of association in multiple independent samples [18]–[23]. Several considerations have prompted differing approaches to 1) combining and comparing GWA datasets and 2) declaring that association between sets of nearby SNPs and a complex disorder is “replicated” in the absence of genome wide significance for any result. Underlying functional haplotypes contributing to disease vulnerability may be tagged differently by different SNP sets in different samples. Allelic heterogeneity can result in 1) contributions of different variants within the same gene and 2) differences in the predominant variants in a gene that influence the phenotype in different samples. Meta-analyses often combine data from studies that have examined alleles of different sets of SNPs.
