One thing that often gets overlooked is the fact that even in human populations, isolation was the norm and not the exception. During my lifetime I was lucky enough to get glimpses into such populations, in Appalachia, among the Amish and Mennonites in Pennsylvania, in the small islands along the Eastern Shore of the Chesapeake Bay, the Outer Banks and Coastal North Carolina, on Dafuskie Island and reservations in Arizona. Even in a city like Baltimore many folk were born, lived their lives and died within a few block area and married from within that population as had their parents and as would their children.
America was a land of small towns and travel outside was just something that didn't happen all that often. There were no interstates, air planes and trains were how the other folk got around. Cars were not all that reliable and every journey, even a day trip, was an adventure. To get from Baltimore to the Eastern Shore you either drove up to Delaware and down the coastal back roads or took a ferry across the Bay.
People married people that lived in their community. In many towns most everyone was related and second third and kissin cousin made sense.
In addition, infant mortality from all causes was higher than today.
In such settings marrying someone closely related would not be all that unusual and if you look at the charts in
The 44 Chromosome Man you see that it is possible to have viable kids that pass on the trait. And it's those successes that count.
Anyone so limited that they can only spell a word one way is severely handicapped!