er will see various terminologies and definitions used by contributing authors to address the interface between genetics and public health.
One may wonder what is the meaning of prevention in the context of genetics and public health? Juengst (19) used the terms "genotypic prevention"-the interruption of genetic trait transmission from one generation to the next through reproductive counseling, carrier testing, prenatal diagnosis and pregnancy termination-and "phenotypic prevention"-the prevention of disease and death among people with specific genotypes (19). In its strategic plan (11), CDC clearly endorses the concept of phenotypic prevention as the strategy for public health-driven programs. Phenotypic prevention can be achieved by interrupting harmful interaction of environmental cofactors with human genetic variation or by using gene therapy to correct deficiencies in gene products.
Although public health-driven prevention may be clear-cut for adult-onset multifactorial conditions such as cancer and heart disease, its role is more problematic for early-onset, lethal single-gene conditions such as Tay Sachs disease and other severe conditions affecting children. It has been argued that an important role still exists for public health activities in prenatal diagnosis and genotypic prevention of severe or lethal conditions (20). Assurance of individuals' and couples' access to reproductive risk information could improve their own well-being, a goal that could be encompassed within a broad definition of health. For many couples, the availability of prenatal diagnosis provides a level of reassurance that permits them to proceed with childbearing they would otherwise forego. Assuring the availability and voluntary access to prenatal services and evaluating the impact of these services would fall within the purview of public health practice; nevertheless, while the public heath community should take an active role in promoting the use of genetic tests and services when there are proven and cost-effective interventions to prevent disease, death or disability (e.g., phenylketonuria or PKU), public health agencies cannot play a role in promoting the use of genotypic prevention as a method to improve the public's health.
Another philosophical issue in the integration of genetics into public health is what defines a genetic condition. Geneticists often make a distinction between single-gene disorders and susceptibility genes that are disease risk factors along with other genes and environmental exposures involved in disease development. This distinction is embodied in the concept of genotype penetrance. The distinction tends to fade when one realizes that all human disease is the result of interactions between genetic variation and the environment (broadly defined to include dietary, infectious, chemical, physical and social factors). Even many of the classical single-gene metabolic disorders are the result of a deficiency in a nutritional enzyme combined with dietary exposure to one or more chemicals (e.g., phenylalanine and phenylalanine hydroxylase deficiency in PKU; and iron intake and mutations in the HFE gene in hereditary hemochromatosis (17). As Rothman puts it, "It is easy to show that 100% of any disease is environmentally determined and 100% is genetically determined as well. Any other view is based on a naive understanding of causation" (21). Perhaps the wide range of penetrance with respect to clinical disease could be due, in part, to variations in the prevalence of interacting cofactors (e.g., other genes and modifiable risk factors). Universal exposure of newborn infants to phenylalanine through their diets leads to a high incidence of mental retardation among those who inherit a deficiency in phenylalanine hydroxylase.
If we accept the fundamental premise that genetic variation is associated with all human disease, there is no compelling reason to label a disease as genetic or not. For example, one can label hereditary hemochromatosis as an iron overload disorder resulting from the interaction between an inheri