Considerable progress is being made in understanding how genetic variation translates intophenotypic variation, and how this translation is affected by the environment. We are beginning to understand the causes of human variation, whether it be for height, skin color, or susceptibility to disease. Analogously, breeders are dissecting the basis for variation in crop yield and resistance to pests, and evolutionary biologists the basis for natural adaptation. The possibilities have caught the attention not only of Hollywood, which, in movies like “GATTACA” (1979), envision a dystopian future of genetic determinism, but also of Wall Street, which touts “personalized medicine” as the future of healthcare.

But what are the biological realities behind this? To what extent can we actually predict phenotype from genotype? Beyond prediction, will it be possible to manipulate the genome to change the phenotype in predictable ways? What are the technical, fundamental biological, and ethical limitations?