Scientists have long used the study of identical twins to examine the effects of nature versus creation, since the accepted view is that, because they share the same genes, any physical or behavioral difference between these brothers is due to outside influences.
Identical twins come from a single fertilized egg, or zygote.
In any embryo, cell division can lead to mutations, but that kind of genetic difference has not been measured previously between identical twins.
Scientists sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track the mutation divergence. The authors found that the twins differed by 5.2 initial developmental mutations, on average.
In approximately 15% of the twin pairs, one brother carried a large number of these mutations that the other twin did not have.
A genetic mutation is a mistake or a 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 like cancer. Mutations can also affect physical attributes like hair color.
This is not the first study to suggest differences between so-called identical twins. An article published in The American Journal of Human Genetics in 2008 revealed some genetic differences between the brothers. However, the new research goes a step further, including the DNA of the extended family.
Genetic differences
Some of the research subjects revealed surprising differences, study co-author Kari Stefansson told CNN.
The implications of this are significant, according to Stefansson, as the research led the team to conclude that “the role of genetic factors” in forming the observed differences between monozygotic twins “has been underestimated.”
He acknowledged that both science and society in general are fascinated by identical twins, adding, “There is something magical about the connection between identical twins.”
His team’s research, however, is more about what divides them than what unites them.
“Imagine you have identical twins who are raised separately. If one of them develops autism, the classic interpretation is that it is due to 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 might be responsible for autism, “said Stefansson.
The “mutation divergence,” he told CNN, could be responsible for a series of “devastating childhood illnesses”, such as severe epilepsy and a series of metabolic disorders.
“It is absolutely incredible how a large percentage of these horrible early childhood syndromes are due to mutations in the genome,” he said.
“This is an extraordinary, exciting and insightful effort to identify the early cellular mechanisms that explain the genetic differences between MZ (monozygotic) twins,” said Nancy Segal, author and professor of psychology who studies twins at California State University, Fullerton, and not was involved in the research.
“It is well known that MZ co-twins are not perfectly similar and that some differences may reflect genetic differences. This study offers new information on the source of the differences between MZ co-twins,” said Segal, who is also director of the CSU Twin Study Center.
The research “did not deny environmental factors in early and later development,” she added, but showed that “some twin models underestimate the genetic effects and require revision”.
The study, according to Segal, also raised questions about how the findings should be applied, such as whether to intervene in prenatal care to correct certain genetic disorders.
“There are many tempting questions raised by this extraordinary study,” she said.