Association studies are usually conducted using a case–control design, and they are performed in order to detect alleles that have an altered frequency in unrelated patients as compared to unrelated control subjects. Association studies have a greater power than linkage studies to detect common risk alleles with small effects (Risch and Merikangas, 1996). In order to conduct a systematic association screen covering the whole genome, it is necessary to genotype hundreds of thousands of polymorphisms. Until recently, systematic association approaches were not possible due to limitations in genotyping technology and limited knowledge of the variability of the human genome, and previous association studies therefore focused exclusively on candidate genes.
