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From: charli@cylixd.UUCP (Charli Phillips)
Newsgroups: net.bio
Subject: Re: Identical twins and handedness
Message-ID: <206@cylixd.UUCP>
Date: Wed, 21-Aug-85 14:52:31 EDT
Article-I.D.: cylixd.206
Posted: Wed Aug 21 14:52:31 1985
Date-Received: Sat, 24-Aug-85 15:45:24 EDT
References: <233@drutx.UUCP>
Reply-To: charli@cylixd.UUCP (Charli Phillips)
Organization: RCA Cylix Communications , Memphis, TN
Lines: 34
Summary: 

In article <233@drutx.UUCP> slb@drutx.UUCP (Sue Brezden) writes:
>andrew@grkermi.UUCP (Andrew W. Rogers) writes:
>>
>>My wife is left-handed, but
>>her identical twin is right-handed... figure that one out!
>>
>
>Actually, I have heard that this is not uncommon among identical
>twins.  One twin will be a mirror image of the other.  I remember
>a book that showed pictures of twin's heads, with the hair growing
>in whorls in opposite directions.
>
>Does anyone know why this happens?  Are fingerprints reversed?
>Come to think of it, are fingerprints the same on identical twins?
>Why does this happen only sometimes?
>

My understanding (based on the human medical genetics class I took in
college a few years back) is that handedness is determined, not by 
chromosomal genes, but by genetic material in the cytoplasm of the ovum.
(This was called "the maternal effect" or something similar by our
professor.)  Since there is no mechanism to make sure everything in the
cytoplasm of one cell is split evenly between the two daughter cells (or
rather, since no such mechanism was known at the time I took the course),
it is possible for the fertilized egg to split "unevenly".  In the case
of twins, when the fertilized egg separated into two cells, and the two
cells separated into two separate zygotes, the genetic material in the
cytoplasm may have been different, resulting in differences in 
handedness, hair whorls, and so on.  

(Of course, I freely admit this information is based on recollection,
and may be incorrect or out of date.)

		charli