It is beyond the scope of this report to review advances in basic and clinical genetics and the relation of genetic structure and function to disease occurrence and outcome. For those involved in social surveys who are not schooled in genetics, a very brief discussion emphasizing the complexity of the situation may be of value. On 23 matched (except for the one pair of sex chromosomes) pairs of chromosomes in the nuclei of each human cell, the human genome contains about 100,000 genes, discrete functional and structural sites that interact with the internal cellular and external environment to direct basic cell growth, activity, and death and to transfer this information to the next generation. Each matched gene may vary somewhat from its mate and from the respective genes at the same site in other individuals. These structural variants, called alleles, may function somewhat differently from each other. The specific genetic makeup of an individual is called the genotype. The process of change in the structure of a gene, often accompanied by changes in function, is called mutation; mutation may occur spontaneously or be accelerated by external environmental forces. Mutations may be harmful or helpful to an organism or be biologically neutral. Not all human genes have yet been identified as to structure and function, but work is progressing rapidly. Determination of the structure and function of genes and the relation of altered gene structure to disease occurrence is made more complex by several recent observations: (1) some genes are not necessarily in one physical location on a chromosome; (2) to the extent that important chronic illnesses are gene-related, there are probably multiple genes involved; (3) the mechanisms of genetic regulation and how environmental factors alter that function are incompletely understood; and, (4) some genetic material (DNA) is located outside the nuclear chromosomes in the cytoplasm of the cell and is probably of maternal origin only. Thus, the search for gene-disease associations is clearly complex and difficult, although extremely important. However, as noted below, there are other potential genetically related applications of population studies.
To find potential applications of existing population studies for addressing genetic hypotheses, it is instructive to indicate some general categories of applications, temporarily leaving aside study methods and logistical issues. These categories are possible through the rapidly expanding ability to identify and characterize many genes within individuals and large population samples. However, as in all other fields of measurement, quality control in the laboratory determination of various alleles is essential, as substantial error can occur in laboratory procedures.
Given the emerging capacity to determine alleles in population samples, the following is a selected list of general genetic research applications in population surveys, recognizing that specific studies have many scientific and methodologic contingencies: