It's been a while since I worked in genetics, but is that page accurate re SNPs? In the "SNPs and disease-causing mutations: Not the same!" section it says: First, to be classified as a SNP, the change must be present in at least one percent of the general population. No known disease-causing mutation is this common.
According to Wikipedia, the SCA SNP is present in about 30% of Sub-Saharan Africans, and there's about 800 million of them, so that would be about 240 million people carrying one of the haplotypes. That would get you to about 3.5% of world population, not counting carriers in other regions.
