Identical twins are ultimately not that identical

It was believed for a long time that identical twins were alike in every way. A study published in the scientific journal Nature Genetics recently skewed this belief. According to the results of this research, which was carried out by a team of Icelandic researchers, identical twins present differences invisible to the naked eye.

Genetic mutations that occur during embryonic development are at the root of these differences. The researchers indicated that, on average, identical twins have genomes that differ by the order of 5.2 mutations.

"What is most surprising is that in most pairs of twins, some mutations are carried by almost all the cells in one of the twins, but are completely absent in the other," said Professor Ziyue. Gao, of the University of Pennsylvania.

Identical Twins Are Ultimately Not That Identical
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The results of this study reveal that about 15% of identical twin pairs have different DNAs.

A study carried out on 387 pairs of twins

This study was made possible by studying the genomes of 387 pairs of twins. As a result of these observations, study lead author Kari Stefansson, who is also the CEO of the CODE genetics company, revealed that one of the twins could carry between 10 to 15 different mutations. For now, researchers do not know in which region of the genome these mutations occur.

As a reminder, twins come from the split of a fertilized egg. The separation of embryos usually occurs between the first and the seventh day of fertilization. In the rarest cases, the split occurs between the eighth and thirteenth day.

Better understand the diseases that affect some twins

The researchers hypothesized that the twins with the most genetic differences are those who separated very early during their embryonic development. This means that all of the mutations that the twins share happened before this split. Those which are different took place afterwards.

Scientists believe this study will help them understand the diseases that affect some twins. According to Kari Stefansson, “The classic model is to use identical twins to help distinguish the influence of genetics from that of the environment in the analysis of disease. So if you take identical twins who were raised separately and one of them develops autism, the classic interpretation is that they are caused by the environment. "

He believes that “this is an extremely dangerous finding, as there is the possibility that the disease is due to an early genetic mutation that occurred in one of the twins. "

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