Maximum number of cigarettes smoked per day was correlated in sibling pairs (r2 = 0.18 ± 0.03, 2796 pairs); the correlation estimate did not vary after adjustment for age and sex (r2 = 0.18 ± 0.03) or age, sex, and year (r2 = 0.16 ± 0.03). Unadjusted correlation estimates were lower for parent-child pairs (r2 = 0.09 ± 0.03, 3037 pairs) and spouse pairs (r2 = 0.13 ± 0.04, 486 pairs). Heritability of this trait was estimated to be 0.21 (SE = 0.03) using the VC approach (p < 0.001), and did not vary with adjustments for sex, age, or year (range, 0.21–0.23). Allowing for a t-distribution did not change VC modeling results (heritability range, 0.21–0.23). Using MCMC oligogenic segregation analysis, the components of variance for maximum number of cigarettes smoked per day were as follows: residual variance 41%, age 3%, sex 5%, and total genetic variance 51%. Two large QTLs were estimated for the trait, with individual contributions of approximately 28% and 20% of the total variance, respectively, which explained approximately 55% and 39% of the genetic variance. MCMC analysis estimated the largest QTL to be overdominant.
