In the past few years, it has become possible to genotype up to a million SNPs throughout the genome in a single experiment—an approach called genome-wide association studies (GWASs). This technique, which is based on the assumption that common genetic variation contributes to disease risk, allows a comprehensive test of association across the genome, rather than testing only one gene at a time. It has been used for many different diseases, with varying success. In particular, the relatively low statistical power of GWASs is a significant hurdle. Thus, the analyses require very large samples because most variations only have small effects; moreover, the multiple testing involved in a GWAS reduces the statistical power to detect associations.
