Identical twins not so identical after all

Identical twins don't always have identical genes, research shows - a new twist in the nature versus nurture debate

Identical twins don't always have identical genes, research shows - a new twist in the nature versus nurture debate

For decades, researchers believed that monozygotic twins - twins that form when a fertilized egg splits in two - were born with essentially the same genetic codes.

He acknowledged that both science and wider society are fascinated by identical twins, adding, "There's something magical about the connection between identical twins".

A closer look at identical twins shows they are not so identical after all. Rather, their genomes often differ in small but significant ways from the very first week of development.

On average, identical twins have 5.2 of these early genetic differences, the researchers found.

The researchers found that monozygotic twins' DNA were actually different by an average of 5.2 developmental mutations, and even higher in 15% of the twins.

© Magnus Andersen/deCODE genetics Kari Stefansson, CEO of deCODE genetics and the lead author of the Nature study.

"The authors conclude that the role of genetic factors in shaping such phenotypic differences has been underestimated", the deCODE news release said.

Identical twins don't always have identical genetics, a study has found, which could explain why one may have autism but the other does not. "But that is an extraordinarily risky conclusion".

The study investigators approximate that in roughly 15 percent of identical twin pairs, one pair is carrying a considerable number of mutations other pairs are not sharing. Before next-generation sequencing became widely available around 2005, it could take 10 years or more to sequence a genome.

At the very earliest stage of development, when the zygote is nothing more than cluster of cells, the egg sometimes splits and develops two babies - the odds of this happening are three in 1,000.

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"As cells replicate, and replicating machinery makes a copy of the genome - it's not flawless", Stefansson said.

According to Gao, one limitation of this research is that the study investigators collected data from blood samples and swabs from the cheek, but they did not gather DNA from sperm or eggs.

© Ott et al./Microbiome, 2020 Transmission electron microscopy image of a D. radiodurans cell splitting.

A genetic mutation is an error or a change in DNA.

"These mutations are interesting because they allow us to explore how twin pregnancies occur", he says.

Fraternal or dizygotic twins are formed by two eggs that are fertilised by two sperm and produce two genetically unique children.

The new study focuses specifically on mutations that occur as or before embryos form from the mass of cells inside the blastocyst, a structure that implants in the uterine wall. But before you can make that interpretation, you'd better make sure that one of them does not have a de novo mutation in an important gene that the other one does not.

"Our results indicate that allocations of cells during development shapes genomic differences between monozygotic twins", they said. "But that is an extraordinarily risky conclusion", the co-author of the paper and head of Iceland's deCode genetics, Kari Stefansson, tells The Guardian.

These mutations may result in one of the twins having autism or other developmental disorders, while the other does not.

"Coincidence, or stochastic mechanism, decides what happens to each twin", Stefansson said.

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