Jennifer Roberson's Cheysuli series.

I remember reading the back cover blurbs - they rang true.

Okay, so I'm American and disapprove of marrying your first cousins. Repeatedly. For generations at a time.

Let's talk about lethal recessives for a while. If you're really unfortunate, I'll also start muttering about haplotypes, and then it's a short slippery slope to LOD scores and other statistical analysis. Ah, genetics!