Technologies for high-throughput assays of thousands, and then tens and hundreds of thousands, of SNPs developed in parallel with the progress of the HapMap, as it became clear that denser maps could effectively capture the majority of human genetic variation (12, 13). These advances have made possible the dense genotyping needed to characterize the SNP variation within an individual, at a sufficiently low cost to allow the large sample sizes needed for comparisons of persons with and without disease. As genotyping platforms expand to include ever more tag SNPs, they capture increasingly larger proportions of the variation in any population, so that even samples of recent African ancestry, characterized by greater variation and shorter stretches of LD (14), have most of the genome covered at high r2 (7).
