observed throughout the genome (Gabriel et al. 2002). Knowing the correlation pattern is critical for the selection of markers and the interpretation of genetic association results. For example, DRD2 spans at least two blocks on the figure. If different studies genotyped SNPs from different blocks, they could reach different conclusions about whether DRD2 was “associated” with outcome, depending on the location of the marker they chose and where the actual associated SNP was. Further, also note that the LD block that contains Taq1A spans the genes DRD2, ANKK1, and TTC12. This makes it very difficult to know which gene is actually important for an observed association. More extensive genotyping across these genes and the other gene located very nearby in the region, NCAM1, has suggested that the association with substance-use phenotypes extends to multiple genes in this region (Dick et al. 2007d, Gelernter et al. 2006, Yang et al. 2008). This underscores the necessity of understanding genomic structure in order to evaluate the role of hypothesized genes of interest.
