ceptualizing and describing these dynamics are highly developed and are clearly central to the taxonomy of human beings.

A key concept is that of genetic distance, of which several measures have been proposed. Basic to these is the probability that a randomly selected allele (see below) from one population will be identical to another allele from the same population but not to one chosen from a different population (see, e.g., Hartl and Clark, 1997). Measures of distance provide metrics that can be used to decide if groups differ sufficiently to be regarded as separate races. But different criteria may be used with respect to how large the genetic distance must be to constitute a racial divide. Some racial classification schemes, therefore, feature many races with small interracial differences, and others identify only a few larger and more separated groups. Each scheme serves its own purposes, and no classification can be regarded as necessarily more correct than others.

In any scheme, the idea of racial identity is a probabilistic one. Different groups are not delineated by clear and unambiguous borders but shade into each other along gradients. Strict categories, exclusive and exhaustive, in which each human being is assignable without ambiguity to one and only one race on the basis of genetic characteristics, are unattainable. Thus, genetics cannot provide a single, definitive human classification scheme with which to address the many facets of health differences.

At the same time, race, as it is socially and politically defined, is a powerful social and political reality. Thus, it is worth asking whether there are identifiable gene differences among socially defined populations or racial groups. The answer to this question requires a distinction between the concepts of genetic loci and genetic alleles. The term locus had its origin at a stage of genetic science when it became evident that the hypothetical elements of Mendelian theory had a physical existence and were located at particular sites, or loci, on chromosomes. A basic postulate of Mendelian theory was that these hypothetical elements (later to be termed genes) could exist in different forms, which came to be termed alleles. Thus, a key question is whether socially defined populations differ in the number or types of loci they possess.

Although it is difficult to prove a universal negative, the evidence is persuasive that all humans possess the same array of genetic loci. The genetic differences among people as individuals consist in the particular array of alleles at these loci; the genetic differences between or among populations can be described in terms of the relative frequencies of the different alleles in the populations. Decades of research in population genetics have documented differences among populations in allelic frequencies for a large number of genetic loci, including many that are health- or disease-related. Of greatest salience for our present topic is the additional observation that the range of genetic differences among individuals within

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