Some identical twins don't have same DNA

Rodiano Bonacci
Gennaio 11, 2021

This is not the first study to suggest differences between so-called identical twins. In approximately 15% of twin pairs, one twin carries a high number of these mutations that the other twin does not have. In the process, they measured embryonic growth and concluded that identical twins differed by an average of 5.2 early developmental mutations, with almost 15 per cent twins having higher number of diverging mutations.

But according to new research published in Nature, identical twins aren't necessarily genetically identical.

Hakon Jonsson and colleagues sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track mutation divergence.

The researchers said some twins may arise when a single cell or a small group of cells splits off from the embryo.

Genetic differences between identical twins can begin very early in embryonic development, according to a study yesterday that researchers said has implications for how these siblings help scientists tease out the effects of nature versus nurture. On average, pairs of twins may differ genetically by 5.2 mutations in early development. One of the things we did is to look at the kind of mutations that happen during development, and we were able to show that the type of mutations that happen early on differ from the mutations that happen later-which is not a surprise to anyone, really.

In one of the pairs of twins studied, for example, a mutation was present in all of cells in one sibling's body - meaning it likely happened very early in development - but not at all in the other twin.

"But our work shows that before you conclude that it's caused by the environment, you have to sequence the genome of the twins to know what could account for the autism", Stefansson said. This way, they were able to have a full picture of the families' genomes and see which mutations were passed down to which twin and then to their offspring. "Mutations also accumulate in the eggs of the mother and sperm of the father". We call it. 'before the primordial germ cell specification.' That is the most important determination of showing that this is an early mutation.

"So if you take identical twins raised apart and one of them developed autism, the classic interpretation has been that that is caused by the environment", explained Kari Stefansson, co-author of the paper and the head of Iceland's deCODE genetics, a subsidiary of the USA pharmaceutical company Amgen. The later the split takes place, the more cells will have accrued when the twins separate.

Twins are two individuals who form in the same uterus. But I think that. when you're trying to understand whatever differences there may be between monozygotic twins, you should take a step or two away from the term 'identical'.

These eggs form separately, in separate sacs adjacent to each other. Indeed, this does not mean they are inherently sharing 100 percent of their DNA by the time they are born.

"Our results indicate that allocations of cells during development shapes genomic differences between monozygotic twins", they said.

But at some point, it splits from one zygote into two.

Furthermore, the authors noted they had no idea which twins were sharing an amniotic sac, chorion, or placenta.

Those that share the same amniotic sac are known as monoamniotic, while those that have their own are diamniotic.

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