The goal of pharmacogenetics, to tailor drug treatments to patients based on molecular genetic markers, is fundamentally premised on drug responses being heritable traits. For studies of disease risk, the traditional genetic-epidemiologic approaches of twin and family studies have convincingly demonstrated a heritable basis for most diseases, motivating subsequent waves of molecular genetic studies to detect the specific genes. However, whether an individual's response to a given drug is heritable has not, in general, been established. This in large part reflects the practical and ethical difficulties inherent in applying twin and family designs to a phenotype that is both difficult to assess and meaningfully defined only in individuals who are both affected by the disease and receive pharmacotherapy. Finding sufficient numbers of concordantly affected relatives to enroll in a drug response study contemporaneously, or who have historically received equivalent exposures to the same drug, is typically not a possibility. Nonetheless, molecular pharmacogenetic studies have proceeded, predicated on the assumption that drug response is heritable.
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