Wednesday, March 12, 2008

One more example of something everyone used to know for sure was true that isn't true at all

True or False? Identical twins are identical because they have exactly the same DNA.

False. According to this week's NYT Science Times, identical twins have very similar genes, but their DNA is not identical.
It is a basic tenet of human biology, taught in grade schools everywhere: Identical twins come from the same fertilized egg and, thus, share identical genetic profiles.

But according to new research, though identical twins share very similar genes, identical they are not. The discovery opens a new understanding of why two people who hail from the same embryo can differ in phenotype, as biologists refer to a person’s physical manifestation.
It's not a trivial difference either. In the study cited by the Times, some of the DNA examined by scientists came from pairs of identical twins that included one who had dementia or Parkinon's disease, while the other did not. The differences involved copy number variations, which were only discovered a few years ago.

For more about copy number variations, which all people -- not just twins -- have, check out this earlier story in Science Daily, "Genetic Variation: We're More Different Than We Thought." The implications of this emerging line of research are huge -- not just for better understanding evolution, but for studying genetic factors in disease.

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