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.
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.