Identical twins aren’t all identical, study finds

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Do you have an identical twin? Are you sure?

New research finds not all identical twins have identical DNA, and the differences can begin early in embryonic development.

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Identical, or monozygotic, twins come from one fertilized egg that splits in two. The pairs have been important in scientific research because physical and behavioral differences were presumed to be caused by the environment, the Guardian reported.

“So if you take identical twins raised apart and one of them developed autism, the classic interpretation has been that that is caused by the environment,” said Kari Stefansson, a co-author of the paper.

His research, published recently in the journal Nature Genetics, might throw a monkey wrench into those views, however.

Stefansson, who is the head of Iceland’s deCODE genetics and his team sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children in order to track genetic mutations.

A mutation is an alteration in the genetic material of a cell that is more or less permanent and that can be transmitted to a cell’s descendants.

The researchers found that twins differed, on average, by 5.2 early developmental mutations.

In about 15% of pairs, one sibling carried a high number of these mutations while the other didn’t.

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In one of the pairs of twins studied, the Guardian reported, a mutation was present in all cells in one sibling’s body — meaning it is likely to have happened very early in development — but not at all in the other twin.

Scientists expect mutations that happen in the first few weeks of development to be widespread not just in an individual but also in their offspring.

Stefansson said that out of the initial mass that would go on to form the individuals, “one of the twins is made out of the descendants of the cell where the mutation took place and nothing else,” while the other was not.

“These mutations are interesting because they allow you to begin to explore the way in which twinning happens,” he told the Guardian.

Stefansson said the term “identical” might be misleading in light of his research.

“I am more inclined to call them monozygotic twins today than identical,” he said.

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