Re: What makes one allele dominant and another recessive?


Alan Meyer wrote:
I am curious about the different reasons why one allele of a gene
may be dominant and another recessive.

One explanation I have seen is:

A dominant allele may code for a product that has some
specific effect while a recessive allele does not produce
that product, or produces a variant of it that does not have
that effect.

If one or both alleles in a particular individual are
"dominant", the effect is produced. It's only if both are
recessive that the effect is not produced.

As examples:

One allele codes for the production of a bright pigment. The
other does not. If there are two dominant alleles, or one
dominant and one recessive, the pigment is produced and is
visible. Only if both are recessive will the pigment not
appear. I think human eye color may be like this with blue
eyes recessive.

One allele (dominant) codes for a protein that acts as a
repressor of some pathway. The other either does not, or
its product does not function as a repressor. If either or
both alleles are dominant, the pathway is repressed. Only if
both are recessive is the pathway allowed.

For any genes that behave in the above fashion, "dominance" does
not have anything to do with "relative strength", or with a
tendency of one allele to suppress the action of the other, or
with a tendency for one allele to be expressed and the other not
to be expressed.

Is that a good explanation?

Are there other causes too?

Thanks.

Alan

I've seen four types of dominance in the literature.

A functional gene product and nonfunctional gene product is one
explanation for dominance where the functional gene product maintains
the gene function in the cell and you only see it if two nonfunctional
alleles are present and you get no function or too little function to
matter.

This brings up the second cause of dominance. The haplo insufficiency
form of dominance happens when one functional copy of a gene is not
enough to produce enough gene product and an abnormal phenotype
results. You only need one bad allele to produce this effect.

There is also a form of dominance that is called parts poisoning. This
happens when the gene product is part of a complex and one bad copy can
mess up the function of the complex even though there are good gene
products around. Enough complexes get poisoned to show a phenotype.
Just think if the gene product had to produce a tetrameric protein to
be functional. What is the chance of getting a functional tetramer
when 1/2 of the parts are defective (1/16th). It could also poison
mutipolypeptide containing complexes that are composed of multiple gene
products.

There is another type of dominance where the mutation affects
developmental timing or what tissue the gene product is produced in.
You just need one mutant copy to produce the function in the new tissue
and produce the new phenotype.

Ron Okimoto


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