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dominant/recessive genes

Keith Robison robison at nucleus.harvard.edu
Thu Nov 16 17:55:16 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.

Someone once posted a very detailed analysis of recessive vs.
dominant; you might find it in the BIOSCI archives --
(one is at http://www.bio.net/ ).

In short, your initial analysis 
	"Dominant genes are expressed, while recessive ones are not."

is overly simplistic, and in particular you need to be careful about
what you mean by "expressed".  For example, one large class of 
dominant vs recessive alleles is those in which the dominant allele
has a biochemical function missing in the recessives, but which
is missing in the protein made from the other allele; however, proteins
are produced from both alleles.  It's really asking for trouble
talking about dominant or recessive _genes_; there are 
dominant and recessive _traits_.

For example, sickling of red blood cells is a recessive trait.  
The sickling allele of beta hemoglobin (Bs) contains 1 amino acid 
difference versus the wild-type allele (B+), and because beta hemoglobin 
dimerizes (well, actually forms a heterotetramer of 2 alphas, 2 betas), 
and red blood cells only sickle if a large proportion of their hemoglobin
is BsBs (i.e. the B+ can stabilize the Bs in a heterodimer, except
under very low oxygen tension).

In some cases, dominance vs. recessive reflects dosage levels.  For some
developmental genes, null alleles (no function) confer a dominant
phenotype, because the cell must have two functional copies 
in order to get the correct dose (such genes are invariably recessive
lethals, which underscores how an allele can be recessive for one
trait and dominant for another, and therefore why my terminological
nitpicking above is actually important).

Yet another variation on dominance vs recessive can be found in
tumor suppressor genes, in which again the null allele shows
a dominant phenotype (tumors).  This is because a cell with no
functional allele of the gene will replicate uncontrollably.  
Under growth conditions, you will eventually pick up a null-function
mutation in one copy of that locus.  In wt/wt homozygotes, losing
one copy is no disaster, and the odds of losing both copies in the
same cell are minimal.  But, in a wt/null heterzygote, the odds of
losing that wt copy somewhere are high, and the selection (tumor
generation) is powerful enough to display that rare event.

So in conclusion, there is no simple answer -- but instead a collection
of explanations.

Keith Robison
Harvard University
Department of Cellular and Developmental Biology
Department of Genetics / HHMI

robison at mito.harvard.edu 

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