(CNN) -- Identical twins may not be quite as identical as we thought; researchers in Iceland have discovered genetic differences that begin at the early stages of embryonic development.
Scientists have long used the study of identical twins to examine the effects of nature versus nurture, as the accepted view has been that, because they share the same genes, any physical or behavioral differences between such siblings must be down to outside influences.
However, this may not be the case, suggested the new research, published Thursday in the journal Nature Genetics.
Identical twins come from a single fertilized egg, or zygote.
In any embryo, cell division can lead to mutations, but this type of genetic difference had not previously been measured between identical twins.
Over the course of a four-year study, a team of Icelandic researchers at DeCode Genetics, a Reykjavík biopharmaceutical company, found that monozygotic, or identical, twins have genetic differences that begin in the early stages of embryonic development
The scientists sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to track mutation divergence. The authors found that twins differed by 5.2 early developmental mutations, on average.
In approximately 15% of twin pairs, one sibling carried a high number of these mutations that the other twin did not have.
A genetic mutation is an error or a change in 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 can lead to illnesses such as cancer. Mutations can also affect physical attributes like 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 the siblings. However, the new research goes a step beyond that by including the DNA of the extended family.
Some of the research subjects revealed startling differences, study co-author Kari Stefansson told CNN.
"We found a pair of identical twins where a mutation was found in all cells in the body of one of them but was not present in the other twin at all. Then we found twins where a mutation was found in all cells in the body in one twin, but in only 20% of the cells of the other," said Stefansson, the founder and CEO of DeCode Genetics, which is a subsidiary of the US pharmaceutical company Amgen.
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 wider society are fascinated by identical twins, adding, "There's something magical about the connection between identical twins."
His team's research, however, is more about what divides than unites them.
"Imagine you have identical twins who are reared apart. If one of them develops autism, the classic interpretation is that it is down to environmental factors. But our work shows that before you conclude that it's caused by the environment, you have to sequence the genome of the twins to know what could account for the autism," Stefansson said.
The "mutation divergence," he told CNN, could account for a range of "devastating childhood illnesses" such as severe epilepsy and a range of metabolic disorders.
"It's absolutely amazing how large a percentage of such horrific syndromes of very early childhood are down to genome mutations," he said.
"This is an extraordinary, exciting and insightful effort to pinpoint 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 was not involved in the research.
"It is well known that MZ co-twins do not show perfect resemblance and that some dissimilarities may reflect genetic differences. The present study offers new information as to 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 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 prenatally to correct certain genetic disorders.
"There are many tantalizing questions raised by this extraordinary study," she said.