No Two Are (Completely) Alike

In a paper recently published in Nature Genetics, Art Petronis of the Center for Addiction and Mental Health in Toronto led a study to look at genome-wide methylation patterns in identical and fraternal twins. Using 12K CpG island microarrays, his team looked at DNA methylation patterns from cheek swabs, blood samples, and gut biopsies across 114 monozygotic twins and 80 dizygotic twins to find that while identical twins did show significant variation in their epigenetic code, they showed less difference than non-identical twins. A story in the Economist wonders how some methylation is escaping pre-fertilization erasure: "Quite a lot of pre-existing methylation is making its way into the new individual -- and thus providing both a complication to those who try to understand the intricacies of inheritance, and a promising new line of inquiry."