A living descendand, even of Jesus, assuming teh best case scenario of 40 year generations, would be 50 generations removed, which means they would have 1/2^50 of his DNA, or roughly 10^-15. Given the mechanisms involved (23 pairs of chromosomes, with occassional drift of genetic base pairs or segments copying from one chromosome to it's companion, what you would really be dealing with is teh chances of any living descendant of Jesus still having any of his DNA remaining. Relic based DNA is a far better chance of a match, given the inherant assumption that there in fact was such a person (which is pretty much inherant to the entire conversation)