I believe it's a combination of bad genes as in low IQ and / or low self esteem. (religion.blogs.cnn.com)
Instead, the embryo replaced the targeted bad gene with a healthy gene from the mother — a conclusion that's been criticized by a group of prominent scientists. (discovermagazine.com)
The closer the parents are related, the more likely it is that both parents will be carriers of the same bad gene. (2ndchance.info)