Nine single-nucleotide polymorphisms (SNPs) were selected in CHRM2 for genotyping, on the basis of evidence of association with one or more externalizing disorders in the COGA sample (Dick, Agrawal, et al., 2007; Wang et al., 2004). A SNP is a DNA sequence variation in which a single nucleotide in the genome differs between individuals or between paired chromosomes in an individual. Some SNPs have functional consequences (e.g., they change the protein product or alter the rate at which the protein is produced), but most have no known consequence. SNPs are used as markers for the surrounding DNA sequence. Multiple SNPs were genotyped across the CHRM2 gene because it is possible that variation at multiple locations could alter the function of the gene and contribute to differential susceptibility. Figure 1 shows the location of the genotyped SNPs and the correlation structure across these SNPs using Haploview (Barrett, Fry, Maller, & Daly, 2005). Because the correlation pattern differs across the SNPs across the gene, one would not expect all SNPs across the gene to yield similar results; rather, the observed pattern of significance across SNPs should map broadly onto the correlation pattern.
