In the article, Differences between germline genomes of monozygotic twins, published in Nature Genetics today scientists at deCODE genetics searched for mutations that were present in only one of monozygotic twins.  

In the study of human genetics the genomes of monozygotic twins are often assumed to be identical, and differences between them have been pinned on the environment rather than genetics. This assumption has been used throughout the centuries to disentangle the contribution of genetics and environment to disease and other phenotypes.

The scientists found two groups of twin pairs, one where twins share mutations and another where the developmental mutations are only present in one of them at high frequency.

"Mutations can be formed when cells divide and the daughter cells may carry a mutation that marks the descendants of the mutated cell within an individual. Mutations that are present in only one of the twins allow us therefore to backtrack to the cell divisions that lead to the development of the twins." says Hakon Jonsson scientist at deCODE Genetics and author on the paper.

Remarkably this backtracking of the mutations that differ between twins suggests that the mutations are formed in the first days of embryonic development when the embryo consists of several cells. 

"These two groups of monozygotic twins give insight into development of the embryo only few divisions after conception when the embryo consists of several cells," says Kari Stefansson CEO of deCODE genetics. "These are exciting times that allows us to use mutations to shed light on the development of humans at these first stages of development."

Hakon Jonsson and colleagues sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track mutation divergence. The authors found that twins differ by 5.2 early developmental mutations, on average. In approximately 15% of twin pairs, one twin carries a high number of these mutations that the other twin does not have.

Physical or behavioral differences between identical twins are usually attributed to environmental factors, according to the assumption that genetic differences between the pair are minimal. However, autism and other developmental disorders—which one twin in a pair might have—are linked to genetic mutations. The authors conclude that the role of genetic factors in shaping such phenotypic differences has been underestimated.