Re: Do identical twins REALLY have identical DNA?

Ms. Brock- We used to think that identical twins were identical in every 
genetic way, but that is not universally thought now.  There are, as I'm 
sure you know, many studies of discordance in identical twins.  There are 
even rare cases of monozygotic twins being different genders and of one 
twin having an extra chromosome, i.e. Down syndrome.  There are hundreds of 
studies to try and isolate the effects of nature versus nurture with twins 
raised apart, etc.  Their blood types are "always" the same, thus proving 
their monozygosity.  (it's dangerous to say always but I know of no cases 
where blood was different)  The genetic differences seem to be from a 
variety of causes, among them spontaneous mutations, non-disjunction during 
meiosis, viruses that may effect one twin in utero and not the other, more 
or less oxygen while in utero etc.  Combine these differences with 
envirnmental differences and "identical" twins may not be so identical!  
Scientific American, Dec. 1990 had an artical by John Horgan that may be of 
interest.  Also, MedLine (on the WEB) can lead you to lots of studies of 
discordance.  I hope this is helpful to you.