The Oddities of Purebloods

So, the question is…what is the effect of all the inbreeding that surely must have gone on for centuries amongst the pureblood families? And what happens when Hermione finds out the Malfoy oddity? Disclaimer: No thought has been given to the science behind this whatsoever – it's just a fun bit of fluff. One-shot, post-war, EWE?