A new study reveals that identical twins do not share 100% of their DNA as many believe. Instead, mutations that occur during their development create slight differences in their DNA.

Identical twins form from the same egg and get the same genetic material from their parents. However, they can pick up genetic mutations in the womb which alter their DNA.

A mutation is an alteration in a sequence of DNA, and is not inherently good or bad but can influence physical features or susceptibility to certain diseases.

Researchers in Iceland sequenced 387 DNA pairs of identical twins, as well as their parents, children and spouses. The extended family was included to determine whether these mutations passed down generations, which would indicate that they are germline mutations meaning they appear in the eggs, sperm and their precursors. If these mutations appeared in the parents of the twins, that would indicate that the mutation appeared during early development.

They found that there were many early mutations that separated the identical twins. On average, pairs of twins may differ genetically by 5.2 mutations in early development.

The paper, published in the journal Nature Genetics on Thursday, January 7, estimates that in about 15% of identical twin pairs, one twin may carry up to 10 to 15 mutations the other does not.

“One particularly surprising observation is that in many twin pairs, some mutations are carried by nearly all cells in one twin but completely absent in the other,” Ziyue Gao, an assistant professor of genetics at the University of Pennsylvania told Live Science.

This may be because the twins split from each other early on in their development. Another reason could be that the egg split after at least a dozen cells accumulated but did not divide evenly between the siblings, meaning one gets a clump of cells carrying common mutations while the other did not.

The study does not specify where in the genome these mutations occur. While some of the mutations are inconsequential, some may lead to diseases, says senior author Kari Stefansson, CEO of deCODE genetics.

However, these genomic differences are still quite rare and it’s unclear how many of these mutations would result in a functional change to how cells work.

“I doubt these differences will have appreciable contribution to phenotypic [or observable] differences in twin studies,” added Gao.

The main takeaway of this study is to show that identical twins do not share 100% of their DNA. This is important, as it cautions against overestimating the influence of the environment on one of the siblings developing a given disease, physical appearance or trait.

Previously, researchers believed the physical differences between identical twins were likely caused by environmental factors like nutrition or lifestyle.

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