In the first part, the article examines the medical roots of the idea of “blood relationship” and traces its development since ancient times. With the emergence of cellular pathology in the 19th century, blood lost its status as a “very special fluid” and therefore also as a generative substance. However, as the second part shows, the link between “blood” and “relationship”, though scientifically discredited, was to persist, and manifested
itself in a modern way in blood group research during the early 20th century.
Blood group tests in paternity trials which quickly became popular in the Weimar Republic, referred to the tradition of blood relationship and led to a new train of thought
towards blood being perceived as a generative substance – even if, paradoxically enough, blood groups could strictly speaking only determine non-paternity and not the definitive
relationship between father and child. Furthermore these blood tests point to a fundamental change in the history of paternity, since they are considered the first step in the biologisation of paternity which today seems finalised in DNA paternity testing.