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We used mitochondria] DNA (mtDNA) and Y-chromosome DNA polymorphisms to analyze the ethnic origin of maternal and paternal lineages in two Amerindian subpopulations from northwestern Argentina. One of the subpopulations was from San Salvador de Jujuy, located 1200 m above sea level. The second subpopulation inhabits the Quebrada de Humahuaca area at altitudes ranging from 2500 to 3500 m. Both subpopulations have the same ethnic background. All mtDNA haplotypes were identified as Amerindian with a frequency of 64.6% of the B form (9-bp deletion in mtDNA region V). Because all Central Andean Amerindian populations studied so far exhibit high frequencies of the B haplotype, we propose that they probably are derived from a common ancestral population that inhabited the Central Andes 6000-8000 years b .p . The presence of paternal directional mating (asymmetric contribution of one parental lineage to interethnic gene mixtures) was demonstrated by the finding of an average introgression of 40.5% Spanish Y chromosomes into our Amerindian sample. This introgression was more evident at low altitude than at high altitude, with frequencies of 64.3% in San Salvador de Jujuy (low altitude) and 27.6% in Quebrada de Humahuaca (high altitude) (p < 0.05). The San Salvador de Jujuy subpopulation also showed a significantly higher Y-chromosome gene variability than the Quebrada de Humahuaca subpopulation. These findings are in good agreement with historical reports indicating that the colonization of South America was undertaken by men who usually practiced polygamous unions with Amerindian women and that San Salvador de Jujuy was the main northwestern Argentinian region of European to Amerindian gene admixture. We found 16.7% of cases with Spanish Y chromosomes and Amerindian family names, and the same percentage with Amerindian Y chromosomes and Hispanic names. The former group probably is the result of unions between Hispanic men, who transmitted the Y chromosome, and Amerindian women, who transmitted the family name to the progeny. The latter group likely illustrates the practice of changing names from Amerindian to Hispanic during the baptism of native Americans in colonial times.