In the link I've posted a few days ago ( http://genome.wellcome.ac.uk/doc_WTD020780.html ), there's an explanation on SCA. It is known and discussed in papers such as those mentioned in #msg-76009125 that rare genetic variants are frequently geographically localized to pockets around the globe rather than being widespread. If such variant has an advantage like resistance to malaria, then its frequency is very high due to selection in certain regions.
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