To exclude any pair of individuals within EAs and AAs who were as closely related as second degree, we calculated estimated genome-wide alleles shared identity-by-descent (IBD) for every pair of individuals using PLINK software and randomly removed one individual from each pair with an IBD value greater than .25 if both individuals had the same lifetime PTSD phenotype; if, within a pair of individuals with IBD larger than .25, one was a PTSD case and the other a PTSD control or PTSD diagnosis unknown, we removed the other subject to maximize the sample size of PTSD cases. A total of 310 EAs and 685 AAs were removed because of relatedness, leaving 1578 EAs and 2766 AAs for analysis. To increase statistical power, we also performed GWAS analysis without removing the related subjects but adjusted the genetic structure using factored spectrally transformed linear mixed models (25). This left 1846 EAs and 3392 AAs for analysis after quality control steps.
