While it has long been assumed that identical twins occurred at random, a new study reveals that this is not actually the case.
Instead, researchers from Vrije Universiteit in Amsterdam claim it’s down to a “signature” in their DNA, which lasts from conception to adulthood.
About 12% of pregnancies begin as a multiple, but only 2% of both babies are carried to term – a condition known as “vanishing twin syndrome”.
Until now, the appearance of identical twins, which are rarely found in a family, was thought to have been completely random, but the team says it is due to their own DNA.
The researchers say this DNA signature allows determining whether an individual has been portrayed as an identical twin, even if they are not known to be a twin. It is not yet possible to determine when these signatures will appear, so they need to investigate further whether they are inherited from the parents or appear randomly when the egg splits.
Jenny van Dongen, Dorit Boomsma and colleagues studied large international twin sets to identify signatures in DNA that were specific to identical twins.
The researchers discovered 834 points across the genome of identical twins, which were born after a fertilized egg splits into two embryos.
They say this DNA measure can be used to determine if someone is an identical twin with 80% accuracy.
This will allow them to identify those who have lost twins in the womb, or were separated from twins at birth. What the team can’t say yet is whether the DNA marks are the reason a person is an identical twin, or whether they occur after the random splitting of a fertilized egg.
However, evidence of genetic markers for identical twins can help treat congenital disorders that disproportionately affect that group.
To find the signs, the team scanned the DNA of more than 3,000 identical twins using blood and cheek cell samples, and compared it to that of fraternal twins.
In total, they examined 400,000 sites in each individual’s genome. This allowed them to find about 800 of those sites with differences in methylation.
This is an important process involved in DNA protection and repair, as well as energy production and cancer prevention.
“It’s possible that something created very early spreads to later cells,” van Dongen told Science.
Some of the changes made sense to the researchers, because they provide markers for genes involved in cell adhesion that can influence the division of an egg into two.
Other changes were less plausible, including some at the ends of chromosomes, because they are linked to aging – but identical twins have a similar lifespan to non-twins.
More extensive research will be required to determine whether these changes are the result of egg division, or if they are present from the start.
Karolinska Institutet’s Nancy Pedersen, who was not involved in the study, said the findings provide “fantastic insights into potential mechanisms in identical twinning” that could shed light on rare disorders involving genetic alterations.
The results were published in the journal Nature Communications.
Source: Daily Mail