Figure 9.

Number of samples required to detect a disease locus where population A ancestry, on average, increases risk, as a function of the proportion of ancestry in each sample. Individuals with population A ancestry between 10% and 90% provide the most power. The power for admixture mapping contributed by a typical African American sample (20% European ancestry; 80% African ancestry) corresponds to a percent population A of 0.2 (European ancestry confers increased risk) or 0.8 (African ancestry confers increased risk). Fewer samples are required if the less common (European) ancestry confers increased risk (e.g., a disease such as MS rather than prostate cancer), although the effect is slight (only 1.2- to 1.3-fold more samples are required to achieve the same power; see fig. 10). We note that this graph assumes perfect information extraction and the same M_i for the two parents of each sample. Deviations from these assumptions—in particular, the imperfect information extraction in real maps such as that described by Smith et al. (2004 [in this issue])—mean that the number of samples required for a practical study would be about twice as high as shown.