A total of 2 446 724 SNPs common to both studies were tested for association with ‘age at first tooth’ and ‘number of teeth at one year’. All analyses were adjusted for gestational age, sex and age, where appropriate (see Materials and Methods). Results from the two studies were combined using fixed effects inverse variance meta-analysis, where effect size estimates are weighted according to the inverse of their standard errors. Q–Q plots indicated little inflation of the test statistics in the individual cohorts and for the meta-analysis overall (‘Age at first tooth’ LAMBDA ALSPAC = 1.04; ‘Age at first tooth’LAMBDA NFBC1966 = 1.05; LAMBDA META = 1.07; ‘Number of teeth’: LAMBDA ALSPAC = 1.02; LAMBDA NFBC1966 = 1.04; LAMBDA META = 1.06) (Supplementary Material, Fig. S1). The genomic inflation factor λ is well known to increase with sample size; we, therefore, also calculated λ1000 values (8) for the ‘Age at first tooth’ (λ1000 = 1.01) and ‘Number of teeth’ (λ1000 = 1.00) meta-analyses. Both values are consistent with little latent population stratification or other systematic biases affecting our results.
