Scientists have long used the study of identical twins to study the effects of nature on upbringing, as it is believed that because they share the same genes, any physical or behavioral differences between such siblings must be due to external influences.
Identical twins come from a fertilized egg or zygote.
In any embryo, cell division can lead to mutations, but this type of genetic difference has not previously been measured between identical twins.
Scientists sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track the divergence of the mutation. The authors found that twins differed on average by 5.2 early mutations in development.
A genetic mutation is a mistake or change in DNA. A mutation occurs when the sequence of the genetic code breaks or changes in some way. Although most mutations are harmless, some can be serious and lead to diseases such as cancer. Mutations can also affect physical characteristics such as hair color.
This is not the first study to suggest differences between so-called identical twins. A paper published in The American Journal of Human Genetics in 2008 revealed some genetic differences between siblings. However, the new study goes a step beyond that, involving the DNA of the extended family.
Some of the subjects found striking differences, study co-author Carrie Stephenson told CNN.
The implications of this are significant, according to Stefanson, as the study led the team to conclude that the “role of genetic factors” in shaping the differences observed between monozygotic twins was “underestimated.”
He acknowledged that both science and the wider society are fascinated by identical twins, adding: “There is something magical about the relationship between identical twins.”
However, his team’s research is more about what divides them than unites them.
“Imagine you have identical twins who were raised by each other. If one of them develops autism, the classic interpretation is that it depends on environmental factors. But our work shows that before you conclude that it is caused by the environment, you have to sequence the twins’ genome to know what autism might explain, ”said Stephenson.
“Mutation mismatches,” he told CNN, could explain a number of “devastating childhood illnesses” such as severe epilepsy and a number of metabolic disorders.
“It’s absolutely amazing how large a percentage of such horrific syndromes from very early childhood are to genomic mutations,” he said.
“It’s an extraordinary, exciting and insightful effort to identify the early cellular mechanisms that explain the genetic differences between MZ (monozygotic) twins,” said Nancy Seagal, author and professor of psychology who studies twins at California State University, Fullerton, and not was involved in the study.
“It is well known that MZ twins do not show a perfect resemblance and that some differences may reflect genetic differences. The current study offers new information on the source of the differences in MZ twins,” said Segal, who is also director of the CSU Twin Center.
The study “does not deny environmental factors in early and late development,” she added, but showed that “some twin models underestimate the genetic effects and require revision.”
The study, according to Segal, also raises questions about how the findings should be applied, such as whether to intervene prenatally to correct certain genetic disorders.
“There are a lot of exciting issues raised by this extraordinary study,” she said.