There are undeniably strong links between the genes a person is born with and her risk of developing schizophrenia, yet the connection is not absolute. If one of your relatives has schizophrenia, it does not mean you will have schizophrenia. It only means that you may have a slightly greater chance than the average person of developing the disease. This depends on how closely you are related to the person with schizophrenia.

The genetic link becomes most striking when twin siblings or both parents have the disease. Two parents with schizophrenia give their children as much as a 46 percent chance of developing the same disorder. A person whose identical twin has schizophrenia has a 50 percent chance of receiving the same diagnosis. These family members have the most genes in common. It is clear that the closer the genetic link, the greater the risk of developing this disorder.

Adoption Studies

Another very strong indication that genetics play a big role in developing schizophrenia comes from studies of adopted infants. Because adopted children are not raised with their biological relatives, key elements of environmental influence are removed from consideration when investigating biological factors that influence the development of the disease.

Comparisons of the incidence of schizophrenia in a child's biological and adoptive families show a strong genetic link. In other words, if an adopted child develops schizophrenia, the biological relatives — who did not raise the child — are more likely to have been diagnosed with schizophrenia or a schizophrenia-like illness than is the child's adoptive family. This indicates that there is something in a patient's genetic makeup that predisposes him to schizophrenia. Researchers at the National Institute of Mental Health estimate that “at least 60 percent of the factors that give rise to schizophrenia may be related to a genetic susceptibility.”

No One “Schizophrenia Gene”

Scientists have identified potential genetic defects on a dozen or so chromosomes that might play a role in increasing someone's chances of developing schizophrenia.

It has been difficult to identify specific genes linked to schizophrenia because the disorder is very complicated and has a wide range of symptoms and potential environmental influences. If specific genes closely associated with the disease were known, it might be possible to develop a genetic test. Individual genes may work in concert to increase a person's susceptibility to the disease rather than condemn them to developing it.

Table 2-1: Genetic Risk of Developing Schizophrenia

Your Relationship to the Personwith Schizophrenia	Your Risk
None	0.6-1%
First cousin, uncle, or aunt	2%
Nephew or niece	4%
Grandchild	4-5%
Half sibling	6%
Parent	6-8%
Full sibling	6-9%
Child with one ill parent	13%
Fraternal twin	17%
Child with two ill parents	39-46%
Identical twin	48-50%

Sources: Adapted from Comer, Fundamentals of Abnormal Psychology and the National Alliance for Research on Schizophrenia and Depression.

Teams of scientists from the International Schizophrenia Consortium and a company called deCODE Genetics in Reykjavik, Iceland, have managed to identify specific variations in the genes of less than 1 percent of patients with schizophrenia compared to the genes of people who are not affected by the disorder. It's important to note that these variations involving missing sections of DNA are rare even in schizophrenia patients. The effect of the deletion, however, is not subtle: it may increase the risk of getting the disease by up to fifteen times the normal risk.

These findings indicate that people with schizophrenia and similar psychotic disorders tend to have one missing section of DNA on chromosome 1 and two missing sections on chromosome 15. Both groups also confirmed a previously identified missing section on chromosome 22.

Scientists working at Cardiff University in the United Kingdom identified three places on patients' DNA molecules that appear to increase the risk for developing schizophrenia. One of these sites is near — and may involve — a gene that controls the activity of other genes. Interestingly, if people with schizophrenia and people with bipolar disorder are both included in the analysis, the link between this genetic site and the risk of mental illness gets stronger. This suggests that different mental disorders may have common genetic risk factors.