Identical Twins Not As Similar As Believed

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Washington, : Contrary to previous belief, identical twins are not genetically identical, a new study found.

The researchers studied 19 pairs of monozygotic, or identical, twins and found differences in copy number variation in DNA. Copy number variation (CNV) occurs when a set of coding letters in DNA are missing, or when extra copies of segments of DNA are produced, the study said.

Humans receive one chromosome from their mother and one from their father, providing for two copies of the genome, the scientists informed.

In some cases, bits of DNA are missing from a chromosome, leaving the offspring with just one copy of that bit of DNA. In other instances, mutations may produce three, four or more copies of a particular bit of DNA, they added.

In most cases, variation in the number of copies likely has no impact on health or development. But in others, it may be one factor in the likelihood of developing a disease.

"The presumption has always been that identical twins are identical down to their DNA," said Carl Bruder, PhD and Jan Dumanski, PhD, of University of Alabama's Department of Genetics and the study"s lead authors.

"That"s mostly true, but our findings suggest that there are small, subtle differences due to CNV. Those differences may point the way to better understanding of genetic diseases when we study so-called discordant monozygotic twins...a pair of twins where one twin has a disorder and the other does not," Sciencedaily quoted them as saying.

Dr Bruder pointed out that one twin might develop a particular disease Parkinson"s, for example while the other does not.

Previously, it was thought that environmental factors were the likely culprits, not genetics. Dr Bruder and Dr Dumanski think their findings indicate that CNV may play a critical role and this can be efficiently studied in identical twins.

"More importantly, changes in CNV may tell us if a missing gene, or multiple copies of a gene, are implicated in the onset of disease," Dr Bruder said. "If twin A develops Parkinson"s and twin does not, the region of their genome where they show differences is a target for further investigation to discover the basic genetic underpinnings of the disease," he added.

The findings of the research were published online in the American Journal of Human Genetics.

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