The ubiquitous answer to these questions in the literature of public health genetics is the prevention of disease, a classic public health goal. This goal is operationalized as the reduction over time in measures of the morbidity and mortality caused by the target disease within the screened population. To flesh out the kinds of interventions that should be counted in those measures, most authors appeal to the public health field's traditional lexically-ordered scheme of primary, secondary and tertiary levels of prevention, and attempt to categorize population genetic screening tests accordingly. Thus, for example, one public health guidance document states:
Primary prevention genetic services are services intended to prevent a birth defect, genetic disorder, or disease before it occurs. Genetic counseling is a form of primary prevention. Genetic counseling provides couples with information about their pregnancy, and reproductive risks and pregnancy options. Secondary prevention genetic services are services intended to prevent the unfavorable sequelae of an existing disorder or genotype. Newborn screening is a classic example of secondary prevention. Tertiary prevention genetic services are services aimed at ameliorating the unfavorable consequences of existing disorders, through enabling services such as parent-to-parent support and empowerment. (Kaye et al.)
Using this scheme provides a logic for shifting tests into the newborn, prenatal and preconception stages, because traditionally "primary prevention" has been considered the ultimate goal of public health interventions.
Unfortunately, this scheme also introduces an important equivocation into public health discourse between two different ways in which genetic screening might be thought to be preventive: genetic screening as a technique for preventing the expression of a genetic disease in an individual and genetic screening as a technique for preventing the inter-generational transmission of disease genes. For convenience, the first kind of prevention may be called phenotypic prevention, since its goal is to prevent the manifestation of a particular clinical phenotype. Similarly, the second sort of prevention may be called genotypic prevention, (or geno-prevention) because its goal is to prevent the birth of people with particular genotypes. Equivocating between these two senses of prevention in discussions of population screening results in the attribution of genotypic preventive goals to public health genetics. That, in turn, generates the deeper questions of public authority, social justice, and professional allegiance that animate bioethical concern in this area.
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