Identical twins have identical genes, right? Well maybe not.

I always accepted the conventional wisdom on this.

The Claim: Identical Twins Have Identical DNA – New York Times

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.

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It has long been known that identical twins develop differences that result from environment. And in recent years, it has also been shown that some of their differences can spring from unique changes in what are known as epigenetic factors, the chemical markers that attach to genes and affect how they are expressed — in some cases by slowing or shutting the genes off, and in others by increasing their output.

These epigenetic changes — which accumulate over a lifetime and can arise from things like diet and tobacco smoke — have been implicated in the development of cancer and behavioral traits like fearfulness and confidence, among other things….

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The specific changes that Dr. Dumanski and his colleagues identified are known as copy number variations, in which a gene exists in multiple copies, or a set of coding letters in DNA is missing. Not known, however, is whether these changes in identical twins occur at the embryonic level, as the twins age or both.

“Copy number variations were discovered only a few years ago, but they are immensely important,” said Dr. Carl Bruder, another author of the study at the university. Certain copy variations have been shown in humans to confer protection against diseases like AIDS, while others are believed to contribute to autism, lupus and other conditions. By studying pairs of identical twins in which one sibling has a disease and the other does not, scientists should be able to identify more easily the genes involved in disease.