Why does incest result in birth defects?

Just to be well and truly fuckin clear. I am not now nor have I ever been nor will I ever be contemplating shagging a family member.

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I'm being pedantic, but if you know you're a carrier (and don't know the carrier status of your sister) then it is 1 in 8

You are correct, and i did realize this some time after posting I'm just too lazy to edit it.

This guy does the math before mating with sis.