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Genetic differences between identical twins can begin very early in embryonic development, according to a study that researchers say has implications for examining the effects of nature versus feeding.
Identical twins – or monozygotes – come from a single fertilized egg that splits in two. They are important research subjects because they are believed to have minimal genetic differences. This means that when physical or behavioral differences occur, it is assumed that environmental factors are the probable cause.
But new research (£), published in the journal Nature Genetics on Thursday, suggests that the role of genetic factors in shaping these differences has been underestimated.
Kari Stefansson, co-author of the paper, said that identical twins have traditionally been used to help researchers try to separate the influence of genetics and the environment in the analysis of diseases and other conditions.
“So, if you take identical twins raised outside and one of them developed autism, the classic interpretation was that it is caused by the environment.
“But this is an extraordinarily dangerous conclusion,” he said, adding that the condition could be caused by 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 or disorders.
Jan Dumanski, a geneticist at Uppsala University in Sweden who was not involved in the new work, praised it as “a clear and important contribution” to medical research. “The implication is that we have to be very careful when we use twins as a model” to destroy the influences of nature and take care of it, he said.
Stefansson, who is the head of genetics at deCODE in Iceland, a subsidiary of the American pharmaceutical company Amgen, and his team sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track genetic mutations.
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 divergent mutations was higher.
When a mutation occurs in the first few weeks of embryonic development, it would be expected to spread to both an individual’s cells and those of their offspring.
In one of the pairs of twins studied, for example, a mutation was present in all the cells in a brother’s body – which means that it probably happened very early in development – but not in the other twin.
Stefansson said that of the initial mass that was to form individuals, “one of the twins consists of the descendants of the cell in which the mutation took place and nothing else,” while the other did not.
“These mutations are interesting because they allow you to start exploring how twinning happens.”
Given the genetic differences found, even the identical term can be misleading to describe siblings. “I’m more inclined to call them monozygotic twins today than identical ones,” 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 with identical twins. This allowed researchers to identify when genetic mutations occurred in two different cell types: those present in a single individual and those inherited from that person’s children.
Nancy Segal, a psychologist who studies twins at California State University Fullerton and was not involved in the paper, called the research “heroic and truly meaningful.”
“This will force scientists to refine our thinking about the influences of genetics and the environment,” she said. “The twins are very similar, but it’s not a perfect resemblance.”
With Agence France-Presse and Associated Press
• This article was corrected on January 8, 2021. Autism is not a disease, as mentioned in an earlier version.
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