Transformation is a process brought about by genes, back then, it was basically unknown if the genetic material was DNA, RNA or protein. DNAse can destroy (denature) DNA , RNAse does the same to RNA and Proteases break down proteins. In the experiment, the idea was to see which one of these was responsible for transformation (and hence would turn out to be the genetic material) and therefore the samples were treated with DNAse, RNAse and protease. (If proteins had been the genetic material, then protease treatment would've induced loss of transforming ability, ditto with the other two)
The assay for the retention of transforming ability revealed that only DNAse treatment resulted in the loss of transforming ability, implying it had to be essential for the transformation to actually occur, thus leading to the conclusion that DNA constitutes the genetic material in the model organism (S.pneumoniae) This result was later extended to other living beings, and as we now know, DNA is the primary genetic material in most organisms.
Regards,
Ankur Chakravarthy

Hello Shirley,
Good question. Biology has so much vocabulary that it’s important to really make sure we use our terms correctly. Tay-Sachs is an example of how an autosomal recessive condition can actually have incomplete dominance at the level of the protein expression phenotype, but complete dominance at the level of the disease phenotype. Codominance is a type of incomplete dominance. But we will get to that later.

Before we answer your question, let's first think a little bit about the concept of dominance. Dominance describes the effects of different gene alleles on an organism’s phenotype. An allele has complete dominance over the phenotype if one copy is sufficient to produce the same effect as two. An allele with incomplete dominance, however, will combine with the other allele to produce some intermediate phenotype. Codominance is a special type of incomplete dominance, when the heterozygous individual simultaneously expresses both allele phenotypes equally.

In general, complete dominance refers to when the heterozygous phenotype is indistinguishable from the homozygous phenotype. When the heterozygotes express an intermediate phenotype that is distinct from the homozygous phenotype, incomplete dominance occurs. In incomplete dominance, the ratios of genotype (1:2:1) and phenotype (1:2:1) are the same, but with complete dominance they are different (genotype 1:2:1 versus phenotype 3:1) (1).

So how do we understand the type of allelic dominance in Tay-Sachs? In this disease, mutations in the gene coding for an enzyme called hexosaminidase A typically cause neurological dysfunction (2). This is because this enzyme normally breaks down lipid by-products called gangliosides in the cell's lysosomes. But when hexosaminidase A does not function properly, lipids build up in the brain and interfere with biological processes.

An individual homozygous for the mutant allele does not produce hexosaminidase A and will suffer from the disease. However, a heterozygous individual has one functional copy and one mutant copy. If you just look at levels of hexosaminidase A activity, these heterozygous "carriers" will have half of the normal level. This means they have an intermediate phenotype arising from each allele's contribution (or lack thereof). Since both the normal and mutated alleles produce the same amount of protein, they are actually codominant, a type of incomplete dominance.



Check out this article on Scitable if you want more on inheritance and dominance patterns:
http://www.nature.com/scitable/topicpage/Genetic-Dominance-Genotype-Phenotype-Relationships-489


(1) Campbell and Reece.2002 Biology 6th ed. Pearson Higher Education.
(2) Pierce, B. 2008. Genetics: A Conceptual Approach. 3rd ed. New York: W.H. Freeman & Co. 138.

Hi Shirley,
Thanks for your question. The terms “dominant” and “recessive” can sometimes be confusing. For any gene, multiple versions of its genetic sequence can exist, and these are called “alleles.” Organisms can have multiple alleles for any one gene. The terms “dominant” and “recessive” describe how the alleles interact to determine the overall phenotype. Only one copy of a “dominant” allele is necessary to control the phenotype, while two copies of a “recessive” allele are necessary for the recessive phenotype to be evident. Also, if a mouse has two copies of the same allele, the genotype is known as “homozygous.” If the alleles differ, the mouse’s genotype for that trait is “heterozygous.” Among simple traits with heterozygous genotypes, the dominant allele will take over the phenotype.

If I understand the problem correctly, it seems like you have an allele, Y, that is both dominant and homozygous lethal. You might be thinking, “How can the same allele be dominant and recessive at once?” The answer lies in which phenotype you choose to examine: lethality or fur color.

First, let’s just consider fur color as our phenotype. Since you said that Y is dominant for fur color, if you have even one copy of the Y allele, then the mouse will always have yellow fur.

However, if you look at lethality, allele Y is recessive. This means that you’re right when you say that YY mice will die. Heterozygous mice, ones that have a genotype of Y-, will survive and have yellow fur. The mice with YY that don’t survive would also have had yellow fur.

You might be interested to know that scientists have figured out why the Y allele that you described is fatal. When two copies of Y are present (YY), they disrupt the activity of another closely linked gene that encodes an RNA-binding protein. If a developing embryo lacks this RNA binding protein during its early growth stages, it dies before fully maturing (1).

For more info on lethal phenotypes. check out this article on the inheritance of lethal genes:
http://www.nature.com/scitable/topicpage/Mendelian-Ratios-and-Lethal-Genes-557

For more info on how Gregor Mendel actually discovered dominant and recessive inheritance patterns, check out:
http://www.nature.com/scitable/topicpage/Gregor-Mendel-and-the-Principles-of-Inheritance-593

For more about how interactions between two alleles can become more complex, check out:
http://www.nature.com/scitable/topicpage/Genetic-Dominance-Genotype-Phenotype-Relationships-489

(1) Michaud E.J., Bultman S.J., Stubbs L.J., Woychik R. P. 1993. The embryonic lethality of homozygous lethal yellow mice (Ay/Ay) is associated with the disruption of a novel RNA-binding protein. Genes & Dev. 7: 1203-1213.

Hi Prosanta,

Your question is definitely intriguing, since the basic structural characteristics of chromosomes are rather difficult to detect under a light microscope. When the chromosomes all closely resemble each other in size and shape, the matched pairs are harder to recognize. Fortunately, techniques have been developed that help us visualize the particular traits of chromosome pairs. One of the most useful of these methodologies is banding.

The first banding procedure was developed in the 1970s by Torbjorn Caspersson. In this procedure, the chromosomes are dyed with quinacrine, a chemical that sticks to DNA. Caspersson and his colleagues were the first to demonstrate that dyeing the chromosomes can produce distinct and reproducible banding patterns that were unique to each homologous pair.

Today, most karyotypes use G-banding. This technique uses Gisema dye instead of quinacrine, produces a better resolution of the individual bands, and can also be analyzed with classic bright-field microscopy. Highly condensed heterochromatin will absorb more of the Gisema dye, resulting in a darker band and looser G-C-rich chromatin will take up less dye, and reveal a lighter band. Because the pattern of heterochromatin and chromatin is unique to each chromosome, banding patterns are distinct and reproducible for each homologous pair. There is a wide range of other banding procedures, but Q-banding and G-banding are the most common techniques used.

Based on the banding principle, investigators have incorporated a technique called FISH, to probe chromosomes for individual genes. Several multicolored probes can be hybridized at the same time to “paint” the chromosomes. These methods can also be very useful in identifying structural mutations, like translocations. For further information on these methods, check out this reference:
Speicher, M. R., Ballard, S. G., & Ward, D. C. 1996. Karyotyping human chromosomes by combinatorial multi-fluor FISH. Nature Genetics 12, 368–375.

On Scitable, we have an article that clearly outlines some of these procedures, called “Karyotyping for Chromosomal Abnormalities”:
http://www.nature.com/scitable/topicpage/Karyotyping-for-Chromosomal-Abnormalities-298

Another Scitable article describes the FISH technique:
http://www.nature.com/scitable/topicpage/Fluorescence-In-Situ-Hybridization-FISH-327

And here is an image of quinacrine-dyed chromsomes
http://www.nature.com/scitable/content/Partial-karyotype-of-six-mitotic-cells-photographed-7790

Thank you for your question!