Mixed Race Studies

Human Genetic Diversity and the Nonexistence of Biological Races

Human Biology
Volume 75, Number 4, August 2003
pages 449-471

Rich Kittles, Associate Professor of Medicine and Epidemiology and Biostatistics
University of Illinois, Chicago

Jeffrey C. Long, Professor of Anthropology
University of New Mexico, Albuquerque

Sewall Wright’s population structure statistic, F_ST, measured among samples of world populations is often 15% or less. This would indicate that 85% of genetic variation occurs within groups while only 15% can be attributed to allele frequency differences among groups. In this paper, we show that this low value reflects strong biases that result from violating hidden assumptions that define F_ST. These limitations on F_ST are demonstrated algebraically and in the context of analyzing dinucleotide repeat allele frequencies for a set of eight loci genotyped in eight human groups and in chimpanzees. In our analyses, estimates of F_ST fail to identify important variation. For example, when the analysis includes only humans, F_ST = 0.119, but adding the chimpanzees increases it only a little, F_ST = 0.183. By relaxing the underlying statistical assumptions, the results for chimpanzees become consistent with common knowledge, and we see a richer pattern of human genetic diversity. Some human groups are far more diverged than would be implied by standard computations of F_ST, while other groups are much less diverged. We discuss the relevance of these findings to the application of biological race concepts to humans. Four different race concepts are considered: typological, population, taxonomic, and lineage. Surprisingly, a great deal of genetic variation within groups is consistent with each of these concepts. However, none of the race concepts is compatible with the patterns of variation revealed by our analyses.

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The Genetic Structure of Admixed Populations

Genetics
February 1, 1991
Volume 127, Number 2
pages 417–428

Jeffrey C. Long, Professor of Anthropology
University of New Mexico, Albuquerque

A method for simultaneously estimating the admixture proportions of a hybrid population and Wright’s fixation index, F_ST, for that hybrid is presented. It is shown that the variance of admixture estimates can be partitioned into two components: (1) due to sample size, and (2) due to evolutionary variance (i.e., genetic drift). A chi-square test used to detect heterogeneity of admixture estimates from different alleles, or loci, can now be corrected for both sources of random errors. Hence, its value for the detection of natural selection from heterogeneous admixture estimates is improved. The estimation and testing procedures described above are independent of the dynamics of the admixture process. However, when the admixture dynamics can be specified, F_ST can be predicted from genetic principles. Two admixture models are considered here, gene flow and intermixture. These models are of value because they lead to very different predictions regarding the accumulation of genes from the parental populations and the accumulation of variance due to genetic drift. When there is not evidence for natural selection, and it is appropriate to apply these models to data, the variance effective size (N_e) of the hybrid population can be estimated. Applications are made to three human populations: two of these are Afro-American populations and one is a Yanomamö Indian village. Natural selection could not be detected using the chi-square test in any of these populations. However, estimates of effective population sizes do lead to a richer description of the genetic structure of these populations.

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