GWAS were made possible by the rapid discovery of many more genetic variants through the International HapMap Project (hapmap.ncbi.nlm.nih.gov), which had the goal of developing a public resource to catalogue normally occurring genome-wide variation in SNPs by annotating the sample genomes of small groups of ethnically homogenous individuals (e.g., Caucasians of European descent, or CEU, are represented by Utah residents with ancestry from northern and western Europe collected in 1980) to create what is known as a reference panel. In addition to providing a database for identification and comparison of the polymorphic nature of SNPs, HapMap also allows for the examination of the extent to which neighboring SNPs are correlated with each other via a population genetics process called linkage disequilibrium (LD). When two or more SNPs are in high LD (e.g., correlations > 0.8), if one of the SNPs is genotyped then the genotype at the others can be probabilistically inferred via imputation. The ability to infer surrounding genotypes based on LD patterns means that we can now cost-efficiently scan the genome with a much smaller subset of markers
