There are many stories of parents bringing their son in for evaluation, and when his ADD is diagnosed and ADD or ADHD is explained to them, the father is amazed to realize that he has it too.
I estimate a 50% chance that my grandfather had it, 2 % chance my father did, 100 % for me and for Duane, my youngest son. ADD or ADHD tends to run in families, especially along the male line. There is less information about in females, but I have seen some suggestion of the same. So it seems clear that there is a genetic component, probably a strong one.