Yes, certainly inbreeding among English royalty and the noble class in England - one of the reason for all the wars in the middle ages but also, oddly, for their amazing brilliance in the arts and law. Sometimes inbreeding creates good minds. But really bad tempers!
The wars were the CAUSE of the brilliance.
Inbreeding only works out if the inept, the weak, and the stupid are kept from breeding. In the Medieval aristocracy, if you were a stupid nobleman, then you were conquered, all your blood line killed, and your lands given to the progeny of your more-capable conqueror.