The first step was to estimate the heritability of the smoking phenotypes. The univariate SNP heritability, the proportion of phenotypic variance explained by GWAS SNPs, was evaluated for each smoking phenotype (Table 2). All the smoking phenotypes have statistically significant SNP heritability (p<0.001). The phenotype with the highest magnitude of estimated SNP heritability (15%) was nicotine dependence. This is approximately double the estimated SNP heritability for the other smoking phenotypes (estimates ranging 6%–8%), suggesting that nicotine dependence, as defined using FTND, has higher SNP heritability than the other smoking phenotypes (p<0.0005, for all two-way comparisons).
