BJHS Themes (2020), 5, 149–165 doi:10.1017/bjt.2020.12 RESEARCH ARTICLE Punnett squares and hybrid crosses: how Mendelians learned their trade by the book Staffan Müller-Wille1* and Giuditta Parolini2 1Department for History and Philosophy of Science, University of Cambridge, Free School Lane, Cambridge, CB2 3RH, UK and 2TU Berlin, Institut für Philosophie, Literatur-, Wissenschafts- und Technikgeschichte, Sekretariat H23, Straße des 17. Juni 135, Berlin, 10623, Germany *Corresponding author: Staffan Müller‐Wille, email:
[email protected] Abstract The rapid reception of Gregor Mendel’s paper ‘Experiments on plant hybrids’ (1866) in the early decades of the twentieth century remains poorly understood. We will suggest that this reception should not exclusively be investigated as the spread of a theory, but also as the spread of an experi- mental and computational protocol. Early geneticists used Mendel’s paper, as well as reviews of Mendelian experiments in a variety of other publications, to acquire a unique combination of experimental and mathematical skills. We will analyse annotations in copies of Mendel’s paper itself, in early editions and translations of this paper, and in early textbooks, such as Reginald Punnett’s Mendelism (1905) or Wilhelm Johannsen’s Elemente der exakten Erblichkeitslehre (1909). We will examine how readers used copies of such works to reproduce the logic behind Mendelian experiments, either by recalculating results, or by retracing the underlying combinatorial reasoning. We will place particular emphasis on the