As i explain, in my little text, those diseases are mostly recessive, you need both bad alleles to have it. So as alleles are inherited, fucking someone your family increase risk. But first, those alleles are rare. Second, even you have both one of those allele, it's just a 1/4 risk that both are inherited for a new child. Third we now have some technical knowledge on how to detect those bad alleles, so we can prevent it (if we found out with genetic screening that a couple have bad allele we can just ask them to use contraception and adopt or use a sperm donor to be safe).
I agree most of the risk is when it's repeat on many generations and bad allele accumulate and can't be evacuate from the genetic pool. That being say, it works also for good allele. Many plants and animals we use for food for example are massively inbred, to keep their selected good characters.
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Also today, sex isn't always linked to procreation, most of the time we have sex just for pleasure, so the argument about risk for babies can be irrelevant.
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So i need to invoke the master again?