Washington – If you are an identical twin who always opposes being called a clone of your siblings, scientists say you have a point.
Genetically identical twins are not identical, new research suggests.
Scientists in Iceland sequenced the DNA of 387 pairs of identical twins – obtained from a fertilized egg – as well as from their parents, children and spouse. That allowed them to “find the twins that separated the initial mutation”, Kari Stephenson, a geneticist at the University of Iceland and co-author of the company Nature Genetics, published in the journal Nature Genetics on Thursday.
A mutation means a change in the sequence of DNA – a small change that is not inherently good or bad, but can affect physiological characteristics or susceptibility to certain diseases. They can occur when a cell divides and makes a slight error in replicating DNA.
Researchers found that of these initial genetic differences, on average, twins are 5.2. Stephenson said that about 15 percent of the twins have a genetic difference, some of them up to 100.
These differences represent a small portion of each twin’s genetic code, but they may affect why a twin is tall or why a 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 paper, praised it as “a clear and important contribution to medical research”.
He said, “This means that we have to be very careful when we are using twins as a model.”
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 involving the DNA of parents, children, and spouses of identical twins. When researchers allowed genetic mutations to occur in two different cell types – those that were present in only one person and that were inherited from that person’s children. They also found mutations that split into two before the developing embryo set the stage for twins.
Stephanson said his team had found pairs of twins where a mutation is present in all cells of a twin’s body, but not found in the other twin at all. However, “Sometimes a second twin may show mutations in some cells, but not all cells.”
The research is called “heroic and really important” by Nancy Sehgal, a psychologist who studies twins at California State University, Fullerton, and was not included in the paper.
“This will force scientists to refine our thinking about genetics and environmental influences,” she said. “The twins are alike, but this is not an ideal likeness.”