To the human eye identical twins look the same, but new research reveals all identical twins may differ genetically to some extent.

Researchers have largely assumed identical twins are genetically identical, reasoning that any variations between them were due to solely environmental factors. For instance, the fingerprints of identical twins differ because they each experience slightly different conditions while in the womb.

Now scientists find that when it comes to the genetics of identical twins" there in fact are tiny differences and that they are relatively common," said researcher Jan Dumanski, a molecular geneticist at the University of Alabama at Birmingham.

In the new study, researchers analyzed 19 pairs of identical twins. Although they did possess nearly identical genomes, closer study revealed they often differed in the number of copies of individual gene segments. For instance, one twin might be missing a segment, or possess more copies of that segment than the other twin.

Such variations could explain why one identical twin can suffer from a disorder while the other remains healthy. Researcher Carl Bruder expected that even identical twins that appear completely alike harbor "copy number variations" when it comes to at least one site in their genomes.

But genetic differences between identical twins might also accumulate after development over a twin's life as well. "I think all our genomes are under constant change," Bruder told LiveScience.

Aside from understanding more about identical twins and other multiples, "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 added.

Charles Lee, Harvard Cancer Center's director of cytogenetics, noted that genetic studies of identical twins could help rapidly identify which genes are linked with a specific disorder, "because everything else is the same."

(Xinhua News Agency February 25, 2008)