Last week The Economist carried a review of the article, which you can read here. According to the review, the study documents a process called epigenesis which may explain physiological differences observed in identical twins. While identical--or monozygotic--twins share the same DNA, they don't always share all the same characteristics.
Very interesting. I wonder if this line of research will shed some light on the similarities and differences in the sexual orientations of identical twins that were noted in the famous 1991 and 1993 Bailey and Pillard studies.
It is not, however, enough for organisms to share DNA in order to share characteristics. Those genes must also behave in the same way. One of the ways that the behaviour of genes is regulated is by the application to their DNA of particular clusters of atoms, known as methyl groups. Methylation shuts a gene down. To the extent that the pattern of methylation is passed from parent to offspring, it forms a second, “epigenetic”, inheritance mechanism parallel to the primary DNA-based one. The importance of epigenetic inheritance is now a matter of hot debate.
Dr Petronis and his team therefore looked at methylation patterns in DNA from cheek swabs, blood samples and gut biopsies that had been collected from 57 pairs of monozygotic twins. They uncovered a significant amount of variation between twins, possibly enough to explain why apparently heritable diseases that require the coincidence of several genetic risk-factors do not, in practice, always appear in both twins. Schizophrenia, for example, has a family component. But if one twin of a monozygotic pair develops it, there is only a 50% chance that the other will too, rather than the 100% chance that you would see if the sequence of genetic “letters” in the DNA were the only cause.
Dr Petronis then looked at whether the amount of difference between the epigenomes of identical twins was similar to that between non-identicals. He studied samples from 80 pairs of twins, half of whom were non-identical, and, once again, created epigenetic profiles for all of them. The results suggest that although monozygotic twins do differ epigenetically, they differ less than dizygotic twins.
Hat tip: the man I love :-)