Much of the early history of developmental and physiological genetics in Germany remains to be written. Together with Carl Correns and Richard Goldschmidt, Alfred Kühn occupies a special place in this history. Trained as a zoologist in Freiburg im Breisgau, he set out to integrate physiology, development and genetics in a particular experimental system based on the flour moth Ephestia kühniella Zeller. This paper is meant to reconstruct the crucial steps in the experimental pathway that led Kühn and his collaborators at the University of Göttingen, and later at the Kaiser Wilhelm Institutes of Biology and Biochemistry in Berlin, to formulate, in their specific way, what later became known as the "one gene - one enzyme hypothesis." Special attention will be given to the interaction of the different parts of Kühn's Ephestia-based project, which were rooted in different research traditions. The paper retraces how, roughly between 1925 and 1945, these elements came to form a mixed experimental set-up composed of genetic, embryological, physiological and, finally, biochemical constituents. Accordingly, emphasis is laid on the development of the terminology in which the results were cast, and how it reflected the hybrid state of an experimental system successively acquiring new epistemic layers.