A new study shows that differences in the DNA of identical twins can occur in the first two weeks after fertilizing an egg
WASHINGTON – 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 the same genetically, new research shows.
Scientists in Iceland have sequenced DNA from 387 pairs of identical twins – those derived from a single fertilized egg – as well as from parents, children and spouses. This allowed them to find “early mutations that separate identical twins,” said Kari Stefansson, a geneticist at the University of Iceland and deCODE genetics, 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 slight mistake when reproducing DNA.
On average, identical twins have 5.2 of these early genetic differences, the researchers found. But about 15 percent of identical pairs of twins have multiple genetic differences, some up to 100, Stefansson said.
These differences represent a small portion of each twin’s genetic code, but could influence why a twin is taller or why a twin has a higher risk for certain types of cancer.
Previously, many researchers believed that physical differences between identical twins were mainly 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 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, “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 with identical twins. This allowed researchers to identify when genetic mutations occurred in two different types of cells – those present in a single individual and those inherited from that person’s children. They also discovered mutations that occurred before the developing embryo split in two, setting the scene for the twins.
Stefansson said his team found pairs of twins in which a mutation is present in all the cells of a twin’s body, but is not found in the other twin at all. However, sometimes the second twin may show mutation in some cells, but not in all cells, he said.
Nancy Segal, a psychologist who studies twins at California State University in Fullerton and was not involved in the newspaper, called the research “heroic and truly significant.”
“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.”
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