William Castle's Guinea Pigs and the Spread of Mendelian Teaching

One of the prime examples of the applicability of Mendel's laws to the animal world involves the characteristics of guinea pig coat color. The paper retraces the history of an especially illustrious example of Mendelian mechanisms in guinea pigs and analyzes its role in the dissemination of genetic theory. Its origins go back to William Castle's cross-breeding experiments conducted in the early 1900s, yet there is a substantial gap between Castle's results in that particular experiment and the canonized form into which they were subsequently molded: a concise chart that simultaneously demonstrates and reaffirms Mendel's laws. The extraordinary appeal of that chart stemmed from scientific, pedagogical, as well as cultural factors, the latter related to the sociopolitical significance of color differences in the context of racial discourse and of concerns about racial mixture. More generally, the guinea pigs chart analyzed here belonged to a family of standardized visualizations that purported to describe empirical findings while actually describing Mendelian theory.