from gloria stephens

robison at robison at
Tue Jul 18 09:33:46 EST 1995

Max Cates (cates at wrote:

:  With VERY few exceptions, this is the way one-allele dominance/recessive 
: cases work.  Incomplete dominance (like in Mendel's pea flowers: 
: red (RR) + white (WW) =pink(RW)) arises when one copy of the gene 
: producing functional protein isn't quite enough to make all the necessary 
: protein, or when the inactive protein can competitively inhibit the 
: active one. 
...or if both alleles make valid enzymes.  The A and B blood antigens
fall into this category (and O blood-type is a defective allele of
the ABO locus which cannot make a functional protein).  AB individuals
make both the A antigen and the B antigen.

BTW, someone once wrote a very nice summary of the various ways
a dominant/recessive system can be explained at the molecular level.
(there are a number of special cases)
It should be in the bionet archives:

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

robison at 

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