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 ,
http://www.dana.org/news/brainhealth/detail.aspx?id=9786
