Figure 4.
Detecting natural selection based on the distribution of ancestry switches. The expected number of ancestry switches is based on the local recombination rate, provided by phase II of the International HapMap Project [19]. The observed number of ancestry switches is based on a sample of 1,018 unrelated African Americans [17]. An excess of ancestry switches on chromosome 6p co-localizes with the major histocompatibility complex (MHC; indicated by the bar), consistent with the well-reported evidence of positive natural selection in this genomic region. Comparing the distributions of the observed and expected numbers of ancestry switches in an admixed sample can reveal genomic regions experiencing either positive natural selection (if there is an observed excess of ancestry switches) or negative natural selection (if there is an observed deficiency of ancestry switches).