The Genetics of Bipolar Disorder.
by Margit Burmeister, Ph.D.
Is Bipolar Disorder an inherited disease? Some time ago, mental illness was thought to be due to “bad mothering”, imposing painful blame on the family members of mentally ill patients. Fortunately, these times are gone. Now, we hear over and over again that inherited (genetic) factors are largely responsible for bipolar disorder.
By making incorrect assumptions, one might miss the true genetic linkage, or “discover” a linkage that, in fact, does not exist.
What is the basis for such claims of a genetic predisposition to bipolar disorder? Observation of families has shown that children of one parent with bipolar disorder are seven to twenty times more likely to have manic depression than children of parents who do not have bipolar disorder. That by itself is no — proof for example, going to Medical School, or going to Law School also runs in families, but nobody argues that genes are responsible for directing children to Medical or Law School. So additional evidence is needed.
To tease out genetic from environmental factors, scientists turn to twin and adoption studies. Adoption studies have shown that it is bipolar disorder in the biological rather than the adoptive parents that is relevant for the increased risk of bipolar disorder in children. Thus, there is strong evidence against the direct influence of parenting. In twin studies, identical twins, who share all their genes, are compared with fraternal twins, who share only about half of their genes, just like other siblings. When one identical twin has bipolar disorder, the other has a 60 to 80% chance of also being affected, whereas the odds are only about 20% for fraternal twins. Taken together, the evidence is thus overwhelming that genes are a major player in bipolar disorder. Nevertheless, these studies also indicate that there is some environmental effect not everybody who has a gene for bipolar disorder becomes manic depressive.
Are there Genes Specifically for Bipolar Disorder?
What these studies indicate is that a predisposition to bipolar disorder is clearly inherited whether a person becomes ill depends on other factors. Is this predisposition specific for bipolar disorder, or is it a predisposition for mental illness in general? It is unusual to find schizophrenia in the biological parents of bipolar patients, suggesting that these are two separate diseases. However, the diagnosis of schizoaffective disorder has features of both diseases, and this diagnosis is made occasionally in the relatives of those who have either bipolar disorder or schizophrenia, indicating that there is some overlap between the two conditions.
Family, twin and adoption studies also show that major depression without manic episodes is commonly found in relatives of bipolar patients. However, depression is quite common and can be caused by environmental as well as genetic factors. There is also some indication that relatives of bipolar patients are more likely to abuse alcohol and other drugs. Thus, many studies indicate that people who carry a genetic risk for bipolar disorder may develop other psychiatric illnesses or may remain well.
These studies, however, do allow geneticists to calculate the probability, within a range, that a person will develop bipolar disorder, given the family constellation. For persons who are interested in finding out more about the genetics of bipolar disorder in their own families, genetic counselors are available to provide this