Identical twins are often remarkably similar, yet they have distinct differences. This might seem unexpected since they share all of the same genes, but scientists have come up with an explanation.

Small differences in which genes are turned on or off, and to what degree, account for the subtle differences in physical appearance or other characteristics, according to a report in the Proceedings of the National Academy of Sciences.

For instance, one twin may be more susceptible to a given disease or have certain features not found in the other twin. One possibility underlying these phenomena is that modifications that affect the expression of genes differ between the twins.

These ‘epigenetic’ modifications determine the level at which a gene’s product is produced, which in turn impacts an individual’s appearance and disease susceptibility among other things.

To investigate, Dr. Manel Esteller, from the Spanish National Cancer Centre in Madrid, and colleagues assessed two epigenetic modifications - DNA methylation (which tends to silence genes) and histone acetylation - in 80 identical twins.

In young subjects, no real difference in the epigenetic modifications was seen between each twin. In older subjects, however, marked differences in DNA methylation and histone acetylation were observed.

Further studies are needed to determine how these epigenetic modifications change in each twin over time, the authors conclude.

SOURCE: Proceedings of the National Academy of Sciences, July 4th early edition, 2005.