January 1, 1970 (Vol. , No. )

Kevin Ahern

A rose is a rose is a rose, but an identical set of twins may be more than advertised. Arising from division and separation of a fertilized egg before implantation in the uterus, identical twins are the closest thing we have to human clones and are relative rarities among mammals. Only a handful of other species give birth to identical twins*. It has been a scientific curiosity, therefore, for years how identical human twins can come down with different diseases seemingly related to genetics, if their DNAs are identical. Such diseases include Parkinson’s and others. The standard explanation to the question has been due to differences in environmental exposure, but the answer now, at least in some cases, appears to be that, in fact, identical twins are in some cases not as identical as we thought they were. By examining the DNAs of 19 sets of “identical” twins exhibiting concordant or discordant phenotypes, collaborating American, Swedish and Dutch researchers were able to establish that the DNAs of such siblings were, indeed, identical, save for the presence of a few small repeated DNAs whose copy numbers varied. Repeated DNAs are factors in DNA fingerprinting and may play roles in control of gene expression and thus explain how the different phenotypes of these twins arise. Yogi Berra once said, “A few thousand dollars here, a few thousand dollars there and pretty soon you’re talking about real money.” The same, it seems, may be true of DNA repeats and genetic differences.

*Interestingly, the nine-banded armadillo is one such species and it almost always gives birth to identical quadruplets!

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