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.
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
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
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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Update: June 2012