Figure 1

Population-specificity of DNA methylation. (a) Heatmap of the clustered methylation data set. Three representative cases are magnified: a site with a clear population difference; a site showing within- but not between-population variability; and a site with little variability within or between populations. (b) We performed pyrosequencing as an independent means to measure methylation of two CpG sites (IGSF2, chromosome 1, base 117345939; PLSCR2, chromosome 3, base 147696535) in our 180 samples. The agreement validates the accuracy of our microarray data. (c) The methylation of many sites differs between CEU and YRI. We performed the nonparametric Wilcoxon test to identify CpG sites differing in methylation between populations. The P-values are skewed towards small values, as shown by comparing to the expected uniform distribution on either a linear (left) or log (right) scale.