Genetic differences between identical twins can begin very early in embryonic development, according to a Thursday study that researchers say has implications for how these siblings help scientists discover the effects of nature versus nurture.
Identical – or monozygotic – twins hatch from a single fertilized egg that splits in two.
They are important research topics because they are thought to have minimal genetic differences.
This means that when physical or behavioral differences occur, environmental factors are believed to be the likely cause.
But the new research, published in the journal Nature Genetics, suggests that the role of genetic factors in shaping these differences has been underestimated.
“The classic model was to use identical twins to help you separate the influence of genetics and environment in disease analysis,” said Kari Stefansson, head of Iceland’s deCODE genetics, a subsidiary of the US pharmaceutical company. Amgen.
“So if you take identical twins that were raised separately and one of them developed autism, the classic interpretation is that it’s caused by the environment.”
“But that’s an extremely dangerous conclusion,” he told AFP, adding that there is a possibility that the disease is due to an early genetic mutation that happened in one of the twins but not the other.
Stefansson and his team sequenced the genomes of 387 identical twins and their parents, husbands, and children to look for genetic mutations.
They measured mutations that occur during embryonic growth and found that identical twins differ an average of 5.2 early developmental mutations.
The number of divergent mutations is higher in 15 percent of twins.
When a mutation occurs in the first few weeks of embryonic development, it would be expected to be widespread in both an individual’s cells and their offspring.
For example, in one of the twin pairs studied, a mutation was present in all cells of a sibling’s body – meaning it likely happened very early in development – but not at all in the other twins.
Stefansson said that out of the original mass that would go on to form the individuals, “one of the twins was created from 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 explore the way twinning takes place.”
Given the genetic differences found, the term can be identically misleading to describe the siblings.
“I’m more likely to call them monozygotic twins these days than identical,” Stefansson said.
© Agence France-Presse