From the 330 extended families of the Framingham Heart Study, 313 were chosen in which there was at least one smoker present in the data from 1970–1971. Table 1 shows some of the characteristics of the sample used in the current study. The smoking rate (SR) phenotype of each individual reported in the study was based on self-reported average number of cigarettes smoked per day during 1970–1971. Information on smoking rate was available for 2493 of the 4522 members distributed across the 313 families. Of these 2493 subjects, 1636 were genotyped for 401 markers at an average spacing of 7.5 cM between markers. For individuals who reported non-smoking, we considered their phenotype as 'zero' and included them in the linkage analysis. On the other hand, we considered the phenotypes of individuals who did not report their smoking status during the survey as 'unknown' and excluded them from the analysis. Skewness and kurtosis for the self-reported average number of cigarettes smoked per day were 1.38 and 5.11, respectively. To minimize the impact of skewness on linkage analysis results, we transformed the
