The evidence of a genetic predisposition to schizophrenia is overwhelming. The frequency of schizophrenia in the general population is less than 1%. However, being related to someone with schizophrenia greatly increases your risk of developing schizophrenia developing schizophrenia.
For example, if your brother or sister has the illness your chance of having schizophrenia is 9%. If your identical twin has the illness, you have a 28% chance of developing schizophrenia. If both of your parents have schizophrenia, you have a 36% likelihood of developing the illness.
We know these family risks are due to genetics rather than family environment because the risks due to family relationships are the same whether a person is raised in the birth family or not. The children of people with schizophrenia are more frequently given up for adoption because their parents are too ill to care for them.
However, genes alone don’t cause schizophrenia. If they did, then identical twins, who share virtually the same genetic code, would have close to 100% likelihood of sharing the illness, rather than 28%.