Genetic differences between identical twins can start very early in embryonic development, according to a study on Thursday that researchers said had implications for how these brothers help scientists discover the effects of nature versus creation.
Identical – or monozygotic – twins come from a single fertilized egg that splits in two.
They are important research objects because they are believed to have minimal genetic differences.
This means that when physical or behavioral differences arise, environmental factors are assumed to be the likely cause.
But the new research, published in the journal Nature Genetics, suggests that the role of genetic factors in the formation of these differences has been underestimated.
“The classic model is to use identical twins to help you separate the influence of genetics from environment in disease analysis,” said Kari Stefansson, head of Iceland’s deCODE genetics, a subsidiary of the American pharmaceutical company Amgen.
“So if you take identical twins raised separately and one of them develops autism, the classic interpretation is that this is caused by the environment.”
“But this is an extraordinarily dangerous conclusion,” he told AFP, adding that there is a possibility that the disease could be caused by an early genetic mutation that happened in one of the twins, but not in the other.
Stefansson and his team sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track genetic mutations.
They measured the mutations that occur during embryonic growth and found that identical twins differ by an average of 5.2 initial developmental mutations.
In 15 percent of twins, the number of divergent mutations is higher.
When a mutation occurs in the first few weeks of embryonic development, it is expected to spread to both the cells of an individual and those of his descendants.
In one of the pairs of twins studied, for example, a mutation was present in every cell in a sibling’s body – meaning that it probably happened very early in development – but not in the other twin.
Stefansson said that of the initial mass that would form the individuals, “one of the twins is made up of the descendants of the cell where the mutation occurred and nothing else”, while the other is not.
“These mutations are interesting because they allow you to start exploring the way twinning happens.”
Given the genetic differences found, the identical term itself can be misleading to describe the brothers.
“I’m more inclined to call them monozygotic twins today than identical ones,” said Stefansson.
© Agence France-Presse