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identical twins

George Gutman ggutman at MEDED.MED.UCI.EDU
Wed Jul 3 15:43:13 EST 1996

"Siblings" refers to offspring of the *same* parents, not just 
genetically identical parents.  The offspring of different mating pairs 
of inbred mice, for example, are not siblings, despite the fact that they 
are genetically identical to each other (as well as to their parents, and 
all other members of the same inbred strain).  So the human offspring in 
question are certainly not "siblings".



* George A. Gutman                                              *
* Department of Microbiology            Tel:(714)824-6593       *
*          and Molecular Genetics       FAX:(714)824-8598       *
* University of California         INTERNET: GAGUTMAN at UCI.EDU   *
* Irvine CA 92717                                               *

On 3 Jul 1996, Michelle Geary-Mallett wrote:

> Brent Gilbert (bg005d at uhura.cc.rochester.edu) wrote:
> : In <4rccbo$1hp at news2.ios.com> knanette at village.ios.com (Nanette Knaster) writes:
> : 
> : >If identical twins marry identical twins, are the offspring of each
> : >marriage (cousins) considered to be siblings genetically since both sets
> : >of offspring share the same gene pools.  Can someone explain this?  It was
> : >a discussion that came up today at the old watercooler, and I'd be much
> : >obliged for a scientific explanation.  Thank you.  Please respond via e-mail.
> : 
> : Not likely. Remember, almost every cell in the human body, save Erythrocytes
> : and sperm/egg cells, have two sets of chromosomes. So, let's say one set of
> : twins has the genotype Bb for eye color ( that is, brown eyes, but a carrier
> : of the blue eyes trait) and the other twin have the same, the offspring can
> : ber either BB, Bb, or bb, hence not identical in this respect.
> Oh, come now.  She did not ask whether these hypothetical cousins would 
> be genetically *identical*, she asked whether they could be considered, 
> genetically, as *siblings*.  And, barring mutations in the germ line of 
> any of the four parents in this example, I think one would have to argue 
> that the cousins are, genetically, akin to siblings.  For any given 
> locus, each mother has the same (maximum) two possible alleles to give to 
> her gametes -- say, c and c' (let's assume a polymorphic situation at the 
> locus). And, each father has the same two alleles at that locus as the 
> other father -- perhaps c'' and c''' -- to donate to his gametes.  Each 
> set of parents, at this and every other locus (again, assuming no new 
> mutations in the germ line of one of these otherwise identical parents), 
> has the same set of *possible* gametes which they can produce, and 
> therefore the same (very large) set of possible genetic combinations in 
> their offspring.  From a genetic standpoint, the cousins would be like 
> siblings.
> Michelle

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