Because many mental illnesses tend to run in families, researchers have been searching for specific genes, passed down through generations (heredity) that may increase a person's risk of developing the illness.
Scientists think there are a set of 'shared' genes that make people more susceptible to developing all mental illnesses involving the symptoms of psychosis.
Many people worry that, because someone in the family has experienced mental illness, they might be next. The risk gradually increases with degree of closeness of relative, but being related to someone with a mental illness does not necessarily mean that you will also develop it.
It is also important to note that while having a relative who has experienced mental illness is a risk factor, a combination of a number of different risk factors including environmental influences - are needed to go on to develop psychosis.
Prof Sir Robin Murray explains more here: