The findings, presented Friday (Nov. 9) here at the American Society of Human Genetics meeting, may partly explain why one twin gets cancer while another stays healthy. The study also suggests that these genetic changes are surprisingly common.

 

"It's not as rare as people previously expected," said study presenter Rui Li, an epidemiologist at McGill University.

 

 While past studies have looked at genetic changes, or mutations, in sperm and eggs, which can be passed on to offspring, very few studies have looked at somatic mutations. These mutations, also called copy errors, can occur early in fetal development, but because they aren't in the sex cells (the X or Y chromosomes) of the fetus, they can't be passed on.

Other studies have shown that chemical modifications, or epigenetic effects, can change which genes are expressed over the years, one factor that renders twins not completely identical. Still, other work has shown that identical twins can have different gene mutations, but this study didn't determine how often they occur.