For the first time, researchers have found a biological cause for the birth of identical twins.

And that is a breakthrough, says researcher Dorret Boomsma. “This is a very big discovery. The origin and birth of identical twins has always been a complete mystery. It is one of the few traits in which genes play no or very modest role.”

monozygotic and dizygotic

Identical twins arise from a single embryo that divides early in pregnancy. This clearly distinguishes them from fraternal twins that arise after double ovulation. This means that two eggs are released, which are then both fertilized, so that two embryos are created. While identical twins look very similar and are always of the same sex, fraternal twins can be very different, both in appearance and gender.

Cause

Previous research has shown that the genetic predisposition of the mother plays a major role in the development of fraternal twins. But much less is known about the origin of identical twins. The new study by Boomsma and colleagues changes that. The scientists show in their study – published in the magazine Nature Communications – shows that identical twins have a unique epigenetic profile.

Epigenetics

While genetics focuses on hereditary characteristics that are recorded in the genetic code (which in turn consists of building blocks of DNA that are placed one behind the other in a specific order), epigenetics focuses on hereditary characteristics that are not in that DNA, but in the DNA. so-called methylation elements are fixed around it. Those elements determine whether genes are on or off. To investigate the origin of identical twins, researchers measured the level of DNA methylation at more than 400,000 sites in the DNA of more than 6,000 Dutch, British, Australian and Finnish twins.

Results

And so the researchers came across a unique epigenetic profile. “We found 834 sites in the DNA where the level of methylation in monozygotic twins was different than in non-twins. These locations in the DNA are involved in functions in early embryonic development,” says researcher Jenny van Dongen. The research not only provides more insight into the origin of identical twins, but may also lead to a better understanding of congenital abnormalities that sometimes occur more often in identical twins.

But the study has another interesting implication. “A particularly surprising finding is that we can determine from the epigenetic profile whether someone is an identical twin,” says Van Dongen. Now, with most identical twins after birth, there is little doubt about that. But it is also quite common that in an early stage of an identical twin pregnancy, one of the fetuses dies and then disappears. “In English, this becomes the ‘vanishing twin syndrome‘ (disappearing twin syndrome),” says Van Dongen. The tissue of the deceased fetus is then taken up by the mother’s body. Today, thanks to early ultrasounds, mothers often know that they have been pregnant with twins and have lost a fetus, but that was different in the past. Taking into account the fact that it vanishing twin syndrome an estimated 1 in 8 twin pregnancies, so there are a lot of people walking around who – without knowing it – are part of identical twins. Until recently, this could not be proven, but this research is changing that. Because the evidence that they are part of identical twins turns out to be permanently stored in their epigenome.