Identical twins aren’t ideal clones, research shows

Pablo Tucker
January 9, 2021

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

The results of the study show that some twins deemed to be "identical" actually have substantial genetic differences. In about 15% of twin pairs, one twin carries a high number of these mutations that the other twin does not have.

His research team sequenced the complete genome of 387 pairs of monozygotic twins as well as that of their parents, spouses and children, to detect genetic mutations. "Then we found twins where a mutation was found in all cells in the body in one twin, but in only 20% of the cells of the other", said Stefansson, the founder and CEO of DeCode Genetics, which is a subsidiary of the USA pharmaceutical company Amgen. The researchers found that, on average, identical twins have 5.2 of these early genetic differences, while around 15 percent of identical twins have more. They can occur when a cell divides and makes a slight error in DNA replication.

By sorting the genital genes of twins and comparing them with close relatives, scientists at Decode Genetics found mutations that exist in only one Monozygotic twins in the first days after conception and detected them.

"It's absolutely wonderful how large a percentage of such horrific syndromes of very early childhood are down to genome mutations", he said.


This study has come years after few scientists discovered genetic differences between identical twins, concluding that twins are alike but not perfectly similar.

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. Mutations can also occur when the morula grows into a blastocyst, an early-stage embryo, less than a week into development.

Hereditary mutations, by contrast, are present in the very first cells that make up a developing embryo, which mean the mutations get passed to each cell.

Stefansson said that out of the initial mass that would go on to form the individuals, "one of the twins is made out of the descendants of the cell where the mutation took place and nothing else", while the other was not. "Twins are very alike, but it is not a ideal similarity".

They share 50 percent of their DNA, as they would with their other non-twin siblings.


"These mutations are interesting because they allow you to begin to explore the way in which twinning happens".

This means that when identical twins are being used to determine the effects of "nature vs. nurture", it shouldn't be assumed that an environmental factor is behind the difference between supposedly identical twins.

If you're an identical twin who has always resisted being called a clone of your sibling, scientists say you're right.

This is not just a study that has relevance when it comes to understanding of the genetics, but also human development: How do we probe early human development in an ethical manner, a non-interventional manner?

H. Jonsson et al., "Differences between germline genomes of monozygotic twins", Nat Gen, doi:10.1038/s41588-020-00755-1, 2021.


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