Figure 2. .
Magnitude of expression differences within and between populations. a, Magnitude of gene-expression differences among individuals, shown as the factor of variation (X-axis) versus Q value (Y-axis). The Q value is a measure of statistical significance in terms of the FDR.22,25 For each gene, the factor of variation is calculated as the ratio of the maximum:minimum log2 expression level across all individuals.5 b, Magnitude of gene-expression differences between populations, shown by a volcano plot of the average log2-fold change between the CEU and YRI samples (X-axis) versus Q value (Y-axis). In panels a and b, the horizontal dashed lines at 0.17 and 0.83 indicate the estimated proportion of truly null hypotheses in tests of differential expression among individuals and between the CEU and YRI samples, respectively.