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Genetic epidemiology is the study of the role of genetic factors in determining health and disease in families and in populations, and the interplay of such genetic factors with environmental factors. In slightly more formal language, genetic epidemiology was defined by Morton as "a science which deals with the etiology, distribution, and control of disease in groups of relatives and with inherited causes of disease in populations". It is closely allied to both molecular epidemiology and statistical genetics, but these overlapping fields each have distinct emphases, societies and journals.

Traditionally, the study of the role of genetics in disease progresses through the following study designs, each answering a slightly different question:


 * Familial aggregation studies: Is there a genetic component to the disease, and what are the relative contributions of genes and environment?
 * Segregation studies: What is the pattern of inheritance of the disease (e.g. dominant or recessive)?
 * Linkage studies: On which part of which chromosome is the disease gene located?
 * Association studies: Which allele of which gene is associated with the disease?

This traditional approach has proved highly successful in identifying monogenic disorders and locating the genes responsible.

More recently, the scope of genetic epidemiology has expanded to include common diseases for which many genes each make a smaller contribution (polygenic, multifactorial or multigenic disorders). This has developed rapidly in the first decade of the 21st century following completion of the Human Genome Project, as advances in genotyping technology and associated reductions in cost has made it feasible to conduct large-scale genome-wide association studies that genotype many thousands of single nucleotide polymorphisms in thousands of individuals. These have led to the discovery of many genetic polymorphisms that influence the risk of developing many common diseases.