Genetic differences between identical twins can begin very early in embryonic development, according to a new study that researchers say has implications for studying 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 Thursday in the journal Nature Genetics, suggests the role of genetic factors in shaping these differences has been underestimated.
“The classic model was to use identical twins to help separate the influence of genetics and environment in the analysis of disease,” said Kari Stefansson, paper co-author and head of Icelandic deCODE genetics, a subsidiary of the US. pharmaceutical company Amgen.
So if you take identical twins who 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 said, adding that the disease may have been due to an early genetic mutation in one twin, but not the other.
A mutation means a change in a DNA sequence – a small change that is not inherently good or bad, but can affect physical characteristics or susceptibility to certain diseases.
Jan Dumanski, a geneticist at Uppsala University in Sweden, who was not involved with the new paper, praised it as “a clear and important contribution” to medical research. “The implication is that we have to be very careful when using twins as a model” to tease the influences of nature and nurture separately, he said.
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 occurred during embryonic growth and found that identical twins differed an average of 5.2 early developmental mutations. The number of divergent mutations was higher in 15% of the 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 in a sibling’s body – meaning it probably happened very early in development – but not at all in the other twins.
Stefansson said that from the initial mass that would go on to form the individuals, “one of the twins is made 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.
Previous studies, including a 2008 paper in the American Journal of Human Genetics, have identified some genetic differences between identical twins.
The new study goes beyond previous work by including the DNA of parents, children, and spouses of identical twins. This allowed the researchers to identify when genetic mutations occurred in two different types of cells: those present in only one person and those inherited by that person’s children.
Nancy Segal, a psychologist who is studying twins at California State University in Fullerton and was not involved in the paper, called the study “heroic and really important.”
“This will force scientists to refine our thinking about the influences of genetics and environment,” she said. “Gemini is very similar, but it’s not a perfect match.”
With Agence France-Presse and Associated Press