Re: What is a recessive gene?


Gary Bohn wrote:
> What is your favourite colour?
> What is the air-speed velocity of an unladen swallow?
>
> Sorry.
>
> I've been playing with the Hardy-Wienberg equation to see how selection
> can actually work in a population and suddenly realized I have no idea
> of what makes a recessive allele recessive. What attributes does the
> recessive allele have that makes it less likely to be expressed or what
> attributes does the dominant allele have that makes it more likely to be
> expressed?

Actually it is not the *gene* that is recessive. It is the *phenotype*
that that gene plays a role in generating. The way to detemine *if* an
allele (one form of a gene) has a dominant or recessive *effect* is to
create a heterozygote, a diploid individual with two *different*
alleles or forms of the same gene (call this the A/a state) and observe
one of the *phenotypes* that that gene is responsible for and compare
that to the *phenotypes* produced when both alleles are the same (the
A/A and a/a states). The allele, which produces the same phenotype in
both the heterozygote and the homozygote (say the A/a and A/A states),
is called "dominant". The other allele is recessive to the dominant A
allele.

The reason for dominance generally has to do with the fact that the
phenotypes being looked at are relatively distant from the genes
themselves and the fact that *functional* genes often have the capacity
to produce the same amount of gene product whether or not it is present
in a single or two copies.

Keep in mind that dominance and recessiveness is purely a phenotypic
description. The same alleles can be dominant for one phenotype,
recessive for another, and intermediate or co-dominant for a third. In
general, the closer the phenotype is to the actual genes and its
products, the greater the probability that the relationship will be
co-dominant. DNA sequence information is almost always co-dominant
(with large deletions being an exception).

Take sickle cell for example:

At the level of DNA sequence or protein, the relationship between S
(the sickle allele) and A (w.t.) is c-odominance. That is, the
heterozygote exhibits the presence of both A and S proteins and thus
differs from either the A/A or S/S state.

The phenotype of sickle cell disease is recessive. That is, the
heterozygote A/S does not exhibit the disease and S is, for this
phenotype, recessive to the w.t. allele A.

Under very low (non-physiologic) level of O2, however, the phenotype of
the heterozygote in terms of sickling of cells resembles that of the
S/S individual.

Thus, depending on the phenotype you are examining, sickle cell is
co-dominant, recessive, or dominant. Those terms are descriptive, not
inherent features of a sequence.

> This of course brings up another question. Which allele gets expressed
> and why when a particular locus is homozygous? Since there are many
> alleles for a given locus within a population, with some presumably
> dominant and others recessive but all slightly different, which allele
> gets expressed if both are dominant (or recessive) but different?
>
>
> --
> Gary Bohn
>
> Science rationally modifies a theory to fit evidence, creationism
> emotionally modifies evidence to fit a specific interpretation of the
> bible.

.

Relevant Pages

  • Re: What is a recessive gene?
    ... >> have no idea of what makes a recessive allele recessive. ... > *phenotype* that that gene plays a role in generating. ... > Keep in mind that dominance and recessiveness is purely a phenotypic ...
    (talk.origins)
  • Re: What is a recessive gene?
    ... >>>have no idea of what makes a recessive allele recessive. ... >>other allele is recessive to the dominant A allele. ... >>Keep in mind that dominance and recessiveness is purely a phenotypic ... The same alleles can be dominant for one phenotype, ...
    (talk.origins)
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  • Re: What is a recessive gene?
    ... >>>other allele is recessive to the dominant A allele. ... >>>Keep in mind that dominance and recessiveness is purely a phenotypic ... The same alleles can be dominant for one phenotype, ...
    (talk.origins)
  • Re: In the News: Professor challenges evolution
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