Incest is based.
There's a chapter in picrel about how in all great civilizations, incest was practiced. I remember Egypt and Greece off the top of my head. Lots of romantic poems between brother/sister.
The thing is, you have to ruthlessly cull all degenerate or weak offspring, like how the Greeks left children exposed over night in the wilderness as a test of their fitness. If you don't cull defectives, then obviously those genes will continue on.