The Habsburg family ruled large parts of Europe over many centuries. One way in which they held onto their considerable estates was to keep some marriages close to the family line. Now a study of the genetics behind these marriages has come to a conclusion about why the Spanish branch died out in 1700: by keeping marriages so close they were effectively inbreeding, and the last Habsburg King of Spain, Charles II, suffered from mental and physical disabilities which were a direct result of marriages earlier in his bloodline. In short “Charles II, was the offspring of a marriage that was almost as genetically inbred as an incestuous relationship between a brother and sister or parent and child”. This conclusion was drawn from a study of sixteen generations of Habsburgs. This article from the Independent
newspaper goes into the science and the medicine in more depth, as well as other genetic problems inherited by the family.