Think identical twins share identical genetics (hence their name)? Think again.

This surprising finding may be of great significance for research on hereditary diseases and for the development of new diagnostic methods. How can it be that one identical twin might develop Parkinson’s disease, for instance, but not the other? Until now, the reasons have been sought in environmental factors.

An international team of researchers have reported that differences between discordant monozygotic twins — where one twin has a genetic disorder and the other does not — are probably due to copy number variations, not environmental factors.

The researchers studied 19 pairs of monozygotic twins and found differences in copy number variations in DNA. Copy number variations (CNVs) occur when a set of coding letters in DNA are missing, or when extra copies of segments of DNA are produced.

“Even though the genome is virtually identical in identical twins, our results show that there in fact are tiny differences and that they are relatively common,” Jan Dumanski from the University of Alabama at Birmingham, Department of Genetics, who led the study.

“This could have a major impact on our understanding of genetically determined disorders.”

“By uncovering these small genetic differences in identical twins where one of them is sick, we have a way of tying specific genetic changes to the genesis of common diseases,” Bruder said.

The researchers think their findings indicate that CNVs 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,” Carl Bruder, a co-researcher, said.

“If twin A develops Parkinson’s and twin B 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.”

The study appeared in the February 14 issue of the American Journal of Human Genetics.

Source: University of Alabama and Uppsala University