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Scientists have long used the study of identical twins to examine the effects of nature versus education, as the accepted view is that because they share the same genes, any physical or behavioral differences between these siblings must be due to outside influences.
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 has not yet been measured between identical twins.
Scientists have sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children to follow the divergence of mutations. The authors found that the twins differed by 5.2 early developmental mutations, on average.
In about 15% of twin pairs, one sibling carried a high number of these mutations that the other twin did not.
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 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 genetic differences between siblings. However, the new research goes a step further by including DNA from 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, Stefansson said, because 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 science and society at large are fascinated by identical twins, adding: “There is something magical about the bond between identical twins.”
However, her team’s research 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 concluding that it is caused by the environment, you have to sequence the genome of the twins to find out what might explain autism, ”Stefansson said.
The “divergence of mutations,” he told CNN, could explain a range of “devastating childhood illnesses” such as severe epilepsy and a range of metabolic disorders.
“It is absolutely amazing how much of a percentage of these horrible syndromes in very early childhood are due to genomic mutations,” he said.
“This is an extraordinary, exciting and insightful effort to identify the earliest cellular mechanisms that explain the genetic differences between MZ (monozygotic) twins,” said Nancy Segal, author and professor of psychology who studies twins at the California State University, Fullerton, and has not been. involved in research.
“It is well known that MZ co-twins do not bear a perfect resemblance and that some differences may reflect genetic differences. The present study offers new insight into the source of the differences in MZ co-twins,” Segal said , also director of the CSU Center for Twin Studies.
The research did “not nullify 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 results should be applied, such as whether to intervene before birth to correct certain genetic disorders.
“There are many tantalizing questions raised by this extraordinary study,” she said.
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