This is common among the Amish in the US. They don't marry outside their religion and the gene pool is way too small. They marry cousins a lot simply because there aren't enough non-related women to marry. I read an article a few years back by a doctor who specializes in genetic diseases. He had identified about 120 diseases in the Amish. Most of these diseases are recessive genes that are very rare. It takes a recessive gene from both parents for it to appear. If the gene pool is very small, the odds of both parents carrying it is greatly increased.
The well known hemophilia in European royals is a great example. Many of them have suffered from it. It's the same thing - they marry within their 'nobility' and their gene pool is way too small.
Maple Syrup disease.
It's common enough that every newborn in PA is tested for it.
Parents who say they have good kids..Usually don't!
