“blocks”) comprised of groups of SNPs where there is high LD across that group of SNPs and low LD with surrounding SNPs located outside the block. This block-like structure is observed throughout the genome (Gabriel et al., 2002). Knowing the correlational 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 attached 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, since the correlational structure means that a signal could originate from any gene in the correlated set (or LD block). More extensive genotyping across these genes, and the other gene located very nearby, NCAM1, has suggested that the association with substance use phenotypes extends to multiple genes in this region This underscores the necessity of understanding genomic structure in order to evaluate the
