Thanks to the efforts of geneticists the story of the extinction of the Spanish Habsburgs is now well known. They are in short a case study in the disastrous consequences of an inbred pedigree. The downsides of inbreeding are to some extent intuitively understood by all, especially consanguineous relations between first order relatives. Though I’m willing to bet that all things equal inbred individuals are not as attractive or intelligent as outbred individuals, the literature in this area for humans is surprisingly thin.
Still, the case that inbreeding results in the expression of deleterious recessive diseases is more straightforward. With inbreeding you are no longer dealing with random breeding.
Read the full article here: The inbred lineage you can improve