Dominant/Recessive Genes

Gerard Tromp tromp at
Mon Jan 25 18:00:42 EST 1999

immu at wrote:
> Well my the problem i confront is pertaining to Genetics..
> I understand that we have Dominant and Recessive Alleles.. Taking hair colour
> for example where dark hair may express itself over light hair. Fine, the
> above may prove true i.e. that the dominant allele is that of the dark hair
> and contra for light hair. What I want to know is what exactly "is it" in an
> allele that makes it dominant or recessive over the other. In a heterozygous
> condition (Dd)why does one dominate and the other not express itself
> phenotypically. Is it something got to do with the nucleotide arrangment?

	It has to do with what the gene (not allele at this point) accomplishes
in the organism. Some simple examples, if the gene codes for an enzyme
and the enzyme is extremely active (low Km, high Kcat, etc), and
therefore only a very little of it is sufficient to perform the required
enzymatic step for the metabolism of the organism (conversion of
substance A to B). As a consequence, the organism requires the
production of the enzyme from one allele only to provide sufficient
enzyme. An allele in this gene that caused non-functional enzyme to be
produced from it, would have no effect unless it was present in
homozygous form (both alleles produced non-functional enzyme). The
hypothetical gene would represent a system that is likely to harbor
mutatant alleles with recessive inheritance. 

	A locus with dominant alleles will typically involve gene products
where half the amount is insufficient to perform the function required
of that gene. 

	These are the simplest models for dominant and recessive alleles,
something like an all-or-none effect. Some systems are additive
(co-dominant). Some genes (loci) can have both dominant and recessive
alleles because their products interact physically with other products
and the exact nature of the interaction determines whether or not the
function required of the gene product can be performed. 

Gerard Tromp, Ph.D.
CMMG, Wayne State University    vox:	313-577-8773
3116, Scott Hall		fax: 	313-577-5218
540 E Canfield Ave		e-mail: tromp at
Detroit, MI 48201                       gtromp at

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