Yeah, sooner or later genetics is gonna bitch-slap ya, but as you noted it's going to take more than a few generations (monster incest babies right from the first time is urban myth). Assuming you don't simply write the issue away (medical phlebotinum!) or just ignore it. There's good things to be said for both writing it realistically and unrealistically.
Considering that only three generations back (great-grandparents) are people that only share 1/8th of the genetic code that makes up you, and that three generations would be about 60 years (more like 90, realistically), yeah, someone several hundred years back in your ancestry isn't going to have much in common with you. At two centuries it's less than 1% (1/1024th) in common with you, which is pretty much 'average stranger' to me. As in, it's not really incest anymore, if that's the closest relation the two people in question have.
Within two generations is probably a good rule of thumb for whether it 'counts as incest'. (first cousins would be two generations) And things like adopted or step-relations don't really count. Half-siblings (same father, different mothers) and comparable relations would count, though. |