Genetic differences between identical twins can begin very early in embryonic development, according to a study Thursday, researchers said that it has implications for the way these siblings help scientists to tease the effects of nature on nurturing.
Identical – or monozygotic – twins come from a single fertilized egg that is divided into two.
These are important researchers because they are thought to have minimal genetic differences.
This means that when physical or behavioral differences emerge, environmental factors are suspected to be the likely cause.
But the new research, published in the journal Natural genetics, suggests that the role of genetic factors in the formation of these differences is underestimated.
“The classic model was to use identical twins to help you separate the influence of genetics on the environment in disease analysis,” said Kari Stefansson, head of Iceland’s deCODE genetics, a subsidiary of the US pharmaceutical firm Amgen, said.
“So if you take a twin that is lifted apart and one of them develops autism, the classic interpretation is that it is caused by the environment.”
“But this is an extremely dangerous conclusion,” he told AFP, adding that the possibility exists that the disease may be due to an early genetic mutation that occurred in one of the twins, but not in the others not.
Stefansson and his team traced the genomes of 387 pairs of identical twins and their parents, spouses and children to detect genetic mutations.
They measured mutations that occur during embryonic growth and found that identical twins differ on average 5.2 early developmental mutations.
In 15 percent of the twins, the number of divergent mutations is higher.
When a mutation occurs in the first few weeks of embryo development, it will be expected that it will be widespread in the cells of the individual as well as in its offspring.
In one of the twin pairs studied, for example, there was a mutation in all the cells in the body of one sibling, which means that it probably took place very early in the development, but not at all in the other twin.
Stefansson said that from the initial mass that would form the individuals, “one of the twins consists of the offspring 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 start investigating the way twins occur.”
Given the genetic differences found, the term can be identically misleading to describe the siblings.
“I am more likely to call them monozygotic twins today than identical,” Stefansson said.
© Agence France-Press