dominant/recessive genes

Douglas Rhoads DRHOADS at MERCURY.UARK.EDU
Thu Nov 16 20:34:34 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.
> 
> Thanks,
> 
> Greg King

This is rarely a question of thermodynamics but rather of kinetics.  
A mutation that makes an enzyme from an allele less active 
kinetically will be recessive to the `wild-type' enzyme.  However, if 
the enzyme activity has some critical threshold then if you only get 
one wild-type activity and one slacker enzyme then you may be at the 
border line of the requisite enzyme activity.  This means that 
environmental consideration which may modify enzyme activities and 
demands become critical.  That explains why two organisms of 
approximately equivalent genotypes and heterozygous for the same two 
alleles can have different phenotypes.  Also the background genotype 
can affect the critical activity need level.  Both of these would 
explain `penetrance' of a particular gene.  Rarely do we deal with 
whether a gene is `expressed' or not.  In actuality, unless the 
mutation is in the promoter/transcriptional control region, all 
alleles are `expressed' as in making an RNA/mRNA.  It is just that we 
may only see a finished-functional product/protein from some alleles. 
 One could envision mutations that destabilize the protein product 
made and therefore the product is rapidly degraded. 

//========================================================\\
||Doug Rhoads              || Dept. of Biological Sciences||
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