Identical twins may share the same DNA, but variations in which genes are active may lead to some important differences.

A new study shows that differences in physical appearance or disease risk found in identical twins may be caused by variations in which of the matching genes are activated.

The results appear in the current edition of Proceedings of the National Academy of Sciences.

Identical twins are produced from a fertilized egg that has divided in two; they share identical genetics. Yet not all of these twins appear identical and disease risk may also differ.

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Patterns of Genetic Activity

In the study, researchers looked at the genetic profiles of 80 sets of identical twins in Spain, who ranged in age from 3 to 74 years old.

Researchers found that in 35 percent of the twin pairs, individual twins had significantly different patterns of active genes. The remaining 65 percent had identical patterns of genetic activity.

The results showed that while young twins were genetically indistinguishable, older twins displayed major differences in the content and distribution of their genetic information, which resulted in unique gene activation profiles.

The study also showed that the greatest differences were seen in twin pairs who had spent less of their lifetimes together or had different medical histories.

Researchers say the results support the theory that environmental factors, including smoking, diet, and physical activity may affect a person’s gene activity and explain some of the differences in disease risk found among identical twins.

The study helps provide insight into the genetic causes of disease risk in identical twins but may be generalized to nontwins.

Read Web MD's "Twins' Close Bonds Lengthen Life."

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCE: Fraga, M. Proceedings of the National Academy of Sciences, July 6, 2005, early online edition.