Although the genomes of monozygotic twins are often assumed to be identical, new research has found that mutations may be present at birth in one ‘identical’ twin, but not the other.

The findings have important research implications, because differences between identical twins, such as autism, are usually assumed to be due to environmental — not genetic — factors, researchers say.

As reported in Nature Genetics, Dr Kari Stefansson of deCODE genetics in Reykjavik, Iceland and colleagues sequenced the genomes of 381 pairs of identical twins and their parents, spouses and children in order to track mutation divergence.

They found that monozygotic twins differ by 5.2 early developmental mutations, on average, and that about 15% have a substantial number of these early mutations that are specific to just one of them.