The findings may partly explain why in some cases one twin gets cancer while another stays healthy.
Even though identical twins supposedly share all of their DNA, they acquire hundreds of genetic changes early in development that could set them on different paths, researchers said.
The findings may partly explain why in some cases one twin gets cancer while another stays healthy. The study also suggests that these genetic changes are surprisingly common, Live Science reported.
"It's not as rare as people previously expected," said study presenter Rui Li, an epidemiologist at McGill University.
To find out how often these mutations occur in early development, researchers led by Li studied the genomes of 92 pairs of identical twins and searched hundreds of thousands of sites in their genomes for differences between twins in base pairs, which are represented by letters that make up DNA.
For instance, one twin may carry an A at one point while another carries a C.
Researchers could only detect differences that would occur very early in foetal development and would show up in most cells in the body.
They then calculated the frequency with which these mutations occurred.
Only two sets of twins had such mutations, which translates to a DNA change occurring once for every 10 million to 10 billion bases that are copied every time a cell divides.
While that may seem like a high accuracy rate, cells in the body divide trillions of times, which means an average twin pair carries 359 genetic differences that occurred early in development.
"Our DNA samples came from blood samples," Li said. "You need to define different rates in different tissues." While past studies have looked at genetic changes, or mutations, in sperm and eggs, which can be passed on to offspring, very few researches have looked at somatic mutations.
These mutations, also called copy errors, can occur early in foetal development, but because they aren't in the sex cells (the X or Y chromosomes) of the foetus, they can't be passed on.
The study findings were presented at the American Society of Human Genetics meeting.
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