Re: What is the ratio between Dominant and Recessive genes?

Great scott!  I must have screwed up in March when I tried to defer this 
question to another mad scientist because my schedule was too tight.  My 
sincerest apologies, Neil.

Now to the question.  I'm going to divide my answer into three sections: a 
quick review of dominance in Mendelian traits, a review of what "dominant" 
means in context, and finally, whether there is a meaningful answer to your 
question.

Dominance is classically defined for a set of alleles at a gene by looking 
at the phenotype of heterozygous individuals and comparing with the 
phenotype of homozygous individuals.  An example you should be aware of is 
the ABO blood group system.  At the ABO locus there are three alleles: A, B 
and O.  If you look at the phenotype of an AO heterozygote, it is 
indistiguishable from an AA homozygote, so A is said to be dominant over O 
and O is said to be recesive to A.  The same relationship holds for B and O: 
B is dominant over O and O is recessive to B.  However, an AB heterozygote 
is phenotypically different from both the AA and the BB homozygotes.  In 
this case, the two alleles are said to be codominant.

In most basic bio classes we are taught the classic Mendelian Dominant/
recessive model, and sent on our way with a few examples such as dark hair 
and eye color is dominant over light, tongue rolling is dominant over non-
rolling, attached earlobes are dominant over separated, Phenyl thio 
carbamide (PTC) tasting is dominant over non-tasting, etc.  Of these four 
examples, only PTC tasting holds up to scrutiny.  There are some rare 
families in which red hair is a dominant trait, but in most families it is 
clear that the hair color of the progeny is somewhere in between the parents 
suggesting a codominant inheritance.  Tongue rolling is an unfortunate piece 
of misinformation which has been propagated for decades.  The original 
author of the tongue rolling paper, Sturtevant, went on to have a brilliant 
career in Drosophila geneitcs, but nobody ever listened to him when he tried 
to retract it.  As for the attached earlobe thing, I've never been able to 
clearly differentiate between classes: there is clearly a large amount of 
variation in how closely attached the lobes are, so it's a lousy example.

As it turns out, there are extremely few traits in normal human beings which 
can be split into trait present/ trait absent categories and therefore 
discussed in terms of dominance.  For example, most of the parameters we use 
to describe a person are quantitative: there is no clear separation of 
categories for height, weight, hair color, eye color, nose length, armspan, 
etc.  

Just to add to the fun, a single allele can contribute to multiple 
phenotypes, with different modes of inheritance for each phenotype.  For 
example, the sickle cell mutation in beta globin contributes to at least two 
phenotypes: malaria resistance and sickle cell anemia.  Both sickle cell 
carriers and homozygotes are resistant to malaria, so the sickle allele is 
dominant with respect to this phenotype.  On the other hand, sickle cell 
carriers do not develop sickle cell anemia, so the sickle allele is 
recessive with respect to anemia.

Thus, the short answer to your question is that there are very few traits in 
humans where dominance applies at all, and even then a single allele can be 
simultaneously recessive and dominant with respect to multiple phenotypes.  
So I'm sorry, but there really is no meaningful answer to your question "In 
an average human, approximately what kind of ratio of dominant to 
recessive genes(traits) exists, and is this more or less a constant ratio?"  
It all depends on which traits you look at, and even then most traits don't 
follow the simple mendelian dominance paradigm.

	Chris Carlson
	peterpan47@earthlink.com