How family charts became Mendelian: The changing content of pedigrees and its impact on the consolidation of genetic theory

This article offers a close examination of a small selection of pedigrees taken from German Mendelian and eugenic scholarship of the 1920s and 1930s. It examines the procedures that became customary for presenting data on human inherited pathologies, as well as the frequent changes in the information content of those charts. Relevant biographical and genealogical data was removed, and important indications regarding the diagnostic methods applied by the investigating scholar were lost, as soon as a pedigree was charted or reproduced. Data on healthy individuals was condensed, leading to an emphasis on the hereditary burden of pathological traits. At times, healthy individuals were entirely omitted, as were exogenous martial partners. These modifications paved the way for further theoretical amendments, including the addition of ‘carrier’ status to chosen individuals along the pedigree. With this addition, these pedigrees changed their ontological status, from empirical records of human reproduction to partially hypothetical illustrations of Mendelian theory itself. This process was complemented by the representation of theoretical genetic models in the format of a human pedigree. A comparison to practices of charting pedigrees still common today suggests that the processes hereby revealed are far from exceptional. In line with the ideas put forward by Ludwik Fleck, they are interpreted as germane to the way scientific ideas are communicated and propagated and to the scientific culture of genetics. The article also offers a refinement to Fleck’s analysis of textbook construction, which highlights the extent to which textbook examples differ from the original data on which they are based.