Well, actually, after 8 generations, each allele has 8 chances to not get inherited. So if the rapist had, say, one copy of the Huntington's allele (an autosomal dominant mutation according to Wiki), then you would have a 1/256 chance of having inherited the disease from that ancestor. Two copies of the allele from the rapist would give you a 1/128 chance. I'd have to say GP's point stands here.