Identical twins or monozygotic twins are born from a single fertilized egg, that splits into two embryos. The reason for the split is unclear, is mostly believed to be caused by a “random” biological process.

Despite all the progress in medical science, the reason why some twins are born looking identical to each other had remained a mystery. On the other hand, a lot of progress has been made in understanding the biological origins of nonidentical twins.

Now, a team of researchers from the Vrije Universiteit in Amsterdam, the Netherlands has made a ground-breaking discovery, providing new insights into the formation of identical twins. The discovery raises hopes for the development of treatments for congenital disorders that afflict identical twins disproportionately.

In a paper published in the journal Nature Communications, the researchers have revealed that the epigenetic information within the chromosomes between identical and not identical twins is different.

These epigenetic differences are found in the chemical marks associated with the DNA code, but not within the DNA itself, and can selectively turn “on” and “off” genes without changing the underlying DNA sequence. The researchers found identical twins from across the world, shared similar marks in their DNA code.

“This is a very big discovery. The origin and birth of identical twins have always been a complete mystery. It is one of the few traits in which genetics plays no or very modest role. This is the first time that we have found a biological marker of this phenomenon in humans. The explanation appears not to lie in the genome, but its epigenome,” said Professor Dorret Boomsma, lead author of the paper.

The paper doesn’t conclusively prove that the chemical marks around the DNA are the cause behind identical twins being born looking like each other. The marks could well be a consequence of the process rather than the cause. However, the discovery might be helpful in further research.

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