More than two-thirds of people with manic-depressive illness have at least one close relative with the disorder or with unipolar major depression, indicating that the disease has a heritable component. Studies seeking to identify the genetic basis of manic-depressive illness indicate that susceptibility stems from multiple genes. Despite tremendous research efforts, however, the specific genes involved have not yet been conclusively identified. Scientists are continuing their search for these genes using advanced genetic analytic methods and large samples of families affected by the illness. The researchers are hopeful that identification of susceptibility genes for manic-depressive illness, and the brain proteins they code for, will make it possible to develop better treatments and preventive interventions targeted at the underlying illness process.
Genetics researchers believe that a person's risk for developing manic-depressive illness most likely increases with each susceptibility gene carried, and that inheriting just one of the genes is probably not sufficient for the disorder to appear. The particular mix of genes may determine various features of the illness, such as age of onset, type of symptoms, severity, and course. In addition, environmental factors are known to play an important role in determining whether and how the genes are expressed.
