In this article we discuss the development of a practice of screening, preventive treatment, and presymptomatic testing for individuals at risk of Familial Adenomatous Polyposis (FAP), a specific hereditary predisposition for colon cancer. We describe this development as a process of co-evolution, showing how this practice has been gradually taking shape in a new network of actors, routines, rules, institutions and technologies. We further argue that, looking at the emergence and transformation of this practice, we can distinguish two different regimes: a regime of prevention and a regime of self-determination. In each of these regimes the autonomy of patients and individuals at risk is shaped in a different way, that is, through a different complex of ideals, procedures, institutions, technologies, and routines. In our view, the interference between these two regimes is an important characteristic of the emergent new genetics and is reflected in the growing debate about non-directivity in genetic counselling. However, as our argument implies, when facing the challenges of the new genetics we should not restrict the debate to the quality and ethics of counselling, but extend our view to the whole complex of elements and activities which shapes individual autonomy in the context of different regimes.