Not exactly the same

Geneticists have assessed discrepancies in the DNA of identical twins

Sergey Vasiliev, Naked Science

The twin method is one of the most well–known approaches to assess the influence of heredity and environmental factors on a particular quality of a person. It is believed that identical (monozygotic) twins are genetically completely identical, which means that any difference between them is the result of differences in living conditions and development. Such studies are carried out even in space: recall that NASA sent astronaut Scott Kelly into orbit while his twin Mark remained on Earth, in order to then compare their condition and more accurately determine the impact of the space environment on the body.

However, it seems that everything is not so clear, and when conducting twin studies, one should keep in mind the small but existing differences in the DNA of identical twins. They were recently evaluated by a team of scientists from the University of Iceland led by Hakon Jonsson, whose article was published in the journal Nature Genetics (Jonsson et al., Differences between germline genomes of monozygotic twins).

The authors performed complete DNA sequencing for more than 380 pairs of twins and compared them, finding that only 38 pairs of genomes were really completely identical. In the overwhelming majority, small differences were revealed that arose during random mutations in the early stages of embryonic development, already independently of each other. On average, identical twins differ from each other by 5.2 mutations.

At the same time, more than a hundred differences were found in 39 couples – this is quite a significant amount, which cannot but influence the conduct of twin studies. According to scientists, the number of mutations that differ from each other is strongly influenced by the moment of separation of twins in the womb: the earlier it occurs, the more different mutations they have time to accumulate.

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