Analyses involving secondary tooth eruption were performed using data from the Danish National Birth Cohort (7). The genotype data were derived from two on-going GWASs of preterm birth (56) and obesity (57). The study combined all observations between age 6 and 14 years (starting with the 6th and stopping with the 14th birthday), the time period when eruption of permanent dentition usually occurs. For each visit to the dentist, the total number of permanent teeth (excluding third molars) was recorded, and regressed on age. The resulting residuals were then standardized, and for each individual the mean residual across all available records was used as the phenotype. Genotypes for the two GWASs were imputed separately using MACH (54). The resulting imputed genotypes were analysed separately and meta-analysed with METAL (55).
