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| keithhudson35 at (no spam) googlemail.com... |
 Posted: Fri Sep 25, 2009 7:03 am |
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What makes one gene dominant over its alternative in the other human
chromosome? I know that in the female case one of the X chromosomes is
completely blanked out, but what happens in the case of the autosomal
chromosomes |
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| Darwin123... |
| Posted: Mon Sep 28, 2009 5:21 am |
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On Sep 25, 1:03=A0pm, "keithhudso... at (no spam) googlemail.com"
<keithhudso... at (no spam) googlemail.com> wrote:
[quote:8bd85076b8]What makes one gene dominant over its alternative in the other human
chromosome? I know that in the female case one of the X chromosomes is
completely blanked out, but what happens in the case of the autosomal
chromosomes
[/quote:8bd85076b8]
I once had the references to this, but I discarded them. However,
as I recall:
It's the role of the enzyme that the gene allele codes for that
makes determines the penetrance of the allele.
For example, in human beings, the brown color of eyes and the
black color of brunette hair is determined by the amount of melanin
being generated in that tissue. There is an allele crucial for making
the melanin in the eye at a certain locus. There is an allele crucial
for making melanin in the hair at another locus.
Lets look at the hair. If a person has even one allele for
making melanin in the hair, maelanin will be made and the hair will be
black. If two alleles code for melanin in the hair, the hair won;t get
any blacker. If no allele at the correct locus codes for melanin, the
hair has little melanin and the hair is blonde.
Lets look at the eyes. If a person has even one allele for
making melanin in the eye, melanin will be made and the eye will be
brown. If two alleles code for melanin in the eye, the hair won't get
any browner. If no allele at the correct locus codes for melanin, the
eye has little melanin and the eye is blue.
This simple model doesn't quite cover the great deal of
variance in the expression of genes, but it's a great heuristic model
as a start.
At some point, I was interested in the problem of partial
penetrance. According to this model, an allele tends to favor
strictly dominant versus strictly recessive alleles. However, some
traites tend to add up in a continuous manner. This is called partial
penetrance. Some hybrids seem to show a 50-50% penetrance.
I found a paper somewhere that described partial penetrance in
terms of cell biochemistry. The paper had a lot of rate equations
showing how the enzyme in question generates products. The conclusion
was that chromosome loci that show a strictly complete dominance
(dominant versus recessive) relate to enzymes that are either at the
very beginning or the very end of a reaction chain. If the enzymes are
at the very end of the reaction chain, then it is unlikely that the
enzyme will affect anything other chemical product than the protein
related to that traite. Then, the action of that allele will be very
simple. Either the enzyme is there or not.
Look up penetrance and biochemistry. |
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| Darwin123... |
| Posted: Tue Sep 29, 2009 6:15 am |
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On Sep 25, 1:03=A0pm, "keithhudso... at (no spam) googlemail.com"
<keithhudso... at (no spam) googlemail.com> wrote:
[quote:b2497bb568]What makes one gene dominant over its alternative in the other human
chromosome?
Some speculation is provided in the following link, which focusses[/quote:b2497bb568]
on the evolution of dominance in genes.
http://digital.library.adelaide.edu.au/coll/special/fisher/93.pdf
The author lists some speculations as to Mendelian allelomorphs.
"Setting aside the suppositions:
(i) that Mendelian allelomorphs are always pairs which can be formally
identified
as the presence or absence of something;
(ii) that mutations are always or usually merely losses or
inactivations of nuclear
material; and
(iii) that such loss or inactivation is in itself a sufficient cause
of recessiveness;
we are in a position to reconsider what the evidence available really
has to tell us as to the incidence, and causes, of the phenomenon of
dominance."
I am sure that there has been more research performed since
then. However, I don't have the references. |
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