After performing linkage analysis on both log-transformed and categorized smoking rate data sets with the EM Haseman-Elston QTL regression method implemented in GENEHUNTER, we found evidence for significant linkage of smoking rate to chromosome 11 in both the log-transformed (p = 0.000018) and the categorized SR data sets (p = 0.000001), based on the threshold suggested by Nyholt [10]. We also found evidence for suggestive linkage of smoking rate to chromosomes 9, 14, and 17 (p < 0.0017; [10]). Further, we found several regions located on chromosomes 7, 15, and 20 to be of potential interest at a significance level of p < 0.01 (Table 2). To confirm these findings, we analyzed the original and transformed SR data sets using the SIBPAL program in S.A.G.E. (v. 4.2) with the option of splitting these extended families into nuclear families, which yielded 1389 sib pairs. Three loci for which SIBPAL provided evidence for suggestive linkage were mapped onto chromosomes 4, 7, and 17. In addition, we found six more loci of potential interest at a significant level of 0.01 (Table 3).
