Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!texbell!uhnix1!uhnix2!davison
From: davison@uhnix2.uh.edu (Daniel B. Davison)
Newsgroups: sci.bio
Subject: Re: A different human genetics question
Message-ID: <1036@uhnix2.uh.edu>
Date: 11 Apr 90 04:29:51 GMT
References: <4070@nmtsun.nmt.edu>
Reply-To: davison@uhnix2.UUCP (PUT YOUR BRAIN HERE)
Organization: University of Houston
Lines: 29


The poster asked, essentially, why siblings are not "even" mixtures of
their parent's traits.

The simple-to-say but complex-to-explain answer is that genes which
determine complicated phenotypes have what is called "variable
penetrance".  The degree to which a phenotype is expressed is the
result of many, many genes interacting, even eye color or voice pitch
(although these two have almost complete penetrance).

So, simply, the genetic background of your cells is not the simple
union of parentral traits (blue eyes or brown eyes) but the
intersection of them (brown eyes + variable activity in
pigment-producing cells + variable activity in pigment transport and
deposition in the iris). [I hope I haven't reversed the intersection
and union concepts, but you get the drift, I hope].

Real human genetics folks who occasionally read this news group like
Ted or Henry may be able to add more definitive information to this,
but "penetrance" is the key concept.


dan
--
dr. dan davison/dept. of biochemical and biophysical sciences/univ. of
Houston/4800 Calhoun/Houston,TX 77054-5500/davison@uh.edu/DAVISON@UHOU

Disclaimer: As always, I speak only for myself, and, usually, only to
myself.