This article presents two case studies in the development of BRCA genetic medicine in relation to specific 'populations' in the United Kingdom and United States, juxtaposing an 'over researched Ashkenazi Jewish population' in the United Kingdom and 'medically underserved populations' in the United States. Each case explores the research and clinical practices, as well as social, cultural and biological histories, through which population and genetics are coproduced (Jasanoff, 2004). In the first case study, ethnographic and qualitative data in the United Kingdom highlight the ways in which genetic medicine, research and disease can become intertwined with specific aspects of Jewish culture and history, and the effect this has on how this population responds to, and takes up, genetic testing and screening programmes. The second case study describes the intersection of genetic medicine in the form of BRCA clinical practices for 'medically underserved populations' with the discourses and practices of health disparities in the United States, and discusses the effects of this intersection on practices of BRCA genetic medicine and research. The juxtaposition of the two cases sheds new light on some of the specific cultural and biological practices that produce and define populations in local contexts, highlighting the contrasting ways in which BRCA genetic medicine is developing in relation to population.