I genuinely did not know that. Consanguinous reproduction on this scale is actually crazy. EVERY person carries at least 1-2 mutated genes, which is not a problem, since most disorders are recessive (you need two mutated genes to actually have the disorder) and you're supposed to marry with non carriers outside of your family, unless you marry your cousin who has 13% of your DNA. Now your child has a proportianally far greater chance to actually develop the recessive genetic disorder. A lot of genetic disorders will cause additional gene deletions, which basically fucks up your childs code even more and makes them more at risk of conceiving a child with rare mutations,
especially if they marry someone with the same conditions from the same bloodline. The important fact is that this basically has a snowballing effect, until after only a few generations you are left with this.
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(and you also have significantly degraded the gene pool)
This is heavily simplified, there are plenty of disorders that can have a minimal to large impact depending on the amount of genes that are malformed for example. Moral of the story is that having 70% consanguinous blood is terrifying. Somewhere I don't want to belive that. Either way, these people need to stop marrying their cousins asap.