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From: emigh@ncsugn.ncsu.edu (Ted H. Emigh)
Newsgroups: sci.bio
Subject: Re: The persistance of homosexuality in a gene pool
Keywords: mutation
Message-ID: <5892@ncsugn.ncsu.edu>
Date: 1 Aug 90 20:17:35 GMT
References: <5914@videovax.tv.tek.com> <1990Jul23.022511.28161@mtcchi.uucp> <8800@uhccux.uhcc.Hawaii.Edu> <19615@orstcs.CS.ORST.EDU>
Reply-To: emigh@ncsugn.UUCP (Ted H. Emigh)
Organization: North Carolina State University, Department of Genetics
Lines: 31

In article <19615@orstcs.CS.ORST.EDU> daver@ECE.ORST.EDU (Dave Rabinowitz) writes:

>....  Scientists have recently reported
>discovering a key gene on the Y chromosome which appears to control the sexual
>development of the fetus, and the articles indicated that there were apparently
>very few other genes, if any, involved in this control.  It is postulated that
>the expression of genes determining sexual characteristics is controlled by 
>proteins transcribed from these control genes, and the activity of these 
>proteins can be affected by other chemicals present in the fetus.  

What has been found is the INITIAL signal to start the embryo along the way
to differentiation between male and female internal/external characteristics.
If the signal is not given (or received) then the individual will develop
(nominally) as a female.  Otherwise, the individual will develop as a male.
Apparently, this signal is used only at this stage.  Other genes on other
chromosomes take over to complete the process.  There are individuals who
have the gene, but have some genetic defects at other places who are XY, but
develop as females (testicular feminization, e.g.).  There are individuals
who lack the gene (XX), but have been subjected to environmental conditions
which bypassed this initial signal who have developed as males (see, e.g.,
work by John Money in the 60s and 70s).  And, of course, there are individuals
who have developed phenotypes which are not classified easily as either male
or female.

These have to do with the physical differentiation of individuals as male
or female.  I know of nothing which relates these to sex preference behaviour.

As a side note, a proper twin study does not look at just MZ twins.  The basis
for deducing a genetic component for the trait in question relies on comparing
the concordance of MZ twins to the concordance of DZ twins.  Of course, DZ
twins do not, necessarily, share a comparable environment to MZ twins.