In <01HXPJTWWTTU001DN0 at ARSERRC.Gov> GKING at ARSERRC.GOV (GREGORY KING)
>In simple discussions of genetics one hears of dominant and recessive
>genes. Dominant genes are expressed, while recessive ones are not.
>What is it exactly that makes one of a complementary pair of genes
>dominant and the other recessive? Also, is the domination complete
>(i.e. dominant gene expressed 100%, recessive gene expressed 0%) or
>is it more of a 90%/10% or 80%/20% situation?
>I know that the laws of thermodynamics must be obeyed, so if you
>can explain this phenomenon using thermodynamic arguments I would
Here is the simplest explanation, which is clearly correct sometimes:
If one allele produces a product, and the other allele produces nothing,
then the one that makes something might be dominant. Two copies that
don't make anything -> no product, one copy that makes the product ->
product, 2 copies that make the product -> product.
Sometimes you can see the difference between one copy and two. If a
flower with two copies of the dominant gene is red while with one copy
it's pink, you might suspect that the red flowers have twice as much
pigment. Obviously the degree of expression of a trait will vary from
trait to trait -- sometimes 50% production gives full expression,
sometimes it gives partial expression, and sometimes there might be a
threshold above 50% and the trait is not expressed.
You can explain most dominance results with this model, if you're clever
enough. And it's the obviously simplest model, which for some people
means that we shouldn't consider any other possibility until this one
clearly fails. But consider....
An organism that could change genes from dominant to recessive and back
could possibly evolve faster. A gene that's currently favorably
selected could spread faster if it was dominant. A gene that's common
but currently unfavorably selected would do less damage if it was
recessive. Also it would survive in the population longer, giving a
greater chance that it would still be present if sometime later it
RA Fisher and Sewall Wright argued this out about 60 years ago, and
Wright won. Modifying genes that affect dominance would have to evolve
at each location, and the selection that would lead to their evolution
is weak. If they mutate at the same rates as the genes whose dominance
they modify, they have little effect on selection. It doesn't work.
Fisher had to agree; the time required to select a dominance-modifying
gene was longer than the lifespan of most species. He made a
half-hearted rear-guard defense that such things could sometimes evolve
over such long times, and he pointed out some sibling species where
opposite versions were dominant, and then he backed off.
But if dominance-modifying genes could be transposed to different
locations at a relatively high rate, they would need to evolve only
once. And if they switch from dominant to recessive and back at a
relatively high rate, say an order of magnitude or two below the
selective rate, they could be selected.
Such a thing would be analogous to phase-change in Salmonella. There an
IS inverts about once each thousand cell divisions. In one orientation
it causes one protein to be initiated and another to be inhibited, in
the other orientation it allows the 2nd protein to be initiated. The
two proteins make a difference in the flegellum, I saw a paper a long
time ago that claimed they work better in media of different viscosity.
The Salmonella system for producing flagella gives a population always
at least a few of each type. If one type is strongly selected agains
they lose less than 0.1% to selection against cells newly changed. If
the rare type is strongly selected for, there are at least a few cells
present that can cash in.
I'm sure lots of plausible molecular mechanisms could be proposed, but
still the last time I did a lit search there were no molecular
mechanisms known that did precisely this, and there were only a few
genes known that transposed to new locations and affected dominance at
the new sites. Of course, the evidence that such things don't exist is
only negative evidence, that they haven't been found and if they exist
they should have been found by now. 8-)