WASHINGTON (AP) – If you’re an identical twin who has always resisted being called a clone of your brother, scientists say you have a point.
Identical twins are not exactly genetically the same, according to new research.
Icelandic scientists sequenced the DNA of 387 pairs of identical twins, those derived from a single fertilized egg, as well as from their parents, children and spouses. This allowed them to find “first mutations that separate identical twins,” said Kari Stefansson, a geneticist at the University of Iceland and the company deCODE genetics, and co-author of the paper. published Thursday in the journal Nature Genetics.
A mutation means an alteration in a DNA sequence: a small change that is not inherently good or bad, but that can influence physical characteristics or susceptibility to certain diseases. They can occur when a cell divides and makes a small mistake in replicating DNA.
The researchers found that, on average, identical twins have 5.2 of these early genetic differences. But about 15 percent of identical twin pairs have more genetic differences, some up to 100, Stefansson said.
These differences represent a small portion of each twin’s genetic code, but they could influence why a twin is taller or why a twin is at higher risk for certain cancers.
Previously, many researchers believed that physical differences between identical twins were primarily related to environmental factors, such as nutrition or lifestyle.
Jan Dumanski, a geneticist at Uppsala University in Sweden, who did not participate in 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 we use twins as a model” to separate influences from nature and nutrition, he said.
Previous studies, including a 2008 document in the American Journal of Human Genetics, they have identified some genetic differences between identical twins.
The new study goes beyond previous work, including the DNA of parents, children, and spouses of identical twins. This allowed the researchers to identify when genetic mutations occurred in two different cell types: those present in a single individual and those inherited by that person’s children. They also found mutations that occurred before the developing embryo split in two, which prepared the ground for the twins.
Stefansson said his team had found pairs of twins where there was a mutation in every cell in one twin’s body, but it was not in the other twin. However, “sometimes the second twin may show the mutation in some cells, but not in all cells,” he said.
Nancy Segal, a psychologist studying twins at California State University (Fullerton) who did not participate 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,” he said. “The twins look a lot alike, but it’s not a perfect resemblance.”
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