and individuals with a pairwise IBD estimate > 25% were assigned to the same family. Self-reported males with X chromosome heterozygosity > 20% and self-reported females with X chromosome heterozygosity < 20% were excluded. Population substructure in the entire sample was evaluated by analysis of the principal components (PCs) of ancestry using Eigensoft20 and the multi-ethnic 1000 Genome reference panel for comparison. Individuals were classified as AA or EA according to the reference panel population to which they were more closely matched. SNP genotype imputation was performed separately in AAs and EAs using the March 2012 1000 Genomes reference panel (1000 Genomes Project, 2012; http://www.1000genomes.org/) and Minimac321 implemented on the Michigan imputation server (https://imputationserver.sph.umich.edu). Genotyping, QC, and imputation procedures for the COGA dataset are described elsewhere22. Analysis was limited to SNPs with an imputation quality score > 0.8 and MAF > 0.03.
