3 generations for rednecks... Cousin marriage is often legal and not unheard of, though most people try to avoid such close relatives. By 3rd cousins - 5 generations - you often don't even know everyone and so it wouldn't be a surprise if you don't even know you are so closely related until long after you have kids. Generally there is enough separation that this isn't a genetic issue. Even first cousins don't typically share enough genes for issues to be much more common than otherwise. (kids of brothers-sister pairs often have genetic issues).