Identical twins are not 100% genetically identical, study suggests

A new study published in the journal Nature Genetics has suggested that genetic differences between identical twins can begin very early in embryonic development. According to researchers, this has implications for studying the effects of nature versus nurture.

On average, identical twins differ by 5.2 genetic changes, researchers wrote in the study. The finding is important because identical twins — also called monozygotic twins because they come from a single fertilized egg — are often studied to determine whether particular traits, diseases or conditions result from genetics or from environmental influences, reported Science News citing the study. 

Identical twins were thought to be genetically the same, so differences in their health were considered to be the product of their environment. The new study says that some genetic changes could also account for differences between twins.

Kari Stefansson, co-author of the paper and the head of Iceland’s deCODE genetics, a subsidiary of the US pharmaceutical company Amgen, said: “The classic model has been to use identical twins to help you to separate the influence of genetics versus environment in analysis of diseases. “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.

“But that is an extraordinarily dangerous conclusion,” he said, adding that there was a possibility the disease could be due to an early genetic mutation in one twin but not the other. Stefansson and his team sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children in order to track genetic mutations, the Guardian reported.

They measured mutations that occurred during embryonic growth and found that identical twins differed by an average of 5.2 early developmental mutations. In 15% of twins, the number of diverging mutations was higher.

When a mutation happened in the first few weeks of embryonic development, it would be expected to be widespread both in an individual’s cells and in those of their offspring.

In one of the pairs of twins studied, for example, a mutation was present in all cells in one sibling’s body – meaning it likely happened very early in development – but not at all in the other twin.

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.”