Identical twins don't necessarily have identical genomes. Anne Casselman at Scientific American writes that the University of Alabama at Birmingham's Carl Bruder has examined the genomes of 19 sets of adult identical twins, finding that some twins had points of genetic differences, including different copy number variants of some genes.
And CNVs have been linked to a number of diseases. Indeed, Casselman notes that one twin in Bruder's study lacked genes linked to leukemia that the other twin had, and the twin without the genes had had the disease.
Bruder says that some such variation could develop as a person ages. "I believe that the genome that you're born with is not the genome that you die with — at least not for all the cells in your body," he says.
Additionally, LiveScience reports on a presentation made at the American Society of Human Genetics meeting that said that identical twins may also have a number of genetic differences that occur early in development. McGill University's Rui Li calculated that "an average twin pair carries 359 genetic differences that occurred early in development," LiveScience says.
"Our DNA samples came from blood samples," Li adds. "You need to define different rates in different tissues."
In Scientific American, Casselman adds that many previously performed twin studies have assumed that identical twins have identical genomes, and that Bruder says that some findings may not hold up. However, the University of British Columbia's Kerry Jang tells Casselman that such new findings of genetic differences between identical twins are "unlikely … to significantly change any of the results found so far."