Some identical twins don’t have identical DNA

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

On average, identical twins differ by 5.2 genetic changes, researchers report January 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 from environmental influences. Identical twins were thought to be genetically the same, so differences in their health were considered to be the product of their environment. The new finding suggests that some genetic changes could also account for differences between twins.

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

Patterns of mutations among twins suggest that embryos don’t split neatly when twins form, the findings suggest. Some twins may arise when a single cell or a small group of cells splits off from the embryo. The number of cells that a twin originates from may determine how genetically different they are from their twin, with more uneven splits of the embryo leading to a greater number of differences between the twins.