Observations of family histories have long confirmed that bipolar disorders are inherited genetically. Adult relatives of someone with Bipolar Disorder I or II are 10 times more likely to also develop bipolar disorder. The risk increases based on the level of relationship connection. Those with a second-degree relationship with someone who has bipolar disorder, such as an aunt or uncle, have a slightly elevated risk. This risk increases when a sibling or fraternal twin is diagnosed. Having one or both parents or an identical twin with bipolar disorder greatly increases one's risk of developing bipolar disorder.
The risk of bipolar disorder II is generally highest among relatives that also have bipolar II disorder, not those with bipolar I or major depressive disorder.
In all, about 80% of people diagnosed with bipolar disorder have a family history of either bipolar disorder or major depressive disorder. However, this does not necessarily mean that someone with such a family history will inherit the genetic predisposition of the disease, or if they do, that their predisposition will eventually develop into symptoms.
Areas of chromosomes 4, 12, 18, 21, 22, and X have been linked to various aspects of bipolar disorder supporting the general notion that the illness is caused by multiple genes. At the present time, however, there are no genes that have been identified as definitively responsible for causing bipolar disorder. It is likely that genes that control bipolar disorders will be identified sometime in the future. However, with the diathesis-stress hypothesis, just having the presence of particular genes alone is not expected to be enough to set the disorder in motion. How the environment reacts with genes is an important component of how bipolar develops and is then maintained.