New Research From deCODE Genetics: Identical Twins Not Always With Identical Genes

An extensive study conducted by deCODE Genetics and published in Nature shows that, despite what we have believed about monozygotic twins for years, they do not always share identical genes.

The study found that “monozygotic twins differ on average by 5.2 early developmental mutations and that approximately 15% of monozygotic twins have a substantial number of these early developmental mutations specific to one of them.”

Researchers sequenced the genomes of 380 pairs of identical twins, two triplets, and their immediate families. As Kári Stefánsson, senior co-author of the study and the CEO of deCODE Genetics told Business Insider, “As cells replicate, and replicating machinery makes a copy of the genome – it’s not flawless. Mutations also accumulate in the eggs of the mother and sperm of the father.”

In their study, deCODE found that in about 15% of the monozygotic twins studied, one twin had a significantly higher number of mutations than the other. These mutations can be somatic—mutations that get passed to every resulting cell but are not necessarily present in every cell in the body—or hereditary, where they are supposedly copied in every cell in the body.

Which twins get which mutations is a matter of “coincidence or stochastic mechanism,” Kári said, referring to components that occur on a molecular level.

The implications of the study are profound. Apart from it up-ending decades of conventional wisdom and scientific study on monozygotic twins, it may have applications in numerous fields, from forensics to hereditary science.

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