It’s been common knowledge for a long, long time that identical twins are exact genetic duplicates of each other. This sameness has made them highly valued study participants when research on genetic issues is conducted.As science is unraveling our DNA structure on an increasingly smaller scale, however, the exactness of identical twins is coming into question, according to Jan Dumanski and Carl Bruder, co-directors of an international study conducted at the Unviersity of Alabama in Birmingham.

Ever-mystifying has been the reason why one twin develops a chronic illness while the other never does. Environmental factors have been thought to be the triggering factor in such diagnoses but no firm conclusions have been established.

The Dumanski-Bruder study, recently published in the American Journal of Human Genetics, however, identifies very small differences in DNA segments between one identical twin and his or her identical sibling. They find that a particular DNA segment may be duplicated or even missing in one twin but no similar sequencing glitch is detected in the other twin.This variance in DNA sequencing may account for the reason one twin becomes sick while the other does not.