Why Are Some Twins Identical? Study Sheds Light on Mysterious DNA Epigenetics

Scientists may have made progress in understanding how identical twins are produced after conception.

The split of a fertilized egg, known as a zygote, into two embryos with highly comparable genes has been studied by Dutch researchers.

The cause for the egg splitting is uncertain, although the most popular idea is that it happens at random.

This differs from how non-identical twins arise due to two different eggs being fertilized by two sperm. It's a process that occurs in families, as opposed to "monozygotic twinning," according to the researchers.

Looking at the Epigenome

The study focused on the epigenome, or the components surrounding DNA's building blocks that govern how genes are expressed and adjusted. These epigenetic markers are "dotted" across the DNA (per Science News) rather than being a part of it (per Genome.gov).

Now, researchers from the Vrije Universiteit Amsterdam think they have solved the mystery. They discovered that identical twins worldwide have comparable chemical markers at 834 locations across their genomes, or complete sets of DNA.

Although it has yet to be proven that chemical markings on DNA induce zygote separation, experts believe it is a reasonable working idea.

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In a New Scientist report, assistant professor Jenny von Dongen said that they may have discovered a process that causes cells to split. These alterations might also occur after the cells have separated.

The researchers anticipate that the discovery of the shared markings will be useful to a wide range of people, including some who feel they had a twin who "vanished" in their mother's pregnancy.

Do You Have A Vanishing Twin?

Researchers said the discovery might help establish whether someone is an identical twin, was separated from their twin at birth, or was unaware they had lost their sibling in the womb.

Although the condition occurs in as many as 12% of pregnancies, only around 2% of moms carry both to term, leaving many individuals uninformed that they have a twin.

In a news release, von Dongen said parts of the DNA have a role in early embryonic development. The new findings might help researchers better understand congenital defects that are more common in monozygotic twins in the future.

Nancy Pedersen of the Karolinska Institute, who was not involved in the study, said in a Daily Mail report that the findings provide fascinating insights into potential processes in identical twinning. She added that the latest study might shed light on uncommon diseases involving genetic alterations.

It will also be used to determine whether someone was ever a twin, since a 'vanishing twin' may appear on an ultrasound in some circumstances, but is often absorbed without leaving a trace.

Researchers reported the study, "Identical Twins Carry a Persistent Epigenetic Signature of Early Genome Programming," in the Nature Communications journal.