dominant/recessive genes

Fri Nov 17 10:36:06 EST 1995

> To:            molecular-evolution at net.bio.net
> From:          JEThomas at ix.netcom.com (Jonah Thomas)
> Subject:       Re: dominant/recessive genes
> Date:          17 Nov 1995 03:36:36 GMT

> writes: 
> >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.

I am not sure this argument or hypothesis could ever hold water.  
Organisms don't change from dominant to recessive or back, traits do. 
Clearly a phenotype can be modified by the environment and the 
penetrance can change with changing environmental pressure just as 
the effects on `fitness' can change.  Normally, a gene that is 
unfavorably selected (what ever that means) will NOT be common unless 
the population has just come out of a situation where the gene was 

> 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. 

Modifying genes that affect dominance do NOT have to be linked to the 
gene.  Why should they??  And what point are YOU trying to make?  
What if the modifying gene was pre-existent in the population.  Multi 
enzyme complexes and receptor signaling process already exist and all 
upstream and downstream processes would have variants in the 

> 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.  

HUH??  (see above)
> 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-)

||Doug Rhoads              || Dept. of Biological Sciences||
||drhoads at mercury.uark.edu || 601 Science Engineering     ||
||drhoads at uafsysb.uark.edu || University of Arkansas      ||
||501-575-3251             || Fayetteville, AR 72701      ||
||     My Dogma Just Got Run Over by Someone's Karma      ||

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