We illustrate the importance of genetic coverage using the example of the Taq1A allele. This polymorphism was originally thought to be in the DRD2 gene, and has an extensive literature of reported associations (and failures to find association) with a number of phenotypes related to substance use, smoking, and a variety of other phenotypes related to impulsivity (Noble, 2000; Dick et al., 2007c). Figure 3 shows a screenshot of output from the program Haploview (Barrett et al., 2005), which uses freely available data from the HapMap project to illustrate the LD structure of the chromosomal region surrounding the DRD2 gene. Along the top of the figure is the base pair position of the chromosomal region, as listed in kilobases (i.e., 1000 nucleotide bases) to give an idea of scale. Directly underneath, the triangles indicate SNPs that were genotyped in the samples on which the Haploview data is based. The SNP highlighted in green is the rs1800497 marker commonly referred to as DRD2 Taq1A. Beneath the SNPs, the genes in the region are listed. The length of the line reflects the
