Monozygotic Twins
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Identical twins have long been touted as an incredible resource for genetics because they're "genetically identical". But new research suggests that might not be quite true.

Identical twins are called monozygotic because they come from a single zygote that has formed from a single sperm and egg. As the cells multiply, they can split into two individuals before they become embryos that have exactly the same genetic material (or so we thought).

Hákon Jónsson of deCODE genetics, Iceland, and colleagues have found that, on average, identical twins differ by about 5.2 developmental mutations. In around 15% of twins, one twin had a high number of these mutations and the other didn't, they report in their paper, published in Nature Genetics.

The team sequenced the genomes of 387 pairs of identical twins and compared their genes with their parents, siblings and offspring, to track the mutations that may have occurred in the womb.

The mutations detected may seem infinitesimal and inconsequential, but the result requires consideration for those undertaking twin studies, especially in cases where one twin has a trait that the other doesn't have.

Because of our understanding of how identical twins formed, the notion that they were genetically identical in basically every way meant all the differences between them, both in behaviour and physical traits, were assumed to be caused by the environment.

But these tiny mutations might be a hidden cause of some quirks.

"This assumes that the contribution of mutations that separate monozygotic twins is negligible; however, for some diseases such as autism and other developmental disorders, a substantial component is due to de novo [from the beginning] mutations," the authors write in their paper.

Looping back to that zygote could explain why these mutations happen.

We have two different types of cell lines: somatic and germline. Somatic cells make up our organs and body and are constantly replicating and changing. Germline cells, like eggs and sperm, carry DNA that will be used in creating offspring; these cells don't experience the same accumulation of mutations after birth because they don't replicate as much.

Contrary to what we learned in high school genetics, germline DNA can mutate even after a zygote is formed. The whole process of this early development can be thought of as giving cells "jobs" and telling them what to become.

When a stem cell is allocated a cell type it will replicate and divide, and the cells born from it will have mostly the same genome and programming.

But every time a cell replicates, there is a chance a mutation will occur. This mostly happens to somatic cells, but at this stage it can happen to the germline ones too, albeit at a very low frequency.

This means that the genetic mutations that amass between identical twins is really dependent on how and when those cells are assigned jobs.

"The accumulation of mutations is a function of the number of cell divisions, regardless of whether the mutation is induced by cell division or DNA damage before cell division," the researchers write.