WASHINGTON – If you are an identical twin who has always resisted being called your brother’s clone, scientists say you are right.
Identical twins aren’t exactly the same genetically, new research shows.
Icelandic scientists sequenced DNA from 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 “early 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 article 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 influence physical characteristics or susceptibility to certain diseases. They can occur when a cell divides and makes a small mistake in DNA replication.
On average, identical twins have 5.2 of those initial genetic differences, the researchers found. But about 15 percent of identical twin pairs have more genetic differences, some up to 100, said Stefansson.
These differences represent a small portion of each twin’s genetic code, but they can influence why one of the twins is taller or because one of the twins is at greater risk of developing 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 the University of Uppsala in Sweden, who was not involved in the new article, 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 separate the influences of nature and creation, he said.
Previous studies, including a 2008 article in The American Journal of Human Genetics, have identified some genetic differences between identical twins.
The new study goes beyond previous work, including DNA from parents, children and spouses of identical twins. This allowed researchers to identify when genetic mutations occurred in two different types of cells – those present in just one individual 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 pairs of twins where a mutation is present in every cell in the body of one of the twins, but was not found in the other twin. However, “sometimes the second twin can show the mutation in some cells, but not in all cells,” he said.
Nancy Segal, a psychologist who studies twins at California State University, Fullerton, and was not involved in the article, called the research “heroic and really 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 is not a perfect resemblance.”