As the cost of genotyping dropped during the 2000s, it became possible to conduct association tests as are done in candidate gene studies, but across the entire genome. Such genome-wide association studies (GWAS) are hypothesis-free as with linkage studies, but have much higher power to detect small effects of common variants. In GWAS, hundreds of thousands to millions of genetic markers known as single nucleotide polymorphisms (SNPs) are genotyped across the genome in an attempt to identify common variants that are associated with a particular outcome (disorder, behavior, etc.), suggesting that a particular genetic variant (or one very nearby) contributes to the outcome. In a sense, GWAS combines the advantages of linkage studies (agnostic screening across the genome) and candidate gene studies (more precise localization of the gene/genetic variant).
