(Spoilers Main) Why are there no deformities from incest?

I'm pretty sure this has been discussed before in the past, but I can't be the only one who's wondered why how with all the incest the has been practiced in Westeros (Targaryens and Lannisters) there are little to no birth defects or physical deformities as a result. Like, the Targs had been marrying each other for so long at that point, I'm honestly surprised that Viserys and Dany weren't born with severe down syndrome or an extra limb or something.

As for Lannisters, Joffrey's insane, but somehow, the other two kids managed to turn out just fine. This becomes even more confusing when you remember that their grandparents were first cousins, add that to the fact that their parents are siblings..................yeah, there's no reason for any of these kids to not have at least some kind of birth defect.