locus display different levels of the phenotype associated with the locus; incomplete penetrance, such that some individuals with the genotype do not show the phenotype at all; and variable time of onset, in which individuals with the same disease genotype vary widely in the age at which symptoms appear.

In respect to health and disease, genetic loci for which particular allelic configurations result with high reliability in a diagnosable disease outcome are often described as “disease genes.” However, particular genotypes at other genetic loci may affect the same general categorical disease state. Thus, many disease genes, though they may be sufficient, are not necessary for the expression of the disease phenotype. In other cases, as in the association between the ε4 allele of the apoE locus and Alzheimer’s disease (discussed below), the identified allele is neither necessary nor sufficient. In different populations, the same disease state may be achieved by different genotypes affecting different mediating mechanisms.

Other disease states may not be categorically distinct from the normal range of variability of some anatomic structure or physiological process but may constitute extremes of such a dimension. The location of an individual in continuously distributed phenotypes is typically due to the action of many genetic loci, as well as many environmental variables. Any particular value of such a continuously distributed phenotype can be achieved by numerous combinations of input variables. The genetic input is described as polygenic. Analysis of such systems is generally statistical, with a central theme being the attribution of proportions of the measured phenotypic variability into two broad categories, genetic and environmental, and, depending on the research design, into more detailed categories, such as additive or nonadditive genetic effects and shared or nonshared environmental influences.

Gene Interactions

In considering ethnic identity and health, genes may be relevant in two broad senses. First, the gene pools of different ethnic groups may contain different frequencies of alleles at some loci that are pertinent to health status or to disease processes. However, such differences by themselves are unlikely to account for broad and pervasive health differences among socially identified racial and ethnic groups.

Second, the phenotype consequent on a given genotype may vary between ethnic groups because of interactions with environmental factors. The environment, in this context, is defined by exclusion, as all influences not coded in DNA. It thus covers all the other factors noted in Chapter 2, including prenatal effects, nutritional influences, the preventive conse-

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