Media release

From: Springer Nature

Genetics: Some identical twins have non-identical genomes

Monozygotic, or identical, twins have genetic differences that begin in the early stages of embryonic development, according to an article published in Nature Genetics.

Identical twins come from a single zygote formed by one sperm fertilizing one egg. Cell division during embryonic growth can lead to mutations, but this source of genetic difference had not been measured between identical twins.

Hákon Jónsson 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 behavioural 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.