filters, leaving 527,829 markers that passed all QC filters. Of these, 515,384 were autosomal and used in the present study. Genotyping was attempted on samples from 7,438 participants. Samples were eliminated if: 1) they had >5000 no-calls; 2) had a low GenCall score; 3) had extreme heterozygosity or homozygosity; or 4) represented a sample mix-up or we could not confirm known genetic relationships. A total of 160 (2.2%) of samples failed quality control filters and were dropped from the present analysis. Only one MZ twin from each MZ twin pair was genotyped. Prior to genomewide genotyping, zygosity had been assigned through questionnaire-based methods (~99% accuracy). With genome-wide genotyping we determined that 1.5% of DZ twins (per the questionnaire) were in fact MZ. Zygosity for these 8 pairs of twins was therefore reassigned as MZ. For all other MZ twins genotypes were wholly imputed from the genotyped cotwin to the non-genotyped cotwin (n = 1127) for a final GWAS sample of 8405 individuals.
