According to Stanford School of Medicine, genetics do play a role in depression and other mental illnesses. However, all cases of depression cannot be attributed solely to genetics. Environmental factors can also lead to depression.
Stanford School of Medicine estimates that heredity is involved in about 40 to 50 percent of major depression cases. However, the extent to which heredity causes depression is not fully understood, and it is not likely that a single gene is responsible. Researchers aren't sure whether people can develop depression from genetic causes alone or whether all cases are caused by both genetic and environmental factors.