Genetics and Bipolar Disorder
The evidence for a genetic predisposition to bipolar disorder is so strong that based on studies of twins, an estimated 80 percent of the risk is inherited. If one of your parents or siblings has this disorder, your chances of having it are four to ten times greater than they would be if no one in your family had it. That sounds like a huge increased risk and cause for worry, but it isn't necessarily as bad as it sounds. In fact, the numbers clearly reflect the fact that the majority (66 percent) of children who have a parent with bipolar disorder will never develop it.
Hiding in the Chromosomes
Humans have twenty-three pairs of chromosomes, the NA-protein structures in our cells that contain all the information needed to create a human being. Genes, built into the molecular structure of DNA, encode instructions for making proteins our bodies depend on to function. From the color of your eyes to features of your personality, genes exert a tremendous — but not complete — influence on your life.
Some of these genes, like a dominant one on chromosome 11, may predispose people to illnesses like bipolar disorder. More than a dozen other chromosomal regions have been implicated as well, including a promising pair on chromosomes 6 and 8. Unfortunately, the data are still not strong enough to link bipolar disorder with any specific genes. There is no bipolar gene, just groups of genes that appear to increase the chances of developing the mood disorder. The same genes, interestingly, also appear on the list of suspected genes predisposing people to schizophrenia, major depression, and other disorders.
How soon might brain changes show up in bipolar disorder?
Perhaps as early as childhood if one MRI study is accurate. It found that the brains of children with bipolar disorder followed the same developmental pattern as the brains of children with a disorder called multidimensional impairment, which has schizophrenia- and bipolar-like symptoms. If true, this would be more evidence for common brain abnormalities underlying mood and other disorders.
Common Genes for Related Illnesses?
An example of genetic risk factors that may predispose some people to a variety of mental disorders was highlighted by a study in the United Kingdom. There, scientists identified three genetic sites that appear to increase the risk of schizophrenia. When the researchers expanded their analysis to include people with bipolar disorder, the association between one of these genetic sites and the risk of developing a mental illness became much stronger. The gene in question may control the expression of other genes.
Additional research is revealing common characteristics of families with a history of bipolar disorder. They've found, in addition to manic and depressive episodes, that obsessive-compulsive disorder occurs with bipolar disorder in some groups.
Several studies indicate that when one member of a family has bipolar disorder, the risks of another family member having bipolar disorder, schizophrenia, or schizoaffective disorder all increase. And the risk of major depressive disorder doubles in such relatives (assuming the depression is not being misdiagnosed and is really bipolar disorder). This supports the suggestion that these illnesses reflect underlying problems in the brain that may manifest themselves differently in different individuals, perhaps in concert with different inherited traits and/or exposure to different environmental factors.