Because bipolar disorder has a genetic component, it tends to run in families. As a result, a family with one bipolar member is likely to have other bipolar members in various generations. There also may be two bipolar members in a single nuclear family, which can't be easy for any of them.
If a family member with bipolar disorder is an identical twin, the other twin is more apt to develop the disorder than are other siblings. A non-identical twin is more likely to develop bipolar disorder than a non-twin sibling is but less likely than an identical twin.
More children of a bipolar 1 or bipolar 2 parents are bipolar 2 than bipolar 1. And first-degree relatives of bipolar people have an increased likelihood of suffering serious depression even if they are not bipolar. They are, unfortunately, also at risk for schizophrenia, alcohol abuse, and drug addiction.
A study at Stanford University concluded that children of bipolar 1 and bipolar 2 parents have a 51 percent likelihood of having a psychiatric disorder like depression or attention deficit hyperactivity disorder (ADHD). Surprisingly (at least to me) children diagnosed with ADHD more often, as adults, have bipolar children than children with ADHD.
I am bipolar NOS (not otherwise specified), a diagnosis for those of us with bipolar symptoms that don't quite fit the bipolar 1 or 2 category. I have not seen any studies that factor bipolar NOS into possible genetic connections, but I feel quite certain that others in my family were bipolar in years before diagnosis and treatment were available.
Bipolar disorder originates primarily in the brain as a result of faulty neurotransmitters. A bipolar episode can be triggered in vulnerable people by events in their lives. Without treatment, a bipolar person may attempt to self-medicate with alcohol, recreational drugs, large doses of caffeine, promiscuous sex, and even to try to end suffering with suicide.
As we come to understand more about our brains and how they work, we can hope to see new approaches to dealing with bipolar disorder in individuals and in families.