For power calculations with Quanto for main effects, according to two recent studies (Saccone and others 2009; Schlaepfer and others 2008), the average genotypic relative risk (GRR) for the chromosome 15 CHRNA5/A3/B4 gene cluster SNPs was about 1.3. In the current study, rs1317286 in the CHRNA3 gene was associated only with SQ and HSI in pooled sample. Another SNP in the CHRNA3 gene, rs8040868, is associated only with SQ in the pooled sample and with all the three adjusted ND phenotypes in the AA sample. The MAFs for rs1317286 and rs8040868 were 0.300 and 0.364, respectively, in the AA sample and 0.410 and 0.457, respectively, in the EA sample. To achieve 80% power for detecting the main effect under the assumption of GRR = 1.3, we would need 1823, 1696 and 1136 case-sibling pairs for MAF = 0.2, 0.30, and 0.40, respectively. Thus, the sample sizes of the current study, ~ 930 sibpairs in the AA sample, and ~ 330 sibpairs in the EA sample, should be sufficient for detecting a GRR of 1.3 for an SNP with an MAF
