I had a psychologist explain once that with bipolar disorder, the genetic link could be quite far removed (by as much as 7 generations), and since most of us don't know that much family history, nor was psychiatric care at all the norm even a couple of generations back so people just weren't diagnosed, knowing the specific genetic connection can be almost impossible. I wish I had the studies he was drawing that information from because I think it would be interesting to read more about it.