The mystery of why some twins are identical may have been solved by scientists, who claim their theory could be used to identify people who lost a sibling before birth.
An estimated 12 per cent of human pregnancies start as multiple embryos, but just under two per cent go on to become fully formed twins – so many people are unaware they fleetingly shared the womb with a potential brother or sister.
However, scientists believe they have now identified a common trait among babies born after a fertilised egg splits into two embryos, calling into question the prevailing theory that it is a random occurrence with no root cause.
International researchers, led by Vrije Universiteit Amsterdam, believe they have found a common “signature” in DNA.
They found that identical twins across the world share similar marks at 834 points across their genome.
This means analysis of a person’s DNA could determine with 80 per cent accuracy if they were an identical twin but – as is the case in some 10 per cent of pregnancies – lost their twin in the womb, or if they were separated from their twin at birth.
It is not yet clear whether the 834 genome marks are the reason someone is an identical twin, or if changes to the genome occur following the random splitting of fertilised eggs.
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But scientists believe it is a plausible working theory which could be confirmed with future research.
It may also be helpful in the treatment of many congenital disorders which disproportionately affect identical twins.
Nancy Segal, a developmental psychologist at California State University, said it was a “very, very important finding” as identical twins are predisposed to a variety of conditions, including spina bifida.
Identical twins are always the same sex
Everyone has the same chance of having identical twins: about 1 in 250.
One third of all twins will be identical and two thirds non-identical. Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in two, creating twins who share the same genes.
Identical twins are always the same sex.
Non-identical (dizygotic) twins occur when two separate eggs are fertilised and then implant into the womb. They are no more alike than any other two siblings. Such twins are more common and can be same or different sexes.
Non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese. Some women inherit a tendency to release more than one egg during ovulation, making non-identical twins more likely.
The research was published in the journal Nature Communications.