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Why are dominant alleles usually better?

by Simfish
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Simfish
#1
May25-07, 12:36 AM
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why is it that dominant alleles are often the “better” ones? Heterosis (matings from different genetic pools) is often considered to be an advantage to the child since it emphasizes the stronger dominant alleles. Whereas inbreeding is dangerous because it increases the risk of two recessive alleles brunching together.

The question is - why is it that certain phenotypic characteristics have been assigned to dominant alleles? Why was it that darker skin/eyes belong to dominant alleles in the first place? What exactly are the origins of such alleles in humans? Of course, recessive alleles are often rare because individuals with both copies of a recessive allele often die before being able to propagate (whereas there may have been many different types of dominant alleles - and the ones that were dominant + beneficial beat out the ones that were beneficial + non-dominant). But this does not explain the origins of the connections between such alleles and phenotypic characteristics. Can someone help me out?

And what of recessive non-dominant traits that are beneficial? I'm talking about traits that are beneficial even when homozygous - so sickle-cell anemia and malaria resistance is not one of them

Thanks!
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Cincinnatus
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May25-07, 02:54 AM
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For most genes, only one copy is necessary to function. So if a single copy of the gene has a "loss of function" mutation we would still see the dominant phenotype in most cases. Only when both copies of the gene have such a mutation do you see the recessive phenotype. So *usually* recessive alleles represent loss of function mutations while the corresponding dominant alleles are wild type.

That said, not all genes/mutations work this way. Some genes need to be present in two functioning copies to show the wild type phenotype. So in this case a loss of function mutation would be dominant.
Moridin
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May25-07, 09:30 AM
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Also note that deleterious dominant alleles are often selected against for obvious reasons. The individuals who have them will have a lesser change of passive them on compared to others.

jim mcnamara
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May25-07, 10:42 AM
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Why are dominant alleles usually better?

Alleles do not have intrinsic value because they are dominant - what Moridin said.

In fact, alleles simply work for/against/neutrally for the survival of a given organism in given environment. In a different environment, an "against" allele may become helpful or harmful for survival. Example: thalassemia and sickle cell anemia.

Note that "neutral" is a really bad term, because gene pairs have functions, even if we humans cannot fathom what they do. So we just think it doesn't do much. No harm, no foul.
Simfish
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May25-07, 02:17 PM
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Also, why is it that "darker skin/hair" is a dominant genotype? Why wasn't it ever the other way around? What factors are behind the formation of which genotypes become dominant, and which genotypes become recessive?

Could there also be multiple recessive genotypes on a particular locus? (given how many mutations are possible, I'm sure that it could happen). Which then begs another question - course a recessive genotype be dominant over an even more recessive genotype?
jim mcnamara
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May25-07, 03:00 PM
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I think you are assigning meaning to something that is the result of random processes. You are free to do that, it just makes it hard to think objectively about this stuff. There is nothing bad/good about recessive in itself. Don't take offense because you think recessive has negative connotations.

Darker skin is good when you live in Africa on the savanna. Humans evolved there. Did you ever hear of the "African Eve" concept - we are all descended from a woman in Africa who lived there circa 230,000 years ago.

Skin color is additive - there are many loci (16 was the last I read). The more melatonin producing (dominant) alleles present the darker the skin.

Dominant vs recessive: A recessive gene usually produces a gene that prevents synthesis of a protein that used in a metabolic pathway - one from point Q to point A. Either it changes the path to point B, or it stops it dead in it's tracks. It may also be that there is an alternate route for the synthesis so that the allele has no net effect at all, except in people who have another gene that also blocks the alternate pathway.

There is no one-to-one relationship between what an allele does/does not do and what can happen in a living organism. There is a phenomenon called partial penetrance (see above about going from point Q to point A)- where a dominant gene is present but does not express itself fully example: A person should two brown-eyed alleles but the person has green eyes, for example.

edit: There was a recent US DNA study of Moms, Dads, and babies, which found that about 1 out of 16 children's dads were not the genetic daddy. That could explain some interesting things, too.

When I get going I'll dig up the reference.

I once read that there are probably 8-12 alleles for every human locus.
I dunno if that's true, but it sounds good.

Histocompatibility loci (32 of 'em) have a big number of alleles.
Simfish
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May25-07, 08:47 PM
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Hm, interesting. I know - assigning meaning to objective processes is not good for objective thinking - it's merely a convenient means of categorization (for a naive person).

Hmm, that all is interesting. As for the loci for skin color - are many of them present on different chromosomes?

Ah yes, the thing you wrote about recessive genes is very enlightening!
Simfish
#8
May26-07, 12:41 AM
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Anyways, the reason why I used the normative distinction between "recessive = bad" and "dominant = good" was because a lot of writers mention that inbreeding = bad due to the magnification of recessive genes in inbreeds, and that breeding between two individuals of diverse populations = good due to the magnification of dominant genes in the offspring thereof.

But most people possess several recessive genes that can prove lethal if they are paired again with some other recessive genes. Most recessive genes are harmless, but the few genes that are lethal are almost always recessive (and can be surprisingly widespread). So my distinction resulted from naive thinking.
Simfish
#9
Jun5-07, 03:34 PM
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http://www.gnxp.com/MT2/archives/003868.html


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