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Sixteen biallelic markers (SRY10831a, SRY10831b, SRY4064, SRY2627, 92R7, P2, P3, M34, M9, M3, M2, YAP, M60, M89, M213, M216) located in the nonrecombinant region of the Y chromosome were analyzed in 209 individuals belonging to six Brazilian populations: four Afro-Brazilian populations, one population of white European descendants, and one popula- tion of Japanese descendants. The results showed that most of the Y chromo- somes of the Afro-Brazilians were from sub-Saharan Africa and that the proportion of Y chromosomes of European origin was greater than that of Y chromosomes of Amerindian origin. No typical African or Amerindian haplogroup was detected among Japanese individuals, and only one white individual showed a typical African haplogroup. Haplogroup P-92R7, which is highly frequent in the Portuguese and Italian populations, was the most frequent among whites (54%), and haplogroup K-M9, which shows wide geographic distribution and is absent in Africa, was the most frequent among Japanese individuals (65.6%). The two semi-isolated Afro-Brazilian popula- tions showed the highest and the lowest genetic diversity, respectively. These differences probably reflect the effect of greater or smaller gene flow be- tween a small isolated group and other populations. These findings show that the process of admixture does not occur homogeneously, with a tendency toward preferential marriages within the ethnic group and a clear direction in unions between European men and Amerindian or African women in the past. The results agree with historical and social data about the formation of the Brazilian population and reveal some of the factors that contribute to its heterogeneity.