Genetic predisposition is proven, and exerts most influence for more severe mood disorders such as bipolar affective disorder.

If one identical twin has bipolar affective disorder, the co-twin will also develop it in about 60 per cent of cases. In non-identical co-twins, the risk is about 20 per cent. The only possible explanation for this difference in concordance rates is closer degree of genetic relation. Hence, this difference is proof of a genetic component in bipolar affective disorder. However, it will be noted that even in identical twins, the concordance rate is not 100 per cent, showing the importance of non-genetic, that is, environmental, factors.

There is also a higher risk of unipolar depression in the relatives of patients with bipolar affective disorder.

In general, it can be said that if an index patient has a mood disorder of any kind, their relatives are at higher risk than the general population of also having a mood disorder of any kind. The lifetime risk of illness in relatives of affected patients is about 10-20 per cent in first-degree relatives.

The more severe the disorder is in the index patient, the more likely that there will be relatives also affected.

The pattern of inheritance is polygenic. Much effort is going into the molecular genetic approach to this; however, clear conclusions as regards aetiology, let alone treatment, are still awaited.

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