There is much talk about data-driven and in silico biology, but how exactly does it work? This essay reflects on the relation of data practices to the biological things from which they are abstracted. Looking at concrete examples of computer use in biology, the essay asks: How are biological things turned into data? What organizes and limits the combination, querying, and re-use of data? And how does the work on data link back to the organismic or biological world? Considering the life cycle of data, the essay suggests that data remain linked to the biological material and the concrete context from which they are extracted and to which they always refer back. Consequently, the transition to data science is never complete. This essay is part of a special issue entitled Histories of Data and the Database edited by Soraya de Chadarevian and Theodore M. Porter.

In 1956, Joe Hin Tjio and Albert Levan published a paper in which they suggested that the number of human chromosomes was 46 and not 48. The story of the recount has been the subject of numerous studies and debates. In this essay I propose to revisit the 1956 paper and the questions surrounding it by considering the chromosome images it contained. Paying attention to the images, including especially the photomicrograph that has come to represent the new chromosome count, offers the opportunity to study the history of an iconic image of genetics. In the course of this history the image moved from proving the quality of Tjio and Levan’s preparations to becoming an object of contention, proof of authorship, example to emulate, manipulable object, recognizable icon, and historical object in its own right. More generally, the essay highlights the role of visual techniques and materials in shaping knowledge and staking claims in human heredity in the mid-twentieth century. The history of postwar cytogenetics has long been overshadowed by dominant accounts of molecular approaches in biology that developed rapidly at the same time. Yet the recognition that, well into the 1970s, chromosome pictures were the most recognizable images of genetics points to the need for new approaches to the historiography.