identical twins

Michelle Geary-Mallett ez045518 at
Wed Jul 3 21:42:28 EST 1996

George Gutman (ggutman at MEDED.MED.UCI.EDU) wrote:
: "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".

I think that perhaps you are missing the intent of the original poster's 
query.  She did not ask whether or not these cousins ought to be *called* 
siblings -- I suspect that most of us understand that siblings are 
properly understood to be the offspring of a given pair of parents -- 
but whether these cousins could be *considered* to be siblings 
*genetically*.  I took this (in my reply) to be asking whether the 
genetic makeup of one cousin would be related to the genetic makeup of 
another cousin (or any possible cousins from these matings) in the same 
way that the genetic makeup of one sibling is related to the genetic 
makeup of another sibling.

Of course, I may have misunderstood her question.

 : : : On 3 Jul 1996, Michelle Geary-Mallett wrote: 
: : > Brent Gilbert (bg005d at wrote:
: > : In <4rccbo$1hp at> knanette at (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 Geary
Plant Biology/ Evolution and Ecology
University of California, Davis

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