There is a strong genetic (inherited) component to bipolar disorder, but that’s not the whole story. This has been clearly demonstrated in studies of identical twins, in which one develops the disorder and the other does not. If genetics were the only factor, both identical twins would always have bipolar disorder. That said, if one identical twin has bipolar disorder, the chance that the other twin will also develop it is greatly increased (between 67 and 87 percent of the time). In fraternal, or non-identical twins, the likelihood that if one twin has bipolar disorder the other will as well goes down to between 22 and 39 percent.
So while it most definitely runs in families, it can also occur in people who have no family history. As the human genome (DNA) is mapped out by scientists, it seems likely that bipolar disorder is associated with more than one gene. One theory that has some evidence to support it is that the different variants of bipolar disorder, and the severity with which they affect a given person, relates to the number of “genetic hits” the individual has.
In other words, multiple genes might be involved in a person who has severe bipolar I disorder, whereas a smaller number may predispose an individual to the less extreme bipolar II disorder, or cyclothymic disorder. Additionally, it appears that even when this is teased apart, genetics will not provide a complete answer for who will and will not develop the illness. Environment, stress, and life circumstances will still factor into the equation.
Particular genes implicated in bipolar disorder are located on chromosomes 22 and 13. Interestingly enough, some of the implicated genes in bipolar disorder are also showing linkages to schizophrenia.