The first biological explanation of OCD is genetics.
Evidence for the genetic basis for OCD comes from evalutations of first degree relatives and twin studies.
NESTADT ET AL identified 80 patients with OCD and 343 of their first degree relatives and compared them with 73 control patients and 300 of their relatives. The results showed a strong familial link for the most common form of this disorder. They found that people with a first degree relative with OCD had a five times greater risk of having the illness themselves.
BILLETT ET AL meta analysis of 14 twin studies of OCD found that identical twins were twice as likely to develop OCD than non identical twins.
The COMT gene
KARAYIORGO discovered the variation in the expression of a specific gene may contribute to OCD. This genetic alteration reduces the production of the enzyme COMT which helps stop the action of dopamine which links to OCD. DNA samples were collected from 73 people with OCD and 148 who had not experienced any mental disorder. The variation in the gene occured in nearly half the men with OCD.
What is inherited?
Genetic factors may lead to abnormal functioning of areas of the brain for example the caudate nucleus. They may also lead to abnormal levels of serotonin and dopamine.
PIGOTT ET AL- Low levels of serotonin are implicated because antidepressant drugs that increase serotonin have been shown to reduce OCD symptoms
JENICKE- whereas antidepressants that have less effect on serotonin do not reduce OCD symptoms.
SZECHTMAN- high doses of drugs that enhance levels of dopamine given to animals, induced stereotyped movements resembling compulsive behaviours found in OCD patients.