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Originally Posted by shakran
Actually they're the direct results of bad genes being allowed to breed period. If an Amish guy has a dominant gene for early onset alzheimers, he's going to pass it on to his kids whether he has the kids with his cousin or a complete outsider.
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This issue with inbreeding is not dominant genes, it's recessive genes. Most genetic disorders are a result of deleterious recessive genes. Deleterious recessive genes are maintained in populations because they are recessive. Dominant deleterious genes are selected out of gene pools very quickly. If you combine deleterious recessive genes with inbreeding, you significantly increase the chance of their expression.