If genes were the only source of bipolar disorder, then whenever one identical twin is bipolar, the other, raised in the same family, would always be bipolar, too. But that doesn't happen. If one twin is bipolar, an identical twin is only 60 to 85 times as likely to be bipolar. Experts differ on the percentage, with the extremes being 25 to 96 percent.
Identical twins do not, of course, have identical lives, and if they are bipolar, they may or may not be in manic, depressed, or normal states at the same time. While stress can trigger a bipolar episode, such episodes also can occur with no discernible trigger, whether or not the bipolar person is a twin.
Not only do identical twins not have identical lives but also families vary greatly, and what triggers bipolar disorder in one family may not trigger it in another family, even when both families carry genes for bipolar disorder.
In fraternal (non-identical) twins, half of their genes are the same, just as half are the same in their other siblings and in each parent. As a result, when one fraternal twin is bipolar, the odds that the other will be bipolar are about 5 to 20 percent, the same odds as for other siblings.
Genetic members of a family in which one member is bipolar are more likely to be bipolar than are individuals in a family in which no one else is bipolar. And adopted members in a family where bipolar disorder is present are less likely to be bipolar than are genetically related members of the family.
The number of genes related to bipolar disorder may be as few as one or two, but genetics are the focus in some studies of twins who grow up in the same home, and we still have much to learn
Sources: http://www.ncbi.nlm.nih.gov/pubmed/23826396, http://www.ncbi.nlm.nih.gov/pubmed/24133464,http://www.ncbi.nlm.nih.gov/pubmed/11914174,