Figure 1 illustrates why. The figure legend discusses factors that predict power: sample size, correction for testing many SNPs, population frequency of the risk allele, and its genotypic relative risk (GRR). Large GRRs (e.g., 5-10-fold increase in risk to carriers) would have produced large linkage signals. Early GWAS analyses with a few hundred cases were powered to search for risk alleles with GRRs above 2. Only a few such effects were detected. (1) The more typical GWAS has included 1,000-2,000 cases plus a similar number of controls, with power to detect risk alleles that are reasonably common and have GRRs of 1.5-2. The small number of robust findings suggested the need to detect smaller GRRs. (2)
