‘Identical’ twins may develop more differences in the womb than previously thought

Identical twins may not be as genetically similar as we had previously thought, according to a new study.

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Researchers found that identical twins can begin to develop genetic differences while still in the embryonic stage, raising significant questions about our understanding of ‘nature versus nurture’.

What was the study?

The study of twins has long been recognised as having particular scientific value, as it allows scientists to measure the impacts of external factors - such as environment and behaviour - on genetically near-identical subjects.

However, the findings of this study demonstrate that identical, or monozygotic, twins can actually have fairly significant genetic differences, which can start to occur very early in development.

The study, carried out by researchers at Iceland’s deCODE genetics project, involved sequencing the genomes of 387 pairs of identical twins as well as their spouses, children and parents, with a view to tracking their genetic mutations.

Researchers found that mutations occurred during embryonic growth, and that identical twins tended to have 5.2 differing early developmental mutations, though in 15 per cent of twins, this number was higher.

A mutation in this context refers to a tiny change within a cell of the body which can influence certain physical characteristics or affect susceptibility to certain diseases later in life.

When mutations did occur early in embryonic development, the changes were usually then widespread throughout the cells of the person, as well as being passed down to their children.

What do the findings mean?

Co-author of the paper and the head of deCODE genetics, Kari Stefansson, said: “The classic model has been to use identical twins to help you to separate the influence of genetics versus environment in analysis of diseases.

“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. But that is an extraordinarily dangerous conclusion.”

The findings also cast doubt on the very terminology we use to describe twins who come from the same egg, with Stefansson saying he will now use the term ‘monozygotic’.

While they are generally known as identical twins, the technical term is monozygotic, as they come from a single fertilised egg which splits into two.