John Crichton is the son of Kenneth Crichton and Baroness Blood. Crichton is worshipped as the Pureblood son of two vampires by Blutsauger, but he has also inherited his father's blood disease which as a result means he can only feast on the blood of his own direct relatives. This limits him to only being able to feed from his mother and his grandmother (and presumably his father when he was revived).