

Genetic drift describes random fluctuations in the numbers of gene variants in a population. Genetic drift takes place when the occurrence of variant forms of a gene, called alleles, increases and decreases by chance over time. These variations in the presence of alleles are measured as changes in allele frequencies.Typically, genetic drift occurs in small populations, where infrequently occurring alleles face a greater chance of being lost. Once it begins, genetic drift will continue until the involved allele is either lost by a population or until it is the only allele present in a population at a particular locus. Both possibilities decrease the genetic diversity of a population. Genetic drift is common after population bottlenecks, which are events that drastically decrease the size of a population. In these cases, genetic drift can result in the loss of rare alleles and decrease the gene pool. Genetic drift can cause a new population to be genetically distinct from its original population, which has led to the hypothesis that genetic drift plays a role in the evolution of new species.

random mating

When mating is random with respect to the trait in question, the likelihood of a mating between two genotypes is a function of the frequency of the genotypes.

randomly amplified polymorphic DNA

Genetic markers made when random sequences are used as primers to amplify DNA.

reaction norms

Pattern of phenotypes that result from genotypes in different environments.

reading frame

Particular way in which a nucleotide sequence is read in groups of three nucleotides (codons) in translation. Each reading frame begins with a start codon and ends with a stop codon.

realized heritability

Heritability measured by response to selection. The ratio of response to one generation of selection to the selection differential of the parents.

reannealing

In DNA, the process by which two complementary single-stranded DNA molecules pair; also called renaturation.

recessive

Refers to a trait that is expressed only when genotype is homozygous; a trait that tends to be masked by other inherited traits, yet persists in a population among heterozygous genotypes.

recessive marker

Recessive genetic trait. Often included on a compound chromososme to detect rare recombinational events.

reciprocal crosses

Crosses in which the phenotypes of the male and female parents are reversed. For example, in one cross, a tall male is crossed with a short female and, in the other cross, a short male is crossed with a tall female.

reciprocal translocation

Reciprocal exchange of segments between two nonhomologous chromosomes.

reciprocal transplant experiment

Experiment designed to demonstrate phenotypic plasticity and genetic differences by raising individuals from different subpopulations in the habitats of all the subpopulations.

recognition helix

The helix in a helix-turn-helix motif that interacts with specific nucleotides in the major groove.

recombinant

An adjective describing offspring of a genetic cross, wherein the offspring has a phenotype resembling neither parent, due to recombination events between linked genes during meiosis.

recombinant congenics

Strains of organisms generated by the inbreeding of the second backcross generation of two separate inbred strains.

recombinant DNA

A type of DNA sequence that is composed of sequences from two or more different sources or organisms, such as human and pig sources, or synthetic (lab-made) sequences and microorganisms. This customized DNA is typically inserted into a different cell's genome for a specific purpose.

recombinant DNA technology

A technology that uses enzymes to cut and paste together DNA sequences of interest. The recombined DNA sequences can be placed into vectors that carry the DNA into a host cell. In this host cell, the customized recombined DNA sequence can be copied or translated.

recombinant gamete

Possesses new combinations of genes.

recombinant inbred

A line of genetically identical animals produced by mating successive generations of sibling animals initially descended from the offspring of a cross between two distinct inbred strains.

recombinant progeny

Possesses new combinations of traits formed from recombinant gametes.

recombination

A process by which pieces of DNA are broken and recombined to produce new combinations of alleles.

recombination frequency

A number that describes the proportion of recombinant offspring produced in a genetic cross between two organisms.

recombination rate

The probability of a crossover event between two loci.

recruitment

In evolutionary biology, a synonym for preadaptation where a trait has a shift in function. In population biology, it refers to when juvenile individuals reach sexual maturity and are added to the breeding population.

recurrence risk

The chances that an inherited disease will be found in another family member.

recurrent mutation

Reoccurrence of a certain type of mutation.

red fluorescent protein

A mutant version of green fluorescent protein that can be used as a reporter construct in live cells.

refuge

A position or place providing protection or shelter.

refugia

Areas where species have survived while becoming extinct in other areas.

regression

In statistics, the mathematical function that best describes the relationship between two variables, where one is independent and one is dependent.

regression coefficient

Statistic that measures how much one variable changes, on average, with a unit change in another variable.

regulated stability

The biochemical mechanisms responsible for controlling the half-life of a protein or RNA molecule. Often, post-translational or post-transcriptional modifications mediate this regulation.

regulator protein

Produced by a regulator gene, a protein that binds to another DNA sequence and controls the transcription of one or more structural genes.

regulatory element

DNA sequence that affects the transcription of other DNA sequences to which it is physically linked.

regulatory gene

DNA sequence that encodes a protein or RNA molecule that interacts with DNA sequences and affects their transcription or translation or both.

regulatory promoter

DNA sequence located immediately upstream of the core promoter that affects transcription; contains consensus sequences to which transcriptional activator proteins bind.

reinforcement

The increase in reproductive isolation due to natural selection for isolation.

relatedness

A measure of genetic similarity.

relative fitness

The ratio of the fitness of a genotype to the mean fitness of a population or competing genotype.

relaxed state

Energy state of a DNA molecule when there is no structural strain on the molecule.

release factor

Protein required for the termination of translation; binds to a ribosome when a stop codon is reached and stimulates the release of the polypeptide chain, the tRNA, and the mRNA from the ribosome.

relict

A population or individual that has survived despite extinction of the rest of the species. Sometimes used as an adjective to describe populations as same.

renaturation

See reannealing.

repetitive DNA

Sequences that exist in multiple copies in a genome.

replacement substitution

A mutation where base pairs are replaced with base pairs that code for an alternate amino acid and therefore a different protein product. Also called a nonsynonymous substitution.

replication

DNA replication is a process by which a double-stranded DNA molecule is copied into two, identical DNA molecules

replication bubble

Segment of a DNA molecule that is unwinding and undergoing replication.

replication error

Replication of an incorporated error in which a change in the DNA sequence has been replicated and all base pairings in the new DNA molecule are correct.

replication fork

Point at which a double-stranded DNA molecule separates into two single strands that serve as templates for replication.

replication licensing factor

Protein that ensures that replication takes place only once at each origin; required at the origin before replication can be initiated and removed after the DNA has been replicated.

replication origin

Sequence of nucleotides where replication is initiated.

replication terminus

Point at which replication stops.

replicative segregation

Random segregation of organelles into progeny cells in cell division. If two or more versions of an organelle are present in the original cell, chance determines the proportion of each type that will segregate into each progeny cell.

replicative transposition

Type of transposition in which a copy of the transposable element moves to a new site while the original copy remains at the old site; increases the number of copies of the transposable element.

replicon

Unit of replication, consisting of DNA from the origin of replication to the point at which replication on either side of the origin ends.

reporter construct

A gene whose expression is easy to observe and is placed upstream to a gene of interest. Observable expression of this upstream gene conveys a report that the downstream sequence has been transcribed. This gene expression can be in the form of color, such as green fluorescent protein or blue beta-galactosidase color in affected cells.

reporter gene

A reporter gene is a DNA sequence that can be attached to a gene under study, which codes for a protein product that can be easily measured

repressible operon

Operon or other system of gene regulation in which transcription is normally on. Something must take place for transcription to be repressed, or turned off.

repressor

Regulatory protein that binds to a DNA sequence and inhibits transcription.

reproduction

The process of making another copy; producing offspring.

reproductive effort

The amount of resources an organism devotes to reproduction instead of its own growth or maintenance.

repulsion

See trans configuration.

resolvase

Enzyme required for some types of transposition; brings about resolution.which is crossing over between sites located within the transposable element. Resolvase may be encoded by the transposable element or by a cellular enzyme that normally functions in homologous recombination.

respiration

In metabolism, a biochemical process inside cells that breaks down organic material and releases energy; occurs in mitochondria and generates ATP, CO2 and H20. In gas exchange, the transfer of oxygen and carbon dioxide in opposite directions across a membrane to regulate oxygen availability inside an organism.

response element

Common DNA sequence found upstream of some groups of eukaryotic genes. A regulatory protein binds to a response element and stimulates the transcription of a gene. The presence of the same response element in several promoters or enhancers allows a single factor to simultaneously stimulate the transcription of several genes.

response to selection

Change in phenotype in one generation as a result of selection.

restriction endonuclease

Technical term for a restriction enzyme, which recognizes particular base sequences in DNA and makes double-stranded cuts nearby.

restriction enzyme

Enzyme that recognizes particular base sequences in DNA and makes double-stranded cuts nearby; also called restriction endonuclease.

restriction fragment length polymorphism (RFLP)

Variation in the pattern of fragments produced when DNA molecules are cut with the same restriction enzyme; represents a heritable difference in DNA sequences and can be used in gene mapping.

restriction mapping

Restriction mapping is a method used to map an unknown segment of DNA by breaking it into pieces and then identifying the locations of the break points

restriction site

The DNA sequence cut by a restriction enzyme. Each restriction enzyme has a particular DNA sequence it recognizes and restricts.

reticulate evolution

Hybridization of related taxa.

retrotransposon

Type of transposable element in eukaryotic cells that possesses some characteristics of retroviruses and transposes through an RNA intermediate.

retrovirus

RNA virus capable of integrating its genetic material into the genome of its host. The virus injects its RNA genome into the host cell, where reverse transcription produces a complementary, double-stranded DNA molecule from the RNA template. The DNA copy then integrates into the host chromosome to form a provirus.

reverse chromosome duplication

Duplication of a chromosome segment in which the sequence of the duplicated segment is inverted relative to the sequence of the original segment.

reverse mutation

Mutation that changes a mutant phenotype back into the wild type.

reverse transcriptase

Enzyme capable of synthesizing complementary DNA from an RNA template.

reverse transcriptase PCR

A polymerase chain reaction that produces multiple copies of a selected RNA sequence rather than a DNA sequence; compare to polymerase chain reaction.

reverse transcription

Synthesis of DNA from an RNA template.

RFLP

Restriction fragment length polymorphisms, or RFLPs, are differences among individuals in the lengths of DNA fragments cut by enzymes.

rho factor

Subunit of bacterial RNA polymerase that facilitates termination of transcription of some genes.

rho-dependent terminator

Sequence in bacterial DNA that requires the presence of the rho subunit of RNA polymerase to terminate transcription.

rho-independent terminator

Sequence in bacterial DNA that does not require the presence of the rho subunit of RNA polymerase to terminate transcription.

ribonuclear protein

Proteins that interact in the nucleus with RNA, often to help catalyze splicing reactions.

ribonucleic acid

RNA; A single-stranded polymer of nucleotides that contain the sugar ribose; made through the process of transcription; three primary types exist, and all three function in the interpretation of the information stored in DNA.

ribonucleoside triphosphate

Substrate of RNA synthesis; consists of a ribose sugar, a nitrogenous base, and three phosphates linked to the 5'-carbon atom of the sugar. In transcription, two of the phosphates are cleaved, producing an RNA nucleotide.

ribonucleotide

Nucleotide containing a ribose sugar; present in RNA.

ribose sugar

Five-carbon sugar in RNA.

ribosomal RNA

RNA that is a component of the ribosome.

ribosome

A complex molecule made of ribosomal RNA molecules and proteins that form a factory for protein synthesis in cells.

ribosomes

Cellular organelles that are the sites for translation of mRNA into protein.

riboswitch

Regulatory sequences in an RNA molecule. When an inducer molecule binds to the riboswitch, it changes the configuration of the RNA molecule and alters the expression of the RNA, usually by affecting termination of transcription or affecting translation.

ribozyme

RNA molecule that can act as a biological catalyst.

ring-X chromosome

Highly unstable X chromosomes in which both ends have deletions and the ends fuse to form a ring.

risk

The possibility of loss or a negative outcome; can be theoretical or measured epidemiologically.

RNA cleavage

Breaking of an RNA strand into smaller pieces, usually by a nucleolytic enzyme such as a ribozyme.

RNA editing

Process in which the protein-encoding sequence of an mRNA is altered after transcription. The amino acids specified by the altered mRNA are different from those predicted from the nucleotide sequence of the gene encoding the protein.

RNA interference

Process in which cleavage of double-stranded RNA produces small interfering RNAs (siRNAs) that bind to mRNAs containing complementary sequences and bring about their cleavage and degradation.

RNA polymerase

Enzyme that synthesizes RNA from a DNA template during transcription.

RNA polymerase I

Eukaryotic RNA polymerase that transcribes large ribosomal RNA molecules (18 S rRNA and 28 S rRNA).

RNA polymerase II

Eukaryotic RNA polymerase that transcribes pre-messenger RNA and some small nuclear RNAs.

RNA polymerase III

Eukaryotic RNA polymerase that transcribes transfer RNA, small ribosomal RNAs (5 S rRNA), and some small nuclear RNAs.

RNA processing

In eukaryotic cells, the process of removing introns and modifying the ends of the precursor RNA transcript prior to leaving the nucleus.

RNA replication

Process in some viruses by which RNA is synthesized from an RNA template.

RNA silencing

Mechanism by which double-stranded RNA is cleaved and processed to yield small single-stranded interfering RNAs (siRNAs), which bind to complementary sequences in mRNA and bring about the cleavage and degradation of mRNA; also known as RNA interference and posttranscriptional RNA gene silencing. Some siRNAs also bind to complementary sequences in DNA and guide enzymes to methylate the DNA.

RNA splicing

Process by which introns are removed and exons are joined together.

RNA-coding region

Sequence of DNA nucleotides that encodes an RNA molecule.

Robertsonian translocation

Translocation in which the long arms of two acrocentric chromosomes become joined to a common centromere, resulting in a chromosome with two long arms and usually another chromosome with two short arms.

rolling-circle replication

Replication of circular DNA that is initiated by a break in one of the nucleotide strands, producing a double-stranded circular DNA molecule and a single-stranded linear DNA molecule, the latter of which may circularize and serve as a template for the synthesis of a complementary strand.

rooted tree

Phylogenetic tree in which one internal node represents the common ancestor to all other organisms (nodes) on the tree. In a rooted tree, all the organisms depicted have a common ancestor.

rubric

A set of expectations or ideal characteristics against which a work is measured; can be used in grading or in the creation of new material that follows set standards.

TOP

S phase

Stage of interphase in the cell cycle. In S phase, DNA replicates.

saltation

A mutation or sudden change in a lineage.

sample

Subset used to describe a population.

sampling error

Deviations from expected ratios due to chance occurrences when the number of events is small.

SAT B1

A human homeodomain-containing transcription factor.

satellite RNA

A form of viral RNA, often found associated with plant viruses, that has self-splicing (catalytic) properties.

scala naturae

A Latin term, translated as the "ladder of nature." An obsolete concept that life was organized in a linear hierarchy, with organisms increasing in perfection from lower forms to higher forms.

scale

To expand in scope, usually exponentially (verb); the spatial relationship of objects or organisms, as the relate to each other or their environment (noun).

second filial cross

Interbreeding of second generation individuals generated from a cross between two parents.

second polar body

One of the products of meiosis II in oogenesis; contains a set of chromosomes but little of the cytoplasm.

secondary immune response

Clone of cells generated when a memory cell encounters an antigen; provides long-lasting immunity.

secondary oocyte

One of the products of meiosis I in female animals; receives most of the cytoplasm.

secondary spermatocyte

Product of meiosis I in male animals.

secondary structure of a protein

Regular folding arrangement of amino acids in a protein. Common secondary structures found in proteins include the alpha helix and the beta pleated sheet.

securin

Molecule that normally binds the enzyme separase, preventing it from cleaving cohesin molecules that hold the sister chromatids together.

segment-polarity genes

Set of segmentation genes in fruit flies that affect the organization of segments. Mutations in these genes cause part of each segment to be deleted and replaced by a mirror image of part or all of an adjacent segment.

segmentation genes

Set of about 25 genes in fruit flies that control the differentiation of the embryo into individual segments, affecting the number and organization of the segments. Mutations in these genes usually disrupt whole sets of segments.

segregating

A population with more than one common allele and thus more than one common phenotype for a given trait.

segregating generation

Generation in which there is more than one expected genotype.

segregating inbred lines

Line developed by inbreeding but in which one or more loci remain heterozygous. This generally requires progeny screening each generation and selection of heterozygous parents.

segregation

Any process of separating one set of things from another; in biology, the separation of alleles into different gametes during meiosis.

segregation distortion

When gene alleles in a heterozygote do not segregate in a 1:1 ratio among gametes; instead, one allele appears in more than 50% of the gametes.

selection

Nonrandom differential survival or reproduction of classes of phenotypically different entities. See also natural selection, artificial selection.

selection coefficient

A measure of the relative fitness of a genotype. A number between 0 and 1 where 1 means the genotype has been entirely selected against and thus makes no contribution to the next generation.

selection differential

The difference between the mean of the parents and the mean of the entire population for a quantitative trait under selection pressure.

selection gradient

A descriptive statistic; used to measure the intensity with which traits are influenced by direct selection.

selection plateau

Also called selection limit. The point where a population is homozygous for the preferred alleles and will no longer respond to selection.;

selective advantage

The increase in fitness due to an allele or trait.

selective agent

An environmental condition or process that exerts selection pressure.

selective breeding

The controlled breeding of specific plants or animals to produce offspring with desirable traits; may involve mutliple generations to achieve results.

selective sweep

When a favorable mutation is strongly selected for and increases in frequency, the corresponding rise in frequency of alleles positioned nearby. Also called "genetic hitchhiking."

self-incompatibility

Plants that are unable to self-fertilize. This promotes outcrossing and the resulting increase in genetic variation.

self-renewal

A stem cell's ability to divide and produce a daughter cell capable of the same.

selfing

Self-fertilization; a cross where the same organism provides both gametes.

selfish DNA

Any sequence of DNA that is able to replicate itself and does not confer any advantage to the organism. It may be deleterious.

semelparous

From the roots "to beget once," a reproductive strategy characterized by only one reproductive event.

semiconservative replication

Replication in which the two nucleotide strands of DNA separate, each serving as a template for the synthesis of a new strand. All DNA replication is semiconservative.

semidominance

The lack of dominance of any specific allele at a locus. Heterozygotes have a phenotype intermediate between the two parents.

semispecies

A group of populations that have reduced gene flow between them yet are not completely reproductively isolated.

sense codon

Codon that specifies an amino acid in a protein.

separase

Molecule that cleaves cohesin molecules, which hold the sister chromatids together.

sequence alignment

Regions of local similarity of DNA, RNA, or amino acid sequences.

sequential hermaphroditism

Phenomenon in which the sex of an individual organism changes in the course of its lifetime; the organism is male at one age or developmental stage and female at a different age or stage.

serial homology

When repeated related structures, such as vertebrae, have similarities in development, function and/or ancestral form.

serotonin

A monoamine neurotransmitter that can act directly on neurons to modulate their activity via ionic or G-protein coupled receptors; chemical name 5-hydroxytryptamine, or 5-HT.

sex

The biological or physiological characteristics that determine whether an organism is male or female or other; the physical act between organisms involving sex organs.

sex chromosomes

Chromosomes that differ morphologically or in number in males and females.

sex determination

Specification of sex (male or female). Sex-determining mechanisms include chromosomal, genic, and environmental sex-determining systems.

sex ratio

The number of males compared to the number of females in a population.

sex-determining region Y gene

On the Y chromosome, a gene that triggers male development; also known as the testis-determining factor (TDF) gene.

sex-influenced characteristic

Encoded by autosomal genes that are more readily expressed in one sex. For example, an autosomal dominant gene may have higher penetrance in males than in females or an autosomal gene may be dominant in males but recessive in females.

sex-limited characteristic

Encoded by autosomal genes and expressed in only one sex. Both males and females carry genes for sex-limited characteristics, but the characteristics appear in only one of the sexes.

sex-linked characteristic

Characteristic determined by a gene or genes on sex chromosomes.

sex-linked dominant

An allele on the X chromosome that encodes a trait that is inherited by one-half of the offspring from an affected mother; an affected father would pass the trait to all of his daughters and none of his sons.

sex-linked recessive

A trait encoded by a gene found on a sex chromosome that requires two copies of the allele to be manifested in the organism. If a sex-linked trait is the result of a recessive allele on the X chromosome, most daughters will not be affected. However, if a male offspring receives a mutant allele from the mother's genetic contribution, he will be affected because he is hemizygous for X-chromosome genes.

sexual reproduction

Production of offspring whose genetic constitution is a mixture of those of two potentially genetically different gametes.

sexual selection

In a population, the force that drives the proliferation of phenotypes that confer a mating advantage.

shifting balance theory

A largely verbal theory of evolution which maintains that the interaction among natural selection, genetic drift, and migration is more important that the action of any single force. Sewall Wright argued that this theory helped to explain how species could effectively search for the global (and not merely local) optimum.

Shine-Dalgarno sequence

Consensus sequence found in the bacterial 5' untranslated region of mRNA; contains the ribosome-binding site.

short interspersed element

Short DNA sequence repeated many times and interspersed throughout the genome.

short tandem repeats

A repeat of a short sequence of DNA where the number of repeats can vary between individuals and be used as molecular markers. Also called simple sequence repeat polymorphisms, simple sequence repeats, microsatellites.

shuttle vector

Cloning vector that allows DNA to be transferred to more than one type of host cell.

sibling species

Species that are difficult to distinguish based on morphological characteristics.

sigma factor

Subunit of bacterial RNA polymerase that allows the RNA polymerase to recognize a promoter and initiate transcription.

sign of disease

A physical symptom that is indicative of a disease.

signal sequence

From 15 to 30 amino acids that are found at the amino end of some eukaryotic proteins and direct the protein to specific locations in the cell; usually cleaved from the protein.

signal transduction

The transfer of extracellular signal to an intracellular process by a series of reactions between membrane-bound and intracellular proteins and other molecules.

silencing

General term for epigenetic negative control of gene expression.

silent mutation

A mutation where a change in a DNA codon does not result in a change in amino acid translation.

silent substitution

A mutation where a change in a DNA codon does not result in a change in amino acid translation. The resulting protein product will also be unchanged. Also called a synonymous substitution.

Simian Virus 40

A DNA polyomavirus that has been linked to cancer.

simple sequence repeat polymorphisms

A repeat of a short sequence of DNA where the number of repeats can vary between individuals and be used as molecular markers. Also called short tandem repeats, simple sequence repeats, microsatellites.

simple sequence repeats

simulation model

Analysis of variables via a computer model to predict outcomes.

single nucleotide polymorphism

A single nucleotide polymorphism, or SNP, is a single base-pair difference in the DNA sequence of individual members of a species; not necessarily a pathological mutation, but commonly studied as a covarying marker of complex disease phenotype.

single-strand binding protein

Binds to single-stranded DNA in replication and prevents it from annealing with a complementary strand and forming secondary structures.

sire

The male parent of a domesticated animal, such as those bred for commercial purposes.

sister chromatids

Two copies of a chromosome that are held together at the centromere. Each chromatid consists of a single DNA molecule.

sister taxa

Two groups descended from an immediate common ancestor that are more closely related to each other than to other groups.

site-directed mutagenesis

Produces specific nucleotide changes at selected sites in a DNA molecule.

small cytoplasmic RNA

Small RNA molecule found in the cytoplasm of eukaryotic cells.

small interfering RNA

Single-stranded RNA molecule (usually from 21 to 25 nucleotides in length) produced by the cleavage and processing of double-stranded RNA; binds to complementary sequences in mRNA and brings about the cleavage and degradation of the mRNA. Some siRNAs bind to complementary sequences in DNA and bring about their methylation.

small nuclear ribonucleoprotein

Structure found in the nuclei of eukaryotic cells that consists of snRNA and protein; functions in the processing of pre-mRNA.

small nuclear RNA

Small RNA molecule found in the nuclei of eukaryotic cells; functions in the processing of pre-mRNA.

small nucleolar RNA

Small RNA molecule found in the nuclei of eukaryotic cells; functions in the processing of rRNA and in the assembly of ribosomes.

small ribosomal subunit

The smaller of the two subunits of a functional ribosome.

SNP

somatic cell

Any cell in an organism that is not a sex cell, not a gamete.

somatic cell nuclear transfer

A cell culture process that yields embryonic stem (ES) cells; involves the transfer of a somatic cell nucleus to an enucleated egg, and growth to the blastocyst stage to yield multiple copies.

somatic gene therapy

A therapy that treats somatic cells in a body, altering genee expression to improve cell or tissue health. The genetic alteration does not transfer to offpsring.

somatic hypermutation

High rate of somatic mutation such as that in genes encoding antibodies.

somatic mutation

Mutation in a cell that does not give rise to a gamete.

somatic recombination

Recombination in somatic cells, such as maturing lymphocytes, among segments of genes that encode antibodies and T-cell receptors.

somatic-cell hybridization

Fusion of different cell types.

SOS system

System of proteins and enzymes that allow a cell to replicate its DNA in the presence of a distortion in DNA structure; makes numerous mistakes in replication and increases the rate of mutation.

Southern blot

A Southern blot is a laboratory method used to detect specific DNA molecules from a mixture of DNA molecules

Southern blotting

A method to identify segments of DNA that have a specific sequence. Briefly, DNA is digested with a specific enzyme and the fragments are separated by gel electrophoresis. The DNA fragments are transferred to a membrane that is probed with a labeled piece of DNA complementary to the DNA sequence of interest.

Sp1 transcription factor

A transcription factor involved in different aspects of development. Sp1 has a zinc finger domain that mediates DNA binding.

spatial structure

The arrangement of individuals in a population in an area.

specialized transduction

Transduction in which genes near special sites on the bacterial chromosome are transferred from one bacterium to another; requires lysogenic bacteriophages.

speciation

Speciation is the evolutionary process by which a new species comes into being

species

A biological species is a group of organisms that can reproduce with one another in nature and produce fertile offspring

species selection

A concept of selection at the species level which proposes that the rates of speciation and extinction are due to differences in characteristics within species.

spectral karyotyping

A molecular cytogenetic technique that allows the visualization of all the chromosomes at once using different colored markers for each chromosome.

speed congenics

A variation on regular congenics. At generation N(2), individual mice (generally males) are genotyped for 60 to 100 microsatellite markersb and the mice with the greatest share of the inbred background chromosomes of interest in are selected as parents. (Males are used because a single male can be backcrossed to many different parental strain females). This process may eventually be further accelerated by microarray screens. The savings can be considerable. Speed congenic technology might produce a congenic strain in 12 to 18 months versus two to three years.

spermatid

Immediate product of meiosis II in spermatogenesis; matures to sperm.

spermatogenesis

Sperm production in animals.

spermatogonium

Diploid cell in the testis; capable of undergoing meiosis to produce a sperm.

spindle

A subcellular structure composed of microtubules that separates chromosomes in a cell during cell division.

spindle fibers

Spindle fibers are protein structures that pull apart the genetic material in a cell when the cell divides

spindle microtubule

Microtubule that moves chromosomes in mitosis and meiosis.

spindle pole

Point from which spindle microtubules radiate.

spliced recombinants

Possible outcome of homologous recombination, consisting of two heteroduplex DNA molecules, with the DNA at each end in combinations different from those originally present.

spliceosome

Large complex consisting of several RNAs and many proteins that splices protein-encoding pre-mRNA; contains five small ribonucleoprotein particles (U1, U2, U4, U5, and U6).

splicing

Removal of introns and connecting of exons in eukaryotic pre-mRNAs.

spontaneous mutation

Arises spontaneously from natural changes in DNA structure or from errors in replication.

sporophyte

Diploid phase of the life cycle in plants.

SR proteins

Group of serine- and arginine-rich proteins that regulate alternative splicing of pre-mRNA.

stability

The ability to resist change or to return to a condition after a change in condition.

stabilizing selection

Selection that decreases genetic diversity and stabilizes the mean of a trait in population around a particular, usually optimal, value.

standard deviation

A measure of the range of the data. The mean of all the means of all the sets of data. The square root of the variance.

stasis

A characteristic of a time period during which few or no changes take place; in evolution, the overall absence of evolutionary change in one or more characters for some period of evolutionary time.

statistical linkage

The estimate that two genes are located on a chromosome together such that they are likely to be inherited together.

statistically significant

Likely due to causes other than chance.

statistics

A mathematical science related to the examination of data and its relevance.

stem cells

In animals, undifferentiated cells that are capable of extensive proliferation. A stem cell generates more stem cells and a large clone of differentiated progeny cells.

steric interference

Interrupts a biochemical process by physically preventing the process from occurring.

stochastic

Having a random probability distribution; hard to predict.

stop codon

Codon in mRNA that signals the end of translation; also called nonsense codon or termination codon. There are three common stop codons: UAA, UAG, and UGA.

strain

An artificial line of a species; usually not pure-breeding.

strand slippage

Slipping of the template and newly synthesized strands in replication in which one of the strands loops out from the other and nucleotides are inserted or deleted on the newly synthesized strand.

strata

Layers of material; used to describe sedimentary rock in geology, tissues in biology, and ranges of vegetation in ecology.

structural gene

DNA sequence that encodes a protein that functions in metabolism or biosynthesis or that plays a structural role in the cell.

structural genomics

Area of genomics that studies the organization and sequence of information contained within genomes; sometimes used by protein chemists to refer to the determination of the three-dimensional structure of proteins.

subfunctionalization

Evolution of redundant genes where each one retains some of the functionality of the original gene.

submetacentric chromosome

Chromosome in which the centromere is displaced toward one end, producing a short arm and a long arm.

subpopulation

A small localized interbreeding group within a larger population.

subspecies

A subset of a species.

substrain

Two inbred lines from a common origin may be considered substrains. This implies that the substrains differ at several loci.

succession of species

The predictable and natural series of changes that occur to assemblages of species in an ecosystem.

sucker shoot

An informal term for basal shoots arising from the meristem of a plant, near the base or below ground level, either from a rhizome or root.

sum of squares

An unadjusted measure of variability; for a sample group it is the sum of squared deviations from the mean and the numerator of the variance.

supercoiling

Coiled tertiary structure that forms when strain is placed on a DNA helix by overwinding or underwinding of the helix. An overwound DNA exhibits positive supercoiling; an underwound DNA exhibits negative supercoiling.

superspecies

The major subdivision of a genus or subgenus.

suppressor mutation

Hides or suppresses the effect of another mutation at a site that is distinct from the site of the original mutation.

surface plasmon resonance

The collective oscillation of electrons that occurs when surface plasmon waves are excited by light deflection at a metal-liquid interface; used to investigate protein-protein interactions, such as antibody-antigen interactions, and can serve as a biosensor;abbrevitaed as SPR.

surrogate

An individual that substitutes for another, by appointment, choice, or default.

symbiosis

Literally "living together," a close association between two or more species.

sympatric

Literally "same fatherland" and used to describe populations in the same or overlapping area.

symptom

Physical manifestation of disease.

synapomorphy

A shared derived trait evolved in a common ancestor.

synapsis

Close pairing of homologous chromosomes.

synaptonemal complex

Three-part structure that develops between synapsed homologous chromosomes.

syndrome

A set of symptoms that are exhibited together that are indicative of a specific disease.

synonymous codons

Different codons that specify the same amino acid.

synonymous substitution

A mutation where a change in a DNA codon does not result in a change in amino acid translation. The resulting protein product will also be unchanged. Also called a silent substitution.

syntenic genes

Determined to be on the same chromosome by physical-mapping techniques.

synteny

A term used to describe the state of two or more genes being present on the same chromosome, though not necessarily linked.

TOP

T-cell receptor

Found on the surface of a T cell, a receptor that simultaneously binds a foreign and a self-antigen on the surface of a cell.

T-lymphocyte

A white blood cell of the immune system that has molecules on its surface that recognize specific antigens.

tandem chromosome duplication

Duplication of a chromosome segment that is adjacent to the original segment.

tandem repeat sequences

DNA sequences repeated one after another; tend to be clustered at specific locations on a chromosome.

Taq polymerase

DNA polymerase commonly used in PCR reactions. Isolated from the bacterium Thermus aquaticus, the enzyme is stable at high temperatures, and so it is not denatured during the strand-separation step of the cycle.

target gene

A gene of interest.

targets of selection

The phenotype characters under direct selection.

TATA box

A TATA box is a DNA sequence that indicates the point at which a genetic sequence can be read and decoded

TATA-binding protein

Polypeptide chain found in several different transcription factors that recognizes and binds to sequences in eukaryotic promoters.

taxon

A taxonomic category.

taxonomy category

Any rank of classification, such as a genus or family, in a taxonomy.

TBP-associated factor

Protein that combines with the TATA-binding protein to form a transcription factor.

teleology

The belief that purpose and design are part of nature.

telocentric chromosome

Chromosome in which the centromere is at or very near one end.

telomerase

Enzyme that is made up of both protein and RNA and replicates the ends (telomeres) of eukaryotic chromosomes. The RNA part of the enzyme has a template that is complementary to repeated sequences in the telomere and pairs with them, providing a template for the synthesis of additional copies of the repeats.

telomere

Stable end of a chromosome.

telomere-associated sequence

Sequence found at the end of a chromosome next to the telomeric sequence; consists of relatively long, complex, repeated sequences.

telomeric sequence

Sequence found at the ends of a chromosome; consists of many copies of short, simple sequences repeated one after the other.

telophase

Telophase is the fifth and final phase of mitosis, which is a process that separates the duplicated genetic material carried in the nucleus of a parent cell into two, identical daughter cells.

telophase I

Stage of meiosis I. In telophase I, chromosomes arrive at the spindle poles.

telophase II

Stage of meiosis II. In telophase II, chromosomes arrive at the spindle poles.

temperate phage

Bacteriophage that utilizes the lysogenic cycle, in which the phage DNA integrates into the bacterial chromosome and remains in an inactive state.

temperature-sensitive allele

Expressed only at certain temperatures.

template strand

The strand of DNA that is used as a template during transcription. The RNA synthesized during transcription is complementary and antiparallel to the template strand.

teratogen

Anything, physical or chemical, that can deform or harm a developing embryo.

terminal inverted repeats

Sequences found at both ends of a transposable element that are inverted complements of one another.

termination codon

Codon in mRNA that signals the end of translation; also called nonsense codon or stop codon. There are three common termination codons: UAA, UAG, and UGA.

terminator

Sequence of DNA nucleotides that causes the termination of transcription.

territory

An area of land designated for a specific purpose; an portion of habitat defended by an organism or a group of organisms against others.

tertiary structure of a protein

Higher-order folding of amino acids in a protein to form the overall three-dimensional shape of the molecule.

test cross

A cross between an individual with an unknown genotype and an individual with the homozygous recessive genotype.

testis-determining factor gene

On the Y chromosome, a gene that triggers male development; also known as the sex determining region Y (SRY) gene.

tetrad

The four products of meiosis; all four chromatids of a homologous pair of chromosomes.

tetrad analysis

Genetic analysis of a tetrad, the products of a single meiosis.

tetraploidy

Refers to the possession of four haploid sets of chromosomes (4n).

tetrasomy

Presence of two extra copies of a chromosome (2n + 2).

TFIIB recognition element

Consensus sequence [(G or C)(G or C)(G or C)CGCC] found in some RNA polymerase II core promoters; usually located from 32 to 38 bp upstream of the transcription start site.

TFIIIA transcription factor

A eukaryotic transcription factor involved in the synthesis of 5S RNA. An interesting fact about TFIIIA is that its structure has nine zinc finger domains, making it an archetype for this class of proteins.

theory of clonal selection

Explains the generation of primary and secondary immune responses. Binding of a B cell to an antigen stimulates the cell to divide, giving rise to a clone of genetically identical cells, all of which are specific for the antigen.

theta replication

Replication of circular DNA that is initiated by the unwinding of the two nucleotide strands, producing a replication bubble. Unwinding continues at one or both ends of the bubble, making it progressively larger. DNA replication on both of the template strands is simultaneous with unwinding until the two replication forks meet.

third filial cross

A cross (mating) between individuals generated from the second generation of a parental cross (i.e., F3 = F2 x F2).

threatened species

Any species vulnerable to endangerment.

three-point test cross

Cross between an individual heterozygous at three loci and an individual homozygous for recessive alleles at those loci.

threshold characteristic

Discontinuous characteristic whose expression depends on an underlying susceptibility that varies continuously.

threshold trait

Quantitative traits that are discretely expressed in a limited number of phenotypes (usually two), but which are based on an assumed continuous distribution of factors that contribute to the trait (underlying liability).

thymidine kinase

An enzyme that plays a key role in the synthesis of DNA and, therefore, cell division. Often, thymidine kinase activity is used as a surrogate for measuring the rate of cell division.

thymine

Pyrimidine in DNA but not in RNA.

Ti plasmid

Large plasmid from the bacterium Agrobacterium tumefaciens that is used to transfer genes to plant cells.

tight linkage

Describes two or more loci with a low recombination frequency usually due to being physically close to one another.

topoisomerase

Enzyme that adds or removes rotations in a DNA helix by temporarily breaking nucleotide strands; controls the degree of DNA supercoiling.

totipotent

Refers to the potential of a cell to develop into any other cell type.

trade-off

A trait, mutation or phenotype that confers both a benefit and cost in terms of fitness.

trait

An observed variation in a specific character of an organism.

trans

trans-acting elements are able to diffuse through the cell and thus can affect loci on both strands of DNA in diploid or polyploid organisms

trans configuration

Arrangement in which each chromosome contains one wild-type (dominant) gene and one mutant (recessive) gene.

trans-splicing

The process of splicing together exons from two or more pre-mRNAs.

transcription

A biochemical process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA).

transcription bubble

Region of a DNA molecule that has unwound to expose a single-stranded template, which is being transcribed into RNA.

transcription factor

Transcription factors are proteins that are involved in the process of converting, or transcribing, DNA into RNA

transcription factor dimerization

When two proteins that serve to initiate gene transcription (transcription factors) must associate with each other to gain a conformation that facilitates binding to nucleic acid.

transcription start site

The first DNA nucleotide that is transcribed into an RNA molecule.

transcription unit

Sequence of nucleotides in DNA that codes for a single RNA molecule, along with the sequences necessary for its transcription; normally contains a promoter, an RNA-coding sequence, and a terminator.

transcriptional activator protein

transcriptional antiterminator protein

Protein that binds to RNA polymerase and alters its structure so that certain terminators are ignored, allowing transcription to continue past the terminators.

transcriptome

A transcriptome is the full range of messenger RNA, or mRNA, molecules expressed by an organism

transducing phage

Contains a piece of the bacterial chromosome inside the phage coat.

transductant

Bacterial cell that has received genes from another bacterium through transduction.

transduction

Type of gene exchange that takes place when a virus carries genes from one bacterium to another. After it is inside the cell, the newly introduced DNA may undergo recombination with the bacterial chromosome.

transduction (prokaryotes)

Transduction is a process by which a virus transfers genetic material from one bacterium to another

transesterification

Chemical reaction in some RNA splicing reactions.

transfer RNA introns

Class of introns in tRNA genes. Splicing of these genes relies on enzymes.

transfer-messenger RNA

An RNA molecule that has properties of both mRNA and tRNA; functions in rescuing ribosomes that are stalled at the end of mRNA.

transformant

A cell that has received additional genetic material, either experimentally or via an infection; can be used to refer to a cell that has become malignant.

transformation

Mechanism by which DNA found in the medium is taken up by the cell. After transformation, recombination may take place between the introduced genes and the bacterial chromosome.

transformation (prokaryotes)

Transformation is a process by which foreign genetic material is taken up by a cell. The process results in a stable genetic change within the transformed cell

transforming principle

Term given to the substance that could be transferred from non living cells to living cells, causing the living cell to show characteristics of the non living cell. This term was used by scientists in the 1930s and 1940s before they had isolated DNA and identified it as the actual transforming substance.

transgene

A gene that has been transferred from the genome of one species into that of another.

transgene escape

The movement of a transgene into the natural population, usually when transgenic crops cross-breed with wild relatives.

transgenic mouse

Mouse whose genome contains a foreign gene or genes added by employing recombinant DNA methods.

transgenic organism

An organism whose genome has been artifically modified; can be a microorganism or a more complex one.

transgenic techniques

Any method for moving a gene into the genome of a new species.

transition

Base substitution in which a purine is replaced by a different purine or a pyrimidine is replaced by a different pyrimidine. See also point mutation, transversion.

translation

Translation is the process by which a protein is synthesized from the information contained in a molecule of messenger RNA (mRNA).

translocation

Movement of a chromosome segment to a nonhomologous chromosome or to a region within the same chromosome; also movement of a ribosome along mRNA in the course of translation.

translocation carrier

Individual organism heterozygous for a translocation.

transmissible disease

A disease that can be inherited.

transmission genetics

The field of genetics that encompasses the basic principles of genetics and how traits are inherited.

transposable element

A mobile fragment of DNA that can change position in a genome; a cause of sequence insertions and deletions in some organisms.

transposase

Enzyme encoded by many types of transposable elements that is required for their transposition. The enzyme makes single-strand breaks at each end of the transposable element and on either side of the target sequence where the element inserts.

transposition

Movement of a transposable genetic element from one site to another. Replicative transposition increases the number of copies of the transposable element; nonreplicative transposition does not increase the number of copies.

transversion

Base substitution in which a purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine. See also point mutation, transition.

trihybrid cross

A cross between two individuals that differ in three characteristics (AA BB CC X aa bb cc); also refers to a cross between two individuals that are both heterozygous at three loci (Aa Bb Cc X Aa Bb Cc).

triplet code

Refers to the fact that three nucleotides encode each amino acid in a protein.

triplo-X syndrome

Human condition in which cells contain three X chromosomes. A person with triplo-X syndrome has a female phenotype without distinctive features other than a tendency to be tall and thin; a few such women are sterile, but many menstruate regularly and are fertile.

triploidy

Refers to the possession of three haploid sets of chromosomes (3n).

trisomy

Presence of an additional copy of a chromosome (2n + 1).

trisomy 13

Presence of three copies of chromosome 13; in humans, results in Patau syndrome.

trisomy 18

Presence of three copies of chromosome 18; in humans, results in Edward syndrome.

trisomy 21

trisomy 8

Presence of three copies of chromosome 8; in humans, results in mental retardation, contracted fingers and toes, low-set malformed ears, and a prominent forehead.

tRNA

RNA molecule that carries an amino acid to the ribosome and transfers it to a growing polypeptide chain in translation.

tRNA charging

Chemical reaction in which an aminoacyl-tRNA synthetase attaches an amino acid to its corresponding tRNA.

tRNA-modifying enzyme

Creates a modified base in RNA by catalyzing a chemical change in the standard base.

truncation selection

A breeding technique where a certain value for a selected character is set as the truncation point. Individuals on one side of the point are parents of the next generation (have high fitness) and those on the other side have no progeny (no fitness).

Trypanosoma

A parasitic protozoa with flagella that infects cells of animals and humans causing a variety of diseases, including sleeping sickness and Chagas' disease.

tubulin

Protein found in microtubules.

tumor suppressor gene

Gene that normally inhibits cell division. Recessive mutations in such genes often contribute to cancer.

Turner syndrome

Human condition in which cells contain a single X chromosome and no Y chromosome (XO). Persons with Turner syndrome are female in appearance but do not undergo puberty and have poorly developed female secondary sex characteristics; most are sterile but have normal intelligence.

twin study

A form of experimental design in which investigators examine twins (both monozygotic and dizygotic) who were reared together or apart in order to determine the relative contributions of genetics and environment to a specific trait.

TOP

ubiquitin

A protein that is post-translationally added to other proteins, frequently tagging them for proteasome-mediated degradation.

ultrasonography

Procedure for visualizing a fetus. High-frequency sound is beamed into the uterus. Sound waves that encounter dense tissue bounce back and are transformed into a picture of the fetus.

unbalanced data

Data where each group does not have an equal number of data points.

unbalanced gametes

Gametes that have variable numbers of chromosomes; some chromosomes may be missing and others may be present in more than one copy.

unbalanced translocation

When pieces of chromosomes are rearranged and genetic material is gained or lost in the cell.

underdominance

A homozygote advantage, whereby the heterozygote is selected against, resulting in more homozygous individuals.

unequal crossing over

Misalignment of the two DNA molecules during crossing over, resulting in one DNA molecule with an insertion and the other with a deletion.

uniparental disomy

Inheritance of both chromosomes of a homologous pair from a single parent.

unique-sequence DNA

Sequence present only once or a few times in a genome.

universal genetic code

Refers to the fact that particular codons specify the same amino acids in almost all organisms.

unlinked loci

Genes with a recombination frequency of 0.5 that therefore sort independently. Genes on separate chromosomes or physically far apart on large chromosomes.

unstable equilibrium

A system that will not return to the original state after a disturbance.

untranslated region

Part of an mRNA sequence that is not included as a template for the production of a protein.

up mutation

Mutation that increases the rate of transcription.

upstream

Toward the 5' end of a chain of nucleotides.

upstream control element

Consensus sequence in eukaryotic RNA polymerase I promoters that extends from 107 to 180 bp upstream of the transcription start site and increases the efficiency of the core element; rich in guanine and cytosine nucleotides.

upstream element

Consensus sequence found in some bacterial promoters that contains a number of A-T pairs and is found about 40 to 60 bp upstream of the transcription start site.

uracil

Pyrimidine in RNA but not normally in DNA.

TOP

variable

Some factor, such as number or a trait, that is likely to change or vary; something measureable yet dynamic, such that measurements change depending on impacting forces, such as the passage of time or environmental conditions.

variable expression

Differences between individuals in how a trait manifests itself.

variable number tandem repeats

Within a gene, short sequences of DNA repeated in tandem that vary greatly in number among individuals; also called microsatellites. Commonly used in DNA fingerprinting due to extreme variability among humans; abbreviated as VNTRs.

variance

A measure of the spread of a set of numerical observations; calculated as the average of the squared deviation from the mean.

vector

A piece of DNA that ferries a foreign seqence of DNA into a cell or organism; together with the foreign DNA, the vector forms recombinant DNA. In disease transmission, the organism that carries an infectious agent (virus, parasite) from one host to another.

verbal model

A display of relationships expressed in words instead of mathematical formula.

vertical transmission

The transfer of a condition or disease from one generation to another via genetics or parent to child contact.

vestigial

Describes something occurring in a simpler, less functional state; sometimes a remnant of a larger more robust form.

viability

The ability to live. May refer to the number of a cohort surviving to a given age.

vicariance

The process whereby environmental changes divide a population into separate isolated groups.

virulence

The degree of pathogenicity.

virulence factor

An intrinsic character of infectious bacteria that facilitates its ability to cause disease.

virulent phage

Bacteriophage that reproduces only through the lytic cycle and kills its host cell.

virus

A virus is an infectious agent that can replicate only within a host organism. Viruses infect a variety of living organisms, including bacteria, plants, and animals

visible polymorphism

Describes a trait with only a few distinct versions.

vulva

In humans, the external genitals of females; in nematodes, an orifice found in the ventral epidermis that facilitates egg-laying and mating.

TOP

weather

The state of the atmosphere at any given time, including measures such as temperature, rain, cloud cover, wind and humidity.

Western blot

A Western blot is a laboratory method used to detect specific proteins from a mixture of proteins

Western blotting

Process by which protein is transferred from a gel to a solid support such as a nitrocellulose or nylon filter.

wheat embryo cell-free translation system

A mixture extracted from wheat embryos that has all of the molecules needed to make a protein when an mRNA is added.

whole-genome shotgun sequencing

Method of sequencing a genome in which sequenced fragments are assembled into the correct sequence in contigs by using only the overlaps in sequence.

wiggle format

In bioinformatics, the display format for dense continuous data such a guanine-cytosine content, probability scores, and transcriptome data; abbreviated as WIG.

wild type

The commonly occurring allele or characteristic in a natural population.

wild-type allele

The most prevalent allele in a natural population for a certain gene.

wobble

Describes the redundancy in the genetic code such that the same amino acid may be encoded by multiple codons.

TOP

X chromosome

One of the two sex chromosomes in animals that are determined by the XX-XY system; the only sex chromosome in animals whose sex is determined by the XX-XO system.

X inactivation

The process and result of one of a pair of X chromosomes becoming silent or inactive in a female mammal; typically occurs in early embryonic development and affects the life of the female.

X-linked characteristic

Characteristic determined by a gene or genes on the X chromosome.

X-ray crystallography

A method to determine the three-dimensional organization of biochemicals. Generally, crystals of a molecule are used because they are an array of many molecules of the same compound, thus increasing the strength of diffraction of the X-rays.

X-ray diffraction

Method for analyzing the three-dimensional shape and structure of chemical substances. Crystals of a substance are bombarded with X-rays, which hit the crystals, bounce off, and produce a diffraction pattern on a detector. The pattern of the spots produced on the detector provides information about the molecular structure.

X-Y homologs

Y-linked genes that are similar to genes on the X chromosome.

X:A ratio

Ratio of the number of X chromosomes to the number of haploid autosomal sets of chromosomes; determines sex in fruit flies.

Xist RNA

The RNA that coats the inactive X chromosome.

XX-XO system

In organisms like crickets, grasshoppers, and some other insects, the female is XX and is the homogametic sex. The male is the heterogametic sex but only has one sex chromosome. The male in XX-XO systems produce gametes with (X) or without (O) a sex chromosome.

XX-XY system

In organisms like humans and mammals, the male is the heterogametic sex, producing gametes with either an X or a Y chromosome. All gametes from the female are X. Thus, the sex of the offspring is determined by the class of sperm.

TOP

Z-DNA

Secondary structure of DNA characterized by 12 bases per turn, a left-handed helix, and a sugar-phosphate backbone that zigzags back and forth.

zinc finger

A protein motif consisting of an alpha helix and an anti-parallel beta sheet. Specific amino acids in the helix and sheet interact with a zinc atom, resulting in a finger-like structure.

zygote

The resulting diploid cell from the fusion of two haploid gametes, the result of sexual reproduction.

zygote intrafallopian transfer

The method of transferring an ovum fertilized in vitro to a woman's uterine tube, to assist in human reproduction.

zygotene

Second substage of prophase I in meiosis. In zygotene, chromosomes enter into synapsis.

ZZ-ZW system

In some birds, butterflies, moths, and other organisms, the female is the heterogametic sex. These females are designated ZW, while the males are ZZ.

TOP

Others

-10 consensus sequence

Consensus sequence (TATAAT) found in most bacterial promoters approximately 10 bp upstream of the transcription start site.

-35 consensus sequence

Consensus sequence (TTGACA) found in many bacterial promoters approximately 35 bp upstream of the transcription start site.

10T1/2 cell line

A contact-inhibited fibroblastic cell line derived from a mouse embryo that is commonly used for toxicity screening, transfection, transformation, tumorigenicity, and cloning studies.

3' end

End of the polynucleotide chain in which an OH group is attached to the 3'-carbon atom of the nucleotide.

3' splice site

The 3' end of an intron where cleavage takes place in RNA splicing.

3' untranslated (UTR) region

Sequence of nucleotides at the 3' end of mRNA; does not code for the amino acids of a protein but affects both the stability of the mRNA and its translation.

30S initiation complex

Initial complex formed in the initiation of translation in bacterial cells; consists of the small subunit of the ribosome, mRNA, initiator tRNA charged with fMet, GTP, and initiation factors 1, 2, and 3.

5' cap

Modified 5' end of eukaryotic mRNA, consisting of an extra nucleotide (methylated) and methylation of the 2' position of the sugar in one or more subsequent nucleotides; plays a role in the binding of the ribosome to mRNA and affects mRNA stability and the removal of introns.

5' end

End of the polynucleotide chain in which a phosphate is attached to the 5'-carbon atom of the nucleotide.

5' splice site

The 5' end of an intron where cleavage takes place in RNA splicing.

5' untranslated (UTR) region

Sequence of nucleotides at the 5' end of mRNA; does not code for the amino acids of a protein.

5'-methylcytosine

Modified nucleotide, consisting of cytosine to which a methyl group has been added; predominate form of methylation in eukaryotic DNA.

5-azacytidine

A chemical that can effectively remove the methyl groups from nucleic acids.

70S initiation complex

Final complex formed in the initiation of translation in bacterial cells; consists of the small and large subunits of the ribosome, mRNA, and initiator tRNA charged with fMet.

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