Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!agate!codon3.berkeley.edu!mkkuhner
From: mkkuhner@codon3.berkeley.edu (Mary K. Kuhner;335 Mulford)
Newsgroups: sci.bio
Subject: Re: twins
Message-ID: <19846@agate.BERKELEY.EDU>
Date: 2 Feb 89 18:31:22 GMT
References: <1046@jimi.cs.unlv.edu>
Sender: usenet@agate.BERKELEY.EDU
Reply-To: mkkuhner@codon3.berkeley.edu.UUCP (Mary K. Kuhner)
Organization: University of California, Berkeley
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In article <1046@jimi.cs.unlv.edu> robert@jimi.cs.unlv.edu (Robert Cray) writes:
>I was reading a genetics book a while ago and came across "...almost all
>MZ twins are same sex...".  Thinking about it, I can't figure out how you
>could have MZ twins of different sex, anyone know?  Was the author just
>hedging?

>					--robert

The apparent sex of a human being depends on a lot of things besides the
sex chromosomes present, so this statement does make some sense.  I know
of at least one case in which an infant boy, one of a pair of MZ twins,
had his genitals so severely injured that the decision was made to surgically
"correct" him to female, and he was raised as such.

Hormone treatment of the mother and some other environmental stimuli could
also result in chromosomal sex not matching physiological sex, though having
this happen to just one of a pair of twins seems farfetched.

Mutation to the testis-determining-factor gene could also cause mismatched
twins, but I don't know of an example.  This would be excessively rare.

I think the author was just being grotesquely overcautious.....

Mary Kuhner