Bipolar disorder certainly runs in families. Studies of twins suggest that genetics accounts for about half of the risk of developing bipolar disorder. And some specific aspects of bipolar disorder run in families. For example, whether or not someone with bipolar disorder experiences symptoms of psychosis (hallucinations and delusions) during mood episodes is also influenced by genetics.
But bipolar disorder is also highly variable—not just within families, but even within the same person across different stages of life. And that variation is not very predictable—by genetics or anything else. So we would not necessarily expect that patterns of mood symptoms (slow cycles vs. rapid, predominant depression vs. predominant mixed or manic) would be consistent within families.
We believe that responses to specific treatments, especially medication treatments, is influenced by genetics. If that is true, then we would expect that responses to treatments (like a good or bad response to lithium) would be similar among family members. But the research on this question so far has not produced any clear answers. Most doctors will still ask about family members’ responses to treatments and consider that information when recommending treatment. But we should acknowledge that the scientific evidence for that practice is pretty weak.
Finally, bipolar disorder is probably even more variable and less predictable among children and teens. Follow-up studies show that patterns of mood symptoms often change significantly between adolescence and adulthood. So we should be cautious making any long-term predictions about how symptoms of bipolar disorder will change (or even resolve) over that time.
So I certainly would not tell your child, “You have bipolar disorder, so you’ll grow up to be like Uncle Ken.” But I might say, “People used to be ashamed of things like this. Your Uncle Ken had to learn on his own how to stay healthy and avoid mood swings. Maybe some of the things he learned could help you.”