ALAGILLE SYNDROME A dominantly inherited disorder that is characterized primarily by a scarcity of bile ducts in the liver. Other features include heart, eye, kidney and skeletal abnormalities, as well as defects in the central nervous system.
BARDET–BIEDL SYNDROME (BBS). A rare and genetically heterogeneous disorder that is characterized primarily by obesity, retinal dystrophy, polydactyly, hypogenitalism, learning difficulties and renal malformations.
CONSANGUINEOUS Descended from a recent common ancestor.
ENTERIC GANGLIA Parasympathetic mass of nerve tissue (ganglia) in the colon.
FAMILIAL ADENOMATOUS POLYPOSIS (FAP). The development of numerous adomatous polyps in the colon that might progress to carcinomas.
GENETIC DRIFT Random fluctuations in the allele and, less commonly, the phenotype frequencies, as genes are transmitted from one generation to the next.
GLAUCOMA The abnormally elevated pressure in the liquid that fills the anterior part of the eye (the aqueous humour).
HAPLOINSUFFICIENCY A gene dosage effect that occurs when a diploid requires both functional copies of a gene for a wild-type phenotype. An organism that is heterozygous for a haploinsufficient locus does not have a wild-type phenotype.
HAPLOTYPE ANALYSIS The study of the pattern of descent of a combination of alleles at different sites on a single chromosome (known as a haplotype). It is used for the identification of recombination events between markers and traits during linkage studies, thereby establishing the boundaries of the location of a phenotype-associated locus.
HERITABILITY The proportion of the variation in a given characteristic or state that can be attributed to genetic factors.
LINKAGE DISEQUILIBRIUM The condition in which the frequency of a particular haplotype for two loci is significantly greater than that expected from the product of the observed allelic frequencies at each locus.
LOD SCORE (Base 10 'logarithm of the odds' or 'log-odds'). A method of hypothesis testing. The logarithm of the ratio between likelihoods under the null and alternative hypotheses.
NEPHROTIC SYNDROME Malfunction of the renal glomerular filtration barrier (a structure in the glomerulus that is responsible for protein filtration) that lead to the loss of plasma proteins.
NON-PARAMETRIC LINKAGE Non-parametric approaches are statistical procedures that are not based on models, or assumptions pertaining to the distribution of the quantitative trait.
PARAMETRIC LINKAGE Parametric analyses are statistical tests for linkage that use assumptions such as mode of transmission, allele frequencies and penetrance.
PENETRANCE The proportion of affected individuals among the carriers of a particular genotype. If all individuals who have a disease genotype show the disease phenotype, then the disease is said to be "completely penetrant".
PHENYLKETONURIA An inborn error of metabolism that is caused by lack of the enzyme PAH that converts phenylalanine to tyrosine. If left untreated, it causes abnormally high phenylalanine levels and severe, progressive mental retardation, but can be prevented by neonatal screening and a low phenylalanine diet from an early age.
QUANTITATIVE TRAIT LOCUS (QTL). A genetic locus or chromosomal region that contributes to variability in complex quantitative traits (such as height or body weight), as identified by statistical analysis. Quantitative traits are typically affected by several genes and by the environment.
RETINITIS PIGMENTOSA (RP). A group of both clinically and genetically heterogeneous hereditary retinal degeneration disorders that are caused by the death of both rod and cone photoreceptors, leading to a complete loss of vision.
TETRAHYDROBIOPTERIN Phenylalanine hydroxylase (PAH) is an oxygenase that couples an electron from a tetrahydrobiopterin cofactor (BH4) and an oxygen atom to hydroxylate phenylalanine to form tyrosine. Consequently, any defects in BH4 biosynthesis impair PAH function.
