This question is currently under investigation. Studies conducted by Johns Hopkins Medical Institute have found some possible genetic markers for OCD on several different chromosomes (1, 6, 7, 15, and two markers on chromosome 3). There were more abnormalities in these chromosomes for people who had OCD than there were for people who did not have OCD.
Other studies corroborate the theory that OCD has a genetic component. Identical twins with OCD show about a 65–85 percent concordance rate (if one twin has OCD, the other also does), whereas fraternal twins show only about a 35–45 percent concordance rate. This suggests that the “identical genes” of the identical twins share something that could lead to the development of OCD more so than do the “non-identical genes” of the fraternal twins. Therefore, identical twins with OCD must share something in their genetic code that leads them both to develop OCD more readily than fraternal twins. However, this also shows that it is not purely genetic, because if it was, then the identical twins would have to have a 100 percent concordance rate. Therefore, there must also be some environmental influences that lead to the development of OCD.
To date, no single OCD gene has been found, and there probably is not one specific gene that is responsible for OCD. More likely, a combination of genetic markers and environmental occurrences are required for the development of OCD.