Blood Relations explores the relationships between mid-century blood transfusion and the development of human genetics. It focuses on Britain, which established its first national transfusion service during the Second World War. By then, blood groups were crucial to transfusion, and were fascinating to scientists interested in heredity and race. Blood Relations follows how the transfusion services made vast quantities of blood group data available for human genetics, during the war and two decades after. The book revisits the history of eugenics in light of the local politics of giving blood, and the power relations between donor, nurse, patient, doctor, and scientist. It reveals how the movement of blood carved out networks that made human populations into objects of surveillance and research. It traces how clerks and scientists transformed documents into genetic data, and analyzes the centrality of paper and notations to genetics. It is an account of how human genetics was constructed through wartime public health, and how it was given meaning through narratives of kin, community, nationalism and internationalism. It is a history of genetics that puts blood, bodies, and bureaucracy at center stage. Today, genetics is still understood as a neutral science that can reliably underpin stories about human identities, ancestry and migrationary history; Blood Relations offers a new historical account of how this understanding began.