As one example, the Taq1A allele, originally thought to be in the DRD2 gene, has an extensive literature surrounding it, with reported associations with a number of phenotypes related to substance use, smoking, and a variety of other phenotypes related to impulsivity (Dick et al. 2007d, Noble 2000). Figure 2 shows a screen-shot of output from the program Haploview (Barrett et al. 2005) illustrating the linkage disequilibrium (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 to give an idea of scale. Directly underneath, the triangles indicate SNPs that were genotyped in the samples on which the Haploview data are based. The SNP highlighted in green is the marker commonly referred to as DRD2 Taq1A. Beneath the SNPs, the genes in the region are listed. The length of the line reflects the length of the gene. One will note that the Taq1A allele is actually located in a small gene next to DRD2 called ANKK1. It is not located in
