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Some identical twins do not have identical DNA

Identical twins may not be carbon copies at the DNA level after all.

On average, identical twins differ in 5.2 genetic changes, according to researchers Jan. 7 in Nature Genetics. The finding is important because identical twins – also called monozygotic twins because they come from a single fertilized egg – are often studied to determine whether particular traits, diseases, or conditions result from genetics or environmental influences. Identical twins are believed to have been genetically the same, so differences in their health were considered the product of their environment. The new discovery suggests that some genetic changes could also explain differences between twins.

Researchers in Iceland deciphered the complete genetic composition, or genome, of 381 pairs of identical twins. Of these, 38 pairs were genetic duplicates of each other, but most had some differences in DNA that probably arose very early in development, well before an embryo split to form two or shortly after division. Some of the twins had many genetic differences, including 39 pairs that had more than 100 changes between the twins.

Mutation patterns between twins suggest that embryos do not divide perfectly when twins form, the results suggest. Some twins can arise when a single cell or small group of cells separates from the embryo. The number of cells from which a twin originates can determine the genetic difference of its twin, with more inequality in the embryo leading to a greater number of differences between the twins.

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