A new study shows that differences in the DNA of identical twins can appear within the first two weeks after an egg is fertilized
WASHINGTON – If you’re an identical twin who’s always resisted being called a clone of your sibling, scientists say you have a point.
Identical twins are not exactly genetically identical, new research shows.
Scientists in Iceland have compiled DNA from 387 pairs of identical twins – those derived from a single fertilized egg – as well as from their parents, children and spouses. This enabled 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, which was published in the journal Nature Genetics on Thursday is, said.
A mutation means a change in a series of DNAs – a small change that is not inherently good or bad, but that can affect physical characteristics or susceptibility to certain diseases. This can occur when a cell divides and makes a slight error in the replication of DNA.
On average, identical twins have 5.2 of these early genetic differences, the researchers found. But about 15% of identical twin couples have more genetic differences, some up to 100, Stefansson said.
These differences represent a small portion of the genetic code of each twin, but they may influence why one twin is taller or why one twin is at greater risk for certain cancers.
Previously, many researchers believed that physical differences between identical twins were mostly 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 article, 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 tease the influences of nature and the nurturing of each other.
Previous studies, including a 2008 article in The American Journal of Human Genetics, have identified genetic differences between identical twins.
The new study extends beyond previous work by including DNA from parents, children and spouses of identical twins. This enabled the researchers to determine when genetic mutations occurred in two different types of cells – those that occur in only one individual and those that are inherited by the person of the person. They also found mutations that occurred before the developing embryo split in two, setting the scene for twins.
Stefansson said his team found pairs of twins where a mutation occurs in all cells of one twin, but not at all in the other twin. “Sometimes, however, the second twin may show the mutation in some cells, but not in all cells,” he said.
Nancy Segal, a psychologist who studied twins at California State University, Fullerton and was not involved in the article, called the research ‘heroic and really meaningful’.
“It will force scientists to refine our thinking about the influences of genetics and the environment,” she said. “Twins are very much the same, but it’s not a perfect match.”
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