Gene admixture in human populations: Models and predictions

Brief accounts of methods for estimating proportions of admixture in populations and individuals of hybrid origin are presented with the objective of appraising their underlying assumptions. In view of the uncertainties introduced by assumptions under which admixture estimates are obtained, it is concluded that the reliability of estimates derived from different methods cannot be formally compared. With examples from several admixed populations, it is shown that all methods do not necessarily give discordant results when identical data are used to obtain admixture estimates. Even though past experiences using admixed populations to detect selection or to understand disease etiology have not been very successful, it is believed that admixed human populations can be regarded as a natural experiment. Hence, they are suitable for microevolutionary and epidemiological studies. The proper identification of ancestral populations and the degree of asymmetry in gene flow (sex-biased admixture) are important issues in admixture studies. These aspects can be examined in the statistical properties of allele frequency distributions, but corroboration of particular models should be made from in-depth investigations of historical demography and social structure of admixed populations. Future studies of admixture with DNA polymorphism data may resolve some of the uncertainties associated with current techniques of detecting genetic polymorphisms. Because of the abundance of genetic data, it is argued that morphological traits are of limited use in resolving current problems of human admixture studies.