Admixture Model Research Articles

The use of nonmetric variation in estimating human population admixture: a test case with Brazilian blacks, whites, and mulattos.

Measurements in populations which serve as valid indicators of biological relationship should be proportional to genetic distance. In order to test the utility of discrete cranial traits for estimating genetic distances among populations, estimates of admixture are obtained for gene frequency data and nonmetric cranial data in São Paulo mulattos (M). The gene frequency data serve as a control that the three populations are related as stated: estimates of admixture are obtained by using São Paulo whites (W) and blacks (B) as parental populations and by estimating the parameter of admixture, m, in the model pM = (1 - m) pW + mpB (Elston, 1971) where the p's are either gene frequencies or nonmetric trait frequencies. A test of goodness of fit of the model provides a means of ascertaining whether or not the data fit this linear model. While the gene frequency data indicate distances among the three populations which are highly compatible with the linear model of admixture, the nonmetric data show significant deviations from the model. This implies that the frequencies of the nonmetric traits in the populations used in this analysis are not a linear function of genetic distance. This discourages the use of nonmetric traits in making quantitative conclusions about genetic relationships. It also suggests the need for investigation of the use of other skeletal characters for estimating genetic distance, as well as approaches for such investigations through the study of hybrid individuals.

Techniques for estimating genetic admixture and applications to the problem of the origin of the Icelanders and the Ashkenazi Jews.

A method is introduced for simultaneously using multiple loci to estimate admixture and test goodness of fit of the model of admixture. Deviation of observed frequencies from expectation caused by sources of error such as sampling and/or drift is allowed for all loci in all populations. This allows investigation of the effects of different assumptions about sources of error on the estimates. Admixture is then investigated for Icelanders and Ashkenazi Jews. Results indicate that the Icelanders have a large Norse contribution, and that the Jews may have a small to moderate contribution from the European gene pool. There are some indications that AB0 and G6PD give abnormal estimates of admixture compared to other loci, and that the Jewish gene pool may be derived from additional populations in addition to the populations considered.

Intertribal gene flow between the Ye'cuana and Yanomama: genetic analysis of an admixed village.

Genetic exchange with a neighboring village of Ye'cuana Indians had introduced two alleles, Dia and ACPa, into the Yanomama Indian Village of Borabuk. After several generations, these alleles had reached frequencies of 0.08 and 0.10, respectively. These frequencies are puzzling because they are higher in Borabuk than in the Ye'cuana village from which they were derived. Single allele estimates of ancestral proportions obtained from either of these traits are biologically unrealistic and suggest that admixture is not a good explanation for genetic variation in Borabuk. Nevertheless, multiallelic admixture models are seen to produce credible estimates of ancestral proportions and to explain a large amount of allele frequency variation in Borabuk. When these results are compared with expectations derived froma formal pedigree analysis, good agreement is seen. Comparison of single allele estimates of ancestral proportions obtained from alleles at 11 loci, with multiallelic estimates obtained from the same 11 loci and with the pedigree-derived estimates, demonstrates the superiority of the multiallelic approach.

Evolutionary trees and admixture: phylogenetic inference when some populations are hybridized.

Maximum likelihood estimation of admixture parameters and divergence times for an evolutionary tree in which some populations may be hybridized is developed. A test of treeness against the alternative of hybridization using a hierarchy of admixture models is proposed as an adjunct to the goodness-of-fit test suggested by Cavalli-Sforza & Piazza (1975). The method is illustrated by an application to gene frequency data from Makiritare Indian villages in southern Venezuela.

A Theoretical Study of Pulmonary Capillary Gas Exchange and Venous Admixture

A model of pulmonary capillary gas exchange and venous admixture is presented and the inclusion of this model into a model of the entire respiratory system is discussed. Partial pressure and concentration gradients for nitrogen, helium, oxygen, and carbon dioxide are predicted. The cases of breathing room air and 10% oxygen are studied. In both of these studies the Bohr and Haldane effects are included, and the "physiological" dissociation curves of oxygen and carbon dioxide are predicted for the normal case as blood flows from the venous blood end of the capillary to the arterial blood end. Venous admixture effects are also calculated for both of these cases. The effects of emphysema, pulmonary congestion, and altered cardiac function on the gradients are studied.