Personalized medicine is the latest promise of a gene-centered biomedicine to provide treatments custom-tailored to the specific needs of patients. Although surrounded by much hype, personalized medicine at present lacks the empirical and theoretical foundations necessary to render it a realistic long-term perspective. In particular, the role of genetic data and the relationship between causal understanding, prediction, prevention, and treatment of a disease need clarifying. This article critically examines the concept of information in genetics and its relation to modern-day genetic determinism, using pharmacogenetics, personalized medicine’s core discipline, as a test case. The article concludes that: (1) genetic knowledge does not constitute a privileged basis for personalized medicine because there is an a priori complete causal parity of genetic and nongenetic resources of development; and (2) prediction, prevention, and treatment all depend on a causal-mechanistic understanding that will follow only from integrating data across the whole gamut of developmental factors—genetic and non-genetic. In a future successful personalized medicine, genes will have no special status, either as determinants of phenotype, markers of disease or as targets of treatment.