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identical twins

Michelle Geary-Mallett ez045518 at boris.ucdavis.edu
Wed Jul 3 11:33:14 EST 1996

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 


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