More recently, technological advances have made genomewide association studies possible (39, 67, 109). Rather than genotyping <10,000 variants, these studies typically genotype 100,000 – 1,000,000 variants in each of the individuals being studied. Since >10 million common genetic variants are likely to exist (104), even these detailed studies examine only a fraction of all genetic variants. While in traditional genetic linkage and founder haplotype mapping studies, geneticists expect to identify long stretches of shared chromosome inherited from a relatively recent common ancestor, in genomewide association studies that focus on apparently unrelated individuals, geneticists expect to identify only relatively short stretches of shared chromosome. Remarkably, genotype imputation can use these short stretches of shared haplotype to estimate the effects of many variants that are not directly genotyped with great precision.
