Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site cylixd.UUCP Path: utzoo!watmath!clyde!bonnie!akgua!akgub!cylixd!charli 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 (Of course, I freely admit this information is based on recollection, and may be incorrect or out of date.)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