Population structure was assessed by examining the principal components (PC) of the variance of the genotype profiles, using Eigenstrat software6. Initial examination revealed several clusters of siblings and other close relatives (cousins or similar) whose similarity skewed the axes; where data were available, these identities were in agreement with participant records. After removal of these relatives, analysis of 163 unrelated individuals revealed seven significant eigenvectors. None of these explain more than 5% of the variance, and PC3 through PC7 are heavily weighted by large clusters of SNPs on one or a few chromosomes. As described by others, such axes are commonly observed and do not provide reliable genome-wide estimates of population structure7,8, but it is interesting to note that PC3 distinguishes Ighrem from the other locations (Supplementary Fig. 1a online).
