Not all identical twins are alike
Study finds pairs do not always share 100 percent of their DNA - with the average set differing by 5.2 mutations
A new study reveals identical twins are not carbon copies of each other when it comes to their DNA.
Scientists in Iceland discovered that embryos do not split evenly while twins form in the womb, resulting in one carrying more mutations, or alterations in a sequence of DNA, than another.
The team sequenced DNA of 387 pairs of identical twins, along with their parents, spouse and children, which allowed them to see early mutations unique to each twin.
The results show that, on average, identical twins differ by 5.2 mutations and about 15 percent of all pairs have genetic mutations, with some of them differing by up to 100.
These differences represent a
tiny portion of each twin's genetic code, but they could influence why one twin is taller or why one is at greater risk for certain cancers.
Identical twins, or monozygotic twins, form in the womb, like most babies do, when an egg is fertilized by one sperm
and attaches to the uterus wall.
At the very earliest stage of development, when the zygote is nothing more than cluster of cells, the egg sometimes splits and develops two babies – the odds of this happening are three in 1,000.
Identical twins have played a major role in genetics research, as scientists have long studied what makes them the same, but little work has been about their genomic differences.
The new study analyzed the early development of identical twins and used DNA from across three generations.
A total of 387 pairs of identical twins and two sets of triplets, along with their family members, had their whole genomes sequenced for this experiment.
Not only were scientists able to see which mutations appeared in which twin, the results showed which of the mutations were passed down to the twin's offspring.
If one was passed down, scientists could conclude that it first appeared in the egg and sperm – also known as a germline mutation – which set the stage for twins.
And if that same germline mutation shows up in a parent, it also means it formed during the twin's early development stage. This is called somatic mutation.
Kari Stefansson, a geneticist at the University of Iceland and the company deCODE genetics, said that his team had found pairs of twins where a mutation is present in all cells of the body of one twin, but not found in the other twin at all.
However, 'sometimes the second twin may show the mutation in some cells, but not all cells,' he said.
Using all of these findings, the team was able to identify which mutations surfaced in the early development state and which twin collected it.