This always seems stupid to me. There are so many variables when it comes to what you kids genetic make up will be like. Unless both you and your SO are riddled with diseases, the combination of you guys can dilute crappy genetics. Just date outside the family or maybe even a different culture.
Good post. As I mentioned, no guarantees, even with healthy backgrounds.
There's variation even among siblings- know a few where one brother is bald, the other is not. That's a mild example, but it shows the variation.
Another is a son in a family with Downs Syndrome, while the 4 other children have no issues.