If you are an identical couple who have always resisted being named a clone of your sisters, scientists say you have a point.
Identity twins are not genetically identical, a new study shows.
Scientists in Iceland ordered DNA from 387 pairs of similar twins – those from one fertilized egg – as well as their parents, children and spouse. That allowed them to “find early mutations that separate an identical twin,” said Kari Stefansson, a geneticist at the University of Iceland and the genetics company deCODE, and co-author of the paper published Thursday in the journal Nature Genetics.
Mutation means a change in a sequence of DNA – a tiny change that is neither good nor bad, but affects physical features or is susceptible to certain diseases. They can occur when a cell divides and makes a small mistake in reproducing DNA.
On average, 5.2 of these early genetic differences have a pair of 5.2, the researchers found. But about 15% of identical twins, some up to 100, have more genetic differences, Stefansson said.
These differences represent a tiny fraction of each couple’s genetic code, but they may affect why one couple is taller or why one couple is at greater risk for specific cancers. .
Previously, many researchers believed that physical differences between identical couples were largely related to environmental factors, such as nutrition or lifestyle.
Jan Dumanski, a geneticist at Uppsala University in Sweden, who was not involved in the new paper, praised it as a “clear and important contribution” to medical research.
“This means we have to be very careful when using twins as a model” to separate from the effects of nature and nurture, 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 earlier work by including DNA of parents, children and spouse of an identical couple. That allowed the researchers to identify when genetic mutations occurred in two cell types – those that were present in just one person and those that were inherited by that person’s children. They also found mutations that occurred before the two embryos separated, setting the stage for a twin.
Stefansson said his team had found pairs of twins where mutations are present in all cells of one pair’s body, but not found in the other twins at all. However, “sometimes the second pair may show a mutation in some cells, but not all cells,” he said.
Nancy Segal, a psychologist who studies twins at California State University, Fullerton and was not involved in the paper, said the research was “heroic and very important.”
“This will make scientists refresh our thinking on genetic and environmental impact,” she said. “Couples are very similar, but not at all similar.”