Scientists have long used the study of identical twins to investigate the effects of nature versus nurture, as the accepted view was that because they share the same genes, physical or behavioral differences between such siblings must be due to external influences.
Identical twins hatch from a single fertilized egg or zygote.
Cell division can lead to mutations in any embryo, but this kind of genetic difference had not previously been measured between identical twins.
The scientists sequenced the genomes of 387 pairs of identical twins and their parents, husbands and children to monitor mutation divergence. The authors found that twins differed an average of 5.2 early developmental mutations.
In about 15% of the twin pairs, one sibling carried a large number of these mutations that the other twins did not.
A genetic mutation is an error or a change in the DNA. A mutation occurs when the sequence of the genetic code breaks or changes in some way. While 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 isn’t 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 the siblings. However, the new research takes it a step further by including the DNA of the extended family.
Genetic differences
Some of the subjects showed 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 shaping the differences observed between monozygotic twins “has been underestimated.”
He acknowledged that both science and society in general are fascinated by identical twins, adding, “There’s something magical about the connection between identical twins.”
However, his team’s research is more about what divides them than unites them.
“Imagine you have identical twins raised separately. If one of them develops autism, the classic interpretation is that it has to do with environmental factors. But our work shows that before concluding that it caused by the environment, you have to sequence the genome of the twins to know what could explain the autism, ”Stefansson said.
The “mutational divergence,” he told CNN, could be the cause of a series of “devastating childhood diseases,” including severe epilepsy and a range of metabolic disorders.
“It is absolutely amazing how large a percentage of such horrific very early childhood syndromes are due to gene mutations,” he said.
“This is an extraordinary, exciting and insightful attempt to locate early cellular mechanisms that explain genetic differences between MZ (monozygotic) twins,” said Nancy Segal, an author and professor of psychology who studies twins at California State University, Fullerton, and that was not. involved in the investigation.
“It is known that MZ twins do not have a perfect resemblance and that some inequalities may reflect genetic differences. The current study provides new information about the source of MZ co-twin differences,” said Segal, who is also director of CSU’s Twin Studies Center. .
The research “did not negate environmental factors in early and later development,” she added, but showed that “some twin models underestimate the genetic effects and need to be revised.”
The study also raised questions about how the findings should be applied, such as whether to intervene prenatally to correct certain genetic conditions, according to Segal.
“There are many tantalizing questions raised by this extraordinary study,” she said.