Eye colour is inherited from ones parents in a completely predictable way and so are a number of so-called genetic illnesses. This is not the case for any mental illnesses. For some mental illnesses it does matter who your parents are but you cannot predict in the same way as eye colour what effect this will have.

There have been many studies to show that schizophrenia is found more commonly in children with one parent with the illness and even more so if both parents are affected. At one time there was a belief that upbringing had something to do with developing this illness but studies of twins brought up in different environments show that this theory is not really viable. A twin, especially if he or she is one of an identical pair, is much more likely to be affected if their other half is.

Manic-depression also has a genetic predisposition. In this condition, sufferers (who develop the illness in adult life) are sometimes ‘high’ or overactive and sometimes ‘low’ or depressed.  There is an increased incidence of the illness amongst the children of those affected but by the same token these children are also more likely to be affected by depression without any evidence of mania.

In the case of less severe depression, although there is an increased incidence in the children of sufferers it is less clear that the mode of inheritance is genetic. It may be in these cases that ways of looking at the world, which the child learns when he or she is growing up, play as great a role in their later development of this illness. More women than men suffer from depression and it is not clear why this is.

In the case of alcoholism there is a greater incidence of the disorder in the children of sufferers but the reasons for this are very unclear. There have been many studies; some seem to support the idea that there is a genetic reason, for example there is a higher incidence in the adopted-away children of alcoholics. However exactly what is inherited is less than clear since on the surface alcoholism would appear to be caused not by an illness but by a way of behaving. We simply do not know whether what is inherited is a psychological trait or some biochemical factor in the brain.

There is a genetic component in Alzheimer’s disease (a type of dementia) [link: Dementia – what can be done to help?]. People who develop this illness are more likely to have an affected relative especially if they develop the condition at a younger age. It is also more common in people with Down’s Syndrome or where there is someone with this condition in the family.

Many other psychiatric conditions such as anxiety and obsessive-compulsive disorder (OCD) also have an hereditary component. In the case of anxiety this has a genetic basis but this is less clear in the case of OCD.

In summary then, there is quite a lot of evidence that many psychiatric disorders have an hereditary component but none is passed down in any clear way. Furthermore in every case, more so in some conditions than others, the effect of the environment and childhood experiences also has to be taken into account. Even where there is a genetic component treatment can help and is just as likely to work for people who have relatives with the condition as for others.

Danny Allen