identical twins

Gerard Tromp tromp at
Wed Jul 3 12:35:13 EST 1996

Brent Gilbert 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.
> --
> Brent Gilbert (bg005d at

To paraphrase the question "Are the offspring of two sets of identical twins related 
to one another as if they were from the same sibship?"
For the very reason that you cite the answer is yes, they will one average share 50% of
their alleles in the same way that normal sibs share, on average, 50% of their alleles. 

Consider the diagram below:
		|		|
               / \             / \
              /   \           /   \
             M-----M         F-----F
            B|b   B|b       B|b   B|b
             |_____|_________|     |
               |   |_______________|
               |             |
           1BB 2Bb 1bb     1BB 2Bb 1bb

This will hold for all loci; therefore, on average, the two sibships from the matings of
pairs of identical twins will be indistinguishable. The degree to which recombination
disrupts haplotypes should be the same as for an extended sibship from the same mating.
Notice, however, that sibs normally _may_ share 0% of their alleles and _may_ share as
much as 100% of their alleles (i.e. equivalent of identical twins).

Gerard Tromp, Ph.D.
CMMG, Wayne State University    vox:	313-577-8773
3116, Scott Hall		fax: 	313-577-5218
540 E Canfield Ave		e-mail: tromp at
Detroit, MI 48201                       gtromp at

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