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New data on 17 blood group and protein genetic systems obtained among the Ayoreo and Lengua Indians of Paraguay are presented. They include the first report on the red cell band-3 protein investigated among South American Indians. This information was integrated with previous results available for these two and four other groups. Five of the six populations reside in the Chaco area, while the sixth was included as an outgroup living elsewhere in Paraguay. Four of the five Chaco tribes exhibit good genetic homogeneity, but the Ayoreo are somewhat different. The results confirm the Chaco as a distinct biological (as well as cultural and economic) region, which should be considered in evaluations of genetic variability among South American Indians.

Information related to 31 protein genetic systems was obtained for 307 individuals affiliated with the Cinta Larga, Karitiana, Surui and Kararaô Indians of northern Brazil. In terms of genetic distances the Cinta Larga showed more similarities with the Karitiana (both are Tupi-speaking tribes), while at a more distant level the Surui clustered with the Kararaô. The latter, a Cayapo subgroup, showed a completely different genetic constitution from the other subgroups of this same tribe. Both the Kararaô and Karitiana are small, remnant populations, and their gene pools have presumably been severely affected by random and founder effects. These results were incorporated with those of 25 other Amazonian Indian tribes, and analysis by two multivariate techniques confirmed a previously observed geographical dichotomy, suggesting either that the Amazon river constitutes a barrier to north-south gene flow or that latitudinally different past migrations entered the region from the west.

It has been suggested that average genetic similarity, as assessed by blood genetic markers, could influence mate choice in humans. In the present study, average genetic similarity was assessed in 183 couples submitting to paternity determinations in relation to six blood group systems and three HLA loci. Couples in which the putative father was excluded were compared with those in which such exclusion did not occur, and real couples were compared to random pairs. The differences were all statistically nonsignificant. Possible reasons for the different results found in the previous sample and in the present study are considered.

Data on 31 genetic systems were obtained for 421 individuals belonging to the Arara, Araweté, Mundurucu, and Jamamadi tribes of northern Brazil. The Jamamadi depart farthest, and the Mundurucu least, from South American Indian averages. These data are analyzed together with those of 24 other Amazonian groups. Genetic distances and corresponding dendrograms indicate a cluster of 14 related tribes living north of the Amazon river. These genetic results show only a modest correlation with linguistic and geographic relationships among these groups.

Blood samples from 509 Macushi (3 villages) and 623 Wapishana (11 villages) of Northern Brasil and Southern Guyana have been analyzed with respect to the phenotype and gene frequencies at the following 12 polymorphic loci: ABO, Kell-Cellano, MNSs, Rh, P, Duffy, Kidd, Diego, Lewis, Group-specific component, and the immunoglobulin allotypes of the Gm and Inv systems. The data suggest that 5-6% of the Wapishana gene pool is derived from non-Indians but only 1-2% of the Macushi. Inter- and intratribal genetic distances between villages are calculated for these data in an effort to understand gene flow between the tribes and to account for the unusual distribution of a newly-discovered genetic polymorphism of erythrocyte esterase A thus far limited to these 2 tribes (Neel et al., 1977). The data are puzzling and consistent with the possibility that both the Carib-speaking Macushi and the Arawak-speaking Wapishana have derived the esterase A allele in question from some third group now extinct or thus far undiscovered. Intertribal genetic distances based on gene frequencies at 6 loci are derived for 20 Amerindian tribes (including these 2); the "central" position of these 2 tribes can in part be explained by the active migration matrix connecting them.

Two communities of relatively recently contacted Brazilian Indians, the Pacaás Novos, have been studied in relation to several demographic parameters and 28 genetic systems. The age and sex distribution, fertility and mortality patterns were not very distinct in the two populations, but they differed markedly in relation to the mating, migration and genetic data (six of the 19 variable loci showed differences higher than 10%). This was interpreted as being the consequence of a fission event, one of the many that may periodically occur, sometimes followed by fusions, in populations at this cultural level. The Pacaás Novos also show some distinctive features when previous genetic studies of South American Indians are reviewed, eight alleles of the variable 19 (LMS, R2, R0, Se, Hp1, Gm1,2;21, Gc1F and PGM1(1)), presenting differences varying between 12%-34% from the average obtained considering these earlier investigations.

Results concerning HLA types and 22 other blood genetic systems are reported for the Parakanã Indians of northern Brazil, a tribe that is notable for the light color and pilosity of some of its members. No clear evidence of Caucasoid admixture was found, but the Parakanã show unusual frequencies in the EsD1, PGM1(1) Gc2, CpB, Fya, Dia, and LM genetic markers. In addition, the very rare Rh allele ry is present, as well as what seems to be a new PGM2 variant. There is very limited heterogeneity in the HLA system. All these distinctive features may have arisen through a combination of founder effects and genetic drift. However, low FIS values, as well as higher mean ages in heterozygous as compared to homozygous persons, suggest that an heterotic effect is counteracting these dispersive forces.