dominant/recessive genes

Jonah Thomas JEThomas at ix.netcom.com
Thu Nov 16 22:36:36 EST 1995


>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
>appreciate it.

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 
became favorable.

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-)

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