Our study has focused on the Y chromosome, but can we say anything about whether admixture has been predominantly male-mediated? Some mtDNA studies32,33 find evidence of the characteristic North African haplogroup U6 within the Iberian Peninsula. Although the overall absolute frequency of U6 is low (2.4%33), this signals a possible current North African ancestry proportion of 8%–9%, because U6 is not a common lineage in North Africa itself. If this figure is reliable, it is not dissimilar from the level of paternal ancestry that we find. This might suggest that initial admixture involved movement of approximately equal numbers of males and females. However, because of drift through the differential reproductive success of males and females carrying different lineages, current relative proportions are an unreliable guide to proportions of the past. Comparable mtDNA data reflecting Sephardic Jewish contributions to the various areas of Iberia are not available, but sequence data on hypervariable regions I and II in a sample of 31 Sephardic Jews from Turkey has shown that their sequences and haplogroup frequencies are similar to those of Iberian populations,67 suggesting that admixture might be difficult to detect. Interestingly, analysis of European genome-wide SNP data68 shows the western half of the Iberian Peninsula to display the highest mean heterozygosity values in the continent, an observation that might reflect its history of population admixture from very different sources.