I don't think potentialism (slayerness?) is passed on genetically. After all, if Buffy and Dawn have the exact same blood, but Dawn isn't even a pottential, than what makes Buffy a slayer (or any girl for that matter) can't really be physical. You cant go to a doctor and have your "slayer gland" removed. I think it follows more along the lines of reincarnation of the soul, not the body, which explains why, in the movie, Buffy had the memories of some of the past slayers. The fact that she had only some of them, though, points to the fact that maybe there are different lines, so that each potential is descended from a long line of slayer-souls, but only one is active at any one time.
That's just my theory, though. Joss seems to not follow genetics that much anyway, otherwise, connor would have been a vampire, and buffy's mom would have had to be a potential.