WASHINGTON (AP) – If you are identical twins who have always resisted being called a clone of your sibling, scientists say you have a point.
Identical twins aren’t exactly genetically the same, new research shows.
Scientists in Iceland have sequenced DNA from 387 pairs of identical twins – which come from a single fertilized egg – as well as their parents, children and spouses. That enabled them to “find early mutations that separate identical twins,” said Kari Stefansson, a geneticist at the University of Iceland and the deCODE genetics company, and co-author of the paper. published Thursday in the journal Nature Genetics.
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. They can occur when a cell divides and makes a small mistake in replicating DNA.
On average, identical twins have 5.2 of these early genetic differences, the researchers found. But about 15% 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 can affect why twins are taller or why twins are at higher risk for certain cancers.
Previously, many researchers believed that physical differences between identical twins were mainly related to environmental factors, such as diet or lifestyle.
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.
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. They also found mutations that occurred before the developing embryo split in two, setting the stage for twins.
Stefansson said his team found twins in which a mutation was present in all cells of the body of one twin, but not at all in the other twin. But “sometimes the second twin shows the mutation in some cells, but not all cells,” he said.
Nancy Segal, a psychologist who is studying twins at California State University in Fullerton and was not involved in the paper, called the research “ 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.”
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