Hi. I found this on a reputable medical web site on the subject:
"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."