Identical twins are produced when a fertilized egg or zygote divides completely in two and forms two separate individuals. The two will therefore have identical DNA. But scientists have found something that twins have notably in common with not only each other, but with other identical twins, even those they are not related to otherwise. And this trait does not appear in people who aren't identical twins.
Scientists looking into the cause of twinning took a look at epigenetic factors. These are things that could cause a gene to be switch on or off, because not all genes are read by the cells they inhabit. A new study looked into methyl groups, that is, molecules that stick to genes and keep them from being expressed. Which genes these methyl groups stick to can form a pattern. A study of 6000 people, including identical twins, fraternal twins, and non-twin family members, shows that all the identical twins had the same methyl group pattern that covered 834 genes, while other participants did not.
Whether this pattern is the cause of the embryonic split or not is still unclear. It's possible the split itself caused the pattern, or possibly the pattern somehow came from something unique in the prenatal environment of identical twins. However, scientists suspect that the methyl group pattern may cause the formation of identical twins, as many of the genes affected are those that govern how cells stick to each other. The research shows that this pattern is quite stable over the lifetime of an individual, but we don't yet know if it has any effect on a twin's physiology after birth.
There are many more zygotes that split into identical twins than there are twins born. Often, one of the twins will stop developing and vanish before birth, without the mother ever knowing about it. From this study, one would think it might be possible to find out if a person had a vanished twin before birth by scanning for this methyl group pattern. But that research has yet to be done.
Read about this breakthrough study in epigenetics at Live Science. -via Damn Interesting