To avoid population stratification (Cardon & Palmer, 2003), we conducted our analyses separately by ancestry group. Genetic ancestry principal components were computed from GWAS data using Eigenstrat (Price et al., 2006) and the 1000 Genomes, Phase III reference panel. These principal components reflect continuous variation in allele frequencies representing ancestral differences. Individuals were assigned an ancestry classification (European, African, or Other) based on the first two principal components. Our analyses included participants of European and African ancestry. We note that genetic ancestry refers to the population(s) from which an individual’s biological ancestors originated (Peterson et al., 2019), and ancestry and self-report racial/ethnic background are not identical. In the United States, individuals of European ancestry and African ancestry often self-identify as White/European American and Black/African American, respectively.
