Genetic Epidemiology

Genetic epidemiology is an emerging field that developed initially from population genetics, specifically human quantitative genetics, with conceptual and methodological contributions from epidemiology. One of the early proponents of genetic epidemiology, Morton, defines the field as one that addresses the etiology, distribution, and control of disease in groups of related individuals and the inherited causes of diseases in populations. This definition has by necessity been broadened to include the role of the environment by others who emphasize the role of genetic factors as they interact with environmental factors in the occurrence of diseases in human populations. Khoury, Little, and Burke (2003) have recently coined the term human genome epidemiology to encompass a system of investigations that use the methods of epidemiology in population-based studies of the influences of genetic variation in both health and disease.

It should be noted that the field of molecular epidemiology stands in contrast to genetic epidemiology, as the former grew out of environmental epidemiology. The rationale for the emergence of molecular epidemiology was the need to identify biomarkers of environmental exposures as an application of molecular biology in epidemiology.

There are a number of aspects that distinguish genetic epidemiology from other areas of genetics from which it developed. The first is the populationbased nature of the research, which, together with shared methodological approaches, is one of its key links with epidemiology. Second, newer ways of conceptualizing the field stress the search for combined and interactive effects of genetic and environmental factors. Finally, genetic epidemiology includes the consideration of the biological basis of the diseases into developing models of causation for diseases.

The goals of modern genetic epidemiology have been broadened to include all diseases, whether they are common and complex or supposedly simpler, such as the so-called monogenic disorders. There has been a tendency in recent years in genetic epidemiology to place almost exclusive emphasis on the complex diseases, but some of the best advances in the epidemiology of genetic diseases have indeed involved simple inherited disorders, which are increasingly seen as more complex as we come to a better understanding of epigenetics and interactions of disease genes with environmental factors. In addition, any definition of genetic epidemiology arguably needs to encompass all aspects of the epidemiology of genetic diseases, including studies of prevalence, clinical epidemiology, genotype-phenotype relationships, and disease outcomes and progression. We also need more comprehensive studies of environmental factors that influence outcomes in genetic diseases.

Historically, the field of genetic epidemiology has some of its roots in the interests of medicine [Page 418]concerning the causes and heredity of disease. In the days prior to the field actually having a label (i.e., prior to the 1950s), scientists who perhaps would now be labeled as early genetic epidemiologists were trying to unravel the issues of nature and nurture with respect to human diseases. These activities were in contrast to the early practitioners of medical genetics who tended to be oriented toward the clinical and descriptive aspects of what were recognized as diseases with potential genetic involvement and to genetic counseling based on what was known then about the inheritance patterns of some diseases. Early practitioners of genetic epidemiology often looked for associations between diseases such as stomach ulcers and well-studied genetic traits of the day such as blood groups. One of the authors (F. J. M.) took a genetics course in the early 1960s that had the name ‘Heredity and Eugenics.’ Interestingly, a faculty member with a medical degree (MD) taught the course, most of which was spent examining what was known then about the genetics of human diseases. This was a time period when the field of medical genetics was in its childhood and genetic epidemiology had barely taken hold, and the legacy of eugenics was still alive in the names of journals and organizations.

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