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Identical twins are not so identical, a study suggests Genetics



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 on parenting.

Identical – or monozygotic – twins come from a fertilized egg that splits in two. They are important subjects in the study because they are thought to have minimal genetic differences. This means that when physical or behavioral differences occur, environmental factors are assumed to be the probable cause.

A new study published Thursday in the journal Nature Genetics suggests that the role of genetic factors in shaping these differences is underestimated.

“The classic model is to use identical twins to help you separate the influence of genetics on the environment in disease analysis,”

; said Carrie Stephenson, co-author and head of Icelandic deCODE genetics, a subsidiary of US pharmaceutical company Amgen.

“So if you take identical twins raised and one of them has developed autism, the classic interpretation is that it’s caused by the environment.

“But this is an extremely dangerous conclusion,” he said, adding that the disease was likely to be due to an early genetic mutation in one twin but not in the other.

A mutation means a change in a series of DNA – a small change that is not good or bad in nature, but can affect physical characteristics or susceptibility to certain diseases.

Jan Dumanski, a geneticist at Uppsala University in Sweden who is not involved in the new document, described it as a “clear and important contribution” to medical research. “The bottom line is that we have to be very careful when using twins as a model,” to irritate the influences of nature and upbringing, he said.

Stephenson 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 mutations at the beginning of development. In 15% of twins, the number of divergent mutations is higher.

When the mutation occurs during the first few weeks of embryonic development, it is expected to be widespread in both the individual’s cells and the offspring’s cells.

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

Stephenson said that from the initial mass that would continue to form individuals, “one twin was made by the offspring of the cell where the mutation occurred and nothing else,” while the other was not.

“These mutations are interesting because they allow you to start exploring the way twinning happens.”

Given the genetic differences found, the term identical itself can be misleading to describe siblings. “Today, I’m more inclined to call them monozygotic twins than identical ones,” Stephenson said.

Previous research, including a 2008 report in the American Journal of Human Genetics, has identified some genetic differences between identical twins.

The new study goes beyond previous work, involving the DNA of parents, children and spouses of identical twins. This allowed researchers to determine when genetic mutations occurred in two different cell types: those present in only one individual and those inherited from that person’s children.

Nancy Seagal, a psychologist who studies the twins at Fullerton State University and is not involved in the article, called the study “heroic and truly significant.”

“This will force scientists to refine our thinking about the impact of genetics and the environment,” she said. “The twins are very similar, but that’s not a perfect resemblance.”

With Agence France-Presse and the Associated Press


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