In this paper, I propose a study of the invention and development of the technique of genetic mapping in the 1920's. I show that what is usually taken as one and the same theory (Classical Genetics) is in fact the result of the articulation of various levels of explanations corresponding to two different disciplines, with different methods and representational practices -- namely Mendelian theory and cytology. The merging of these two disciplinary frameworks is embodied in the very rules underlying the construction and interpretation of genetic maps. Moreover, the debates between the geneticists around how to display data within these maps reveal that they have different understanding of the articulation of these disciplines (and different theoretical commitments).