There is evidence for a strong genetic component in obsessive-compulsive disorder (OCD) but the underlying genetic mechanisms are not clearly understood. Associations have been reported for the low enzyme activity allele of the COMT (catechol-O-methyltransferase) gene and MAO-A (monoamine oxidase A), two enzymes involved in the neuronal metabolism of catecholamine transmitters, along with 5-HT1Dβ. MOG-4 (myelin oligodendrocyte glycoprotein-4), a glycoprotein found on the surface of myelinating oligodendrocytes and external lamellae of myelin sheaths, dopamine D4 receptor, and region q34–q35 on chromosome 4 have also been reported as candidate genes but their involvement, if any, in OCD has not yet been determined.
Inheritance of phobia is thought to be autosomal dominant. Few genes have been implicated in phobia and their inheritance and effect on phenotype remain unclear. A duplication on chromosome 15, DUP25, and a polymorphism in the COMT (catechol-O-methyltransferase) gene and another in the upstream region of the serotonin transporter gene (SLC6A4) which reduces transcription, have all been identified as possible susceptibility factors. The involvement of serotonin in phobia has been known for some time and COMT has links with the sympathetic nervous system, however the effect of DUP25 in this condition, if any, is unclear.
Post-traumatic stress disorder (PTSD) has a complex genetic basis that is triggered by exposure to an extremely traumatic environmental event. A deletion of the DYT1 gene, encoding torsin-A, an adenosine triphosphatase, was identified in a family whose predominant phenotype was myoclonic dystonia but in which several members presented with a range of psychiatric illnesses including PTSD. However, the role of DYT1 in PTSD is unclear. Some studies support a possible association between alleles of the dopamine D2 receptor gene and PTSD, while others show no such allelic association.