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Name: Deborah
Status: educator
Grade:
Location: WA
Country: USA
Date: N/A 


Question:
Would you please explain the molecular basis of genetic dominance? The answer on your site referred to recessive genes as being defective mutations. This does not make sense to me since the recessive gene can be fully and successfully expressed in a homozygous recessive offspring. So what causes one allele to be expressed and another not? Also, what occurs on a molecular level in incomplete dominance? I understand that co-dominance represents both alleles being expressed but in incomplete dominance they are blended.



Replies:
The recessive allele is not expressed in the homozygous condition. For example the gene for albinism is not expressed in the homozygous condition. There is no melanin produced. In the case of incomplete dominance, one of the genes is expressed only half as much because only one of the alleles is producing mRNA for the gene in question.

Ron Baker, Ph.D.


It should be noted that often recessive alleles are often nothing - literally. Type "O" blood is recessive because there is no protein produced with that allele while type "A" and :"B" do produce proteins. With either "A" or "B" alleles paired with "O" of course "O" allele does nothing so it is recessive because it does nothing to produce a protein. One could look at type "O" as the default because it is clear it did not arise from a mutation. Types "A" and "B" were mutations. Many recessive genes are default and dominant alleles are commonly mutations.

When a person is showing type "AB" blood this is an example of co-dominance as you mentioned with both alleles showing. Keep in mind we are are talking about the phenotype.

Here is where it gets interesting, in most pink flowers produced by a red allele and a white allele, the pink phenotype is considered to be incomplete dominance, but again we are talking phenotype. BUT, under the microscope one can see clearly (in most cases) that the red and white pigments stand separately which would change it by definition to co-dominance.

Man makes the definitions, not nature.

What I have said is basic and hopefully will help in your understanding. There is a lot more to this, but I do not think that is what you requested.

Steve Sample M.S., C.A.S.



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