The sometimes-preternatural similarity of identical twins is more profound than previously thought. Identical twins, known to science as “monozygotic”, may share more than identical looks and genes, according to new research in the field of epigenetics.

Monozygotic twins form when a single embryo, derived from a single fertilised egg, called a zygote, splits in the womb to form two separate individuals with identical genomes. As such, they have long been a source of fascination for geneticists, as they offer insight into the relative contribution of nature and nurture.

Epigeneticists are now similarly fascinated. Epigenetics is the study of the way in which various molecular mechanisms affect the way genes are expressed. The most common of these is DNA methylation, in which “methyl groups” – molecules derived from methane, containing one carbon atom bonded to three hydrogen atoms – come from the environment and attach to DNA, affecting the way genes function. These markers can lead to profound differences. For example, research from 2017 indicates that the human capacity for speech is of epigenetic, rather than genetic, origin.

Given that epigenetic markers are derived from the environment it’s not surprising that past research in the area has revealed substantial epigenetic difference between monozygotic twins. For example, a 2011 paper by Jordana Bell and Tim Spector of King’s College London, UK, found there was significant variation in epigenetically-linked diseases in identical twins.

Now, however, an international team of researchers led by geneticist Robert Waterland of the US Department of Agriculture Children’s Nutrition Research Centre and Baylor College of Medicine in Houston, Texas, US, has published findings that paint a very different picture.

The team zeroed in on a class of epigenetic markers that are stable and exist in all cell types, called “metastable epialleles” (MEs). The epigenetic variation at MEs is determined randomly and is influenced by many environmental factors, ranging from the nutritional breakdown of the mother’s diet to the season. Consequently, it was expected that levels of epigenetic similarities and differences for MEs would be similar for both identical and fraternal, or non-identical, twins.

What they found was something of a shock.

Their research, published in Genome Biology, shows that monozygotic twins have identical epigenetics at MEs. “We found that the methylation patterns matched almost perfectly in identical twins, a degree of similarity that could not be explained by the twins sharing the same DNA,” says Waterland. “We call this phenomenon ‘epigenetic supersimilarity.’”

The striking finding had a simple explanation: “If, in this group of genes, the epigenetic markers are established before the embryo splits into two, then the markers will be the same in both twins,” says Waterland.

“In essence, both twins inherit an intimate molecular memory of their shared developmental legacy as a single individual. On the other hand, genes at which epigenetic markers are set after the embryo splits can have greater epigenetic differences between the two twins.”

Further study into epigenetic supersimilarity may hold yet more surprises: by teaming up with cancer epidemiologists from the Cancer Council Victoria’s Melbourne Collaborative Cohort Study in Melbourne, Australia, the scientists have also detected a link between methylated MEs and the risk of developing certain types of cancer.

These results agree with Waterland’s earlier research, and suggest that the field of epigenetics may well be crucial to the future of medicine.