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.

In this paper, I explore the origins of medical cytogenetic knowledge and practices in the 1960s and 1970s in Mexico, focusing on the work of the group headed by Salvador Armendares, who spent two years in Oxford, England, with human genetics expert Alan C. Stevenson. Upon Armendares’ return from England in 1966, the first Unit for Research in Human Genetics was created at a medical setting, the Instituto Mexicano del Seguro Social (Mexican Institute of Social Security). Soon after its creation, Fabio Salamanca and Leonor Buentello began to work with Armendares in the implementation of cytogenetics. Some of the research projects showed the embeddedness of these researchers in both public health policy and medical care, as they tackled the effects of malnutrition on chromosome structure, child mortality, chromosome aberrations, and Down syndrome. Armendares, Salamanca, and Buentello had trained at different academic institutions at many different times, and contributed to transforming hospital medical practice into a medical research discipline. By posing malnutrition, one of the main concerns of Mexican post-revolutionary governments, as both a medical and a genetic problem, the unit contributed to positioning cytogenetics as a medical practice and a medical research domain. The focus of this paper will be this set of institutions, physicians, practices, and ideas that began to reshape medical genetics in Mexico. The reconstruction of the early days of cytogenetics in Mexico demonstrates the major roles played by both the clinic and post-revolution public health policies in the origins of medical genetics in Mexico, within a global movement to deliver the benefits of scientific knowledge to the general population.

In this paper I analyze the trajectory of research objects and experiments on tumor cells that led the Swedish geneticist, Albert Levan, through successive collaborations with Theodore Hauschka in Philadelphia (U.S.) and Joe Hin Tjio in Lund (Sweden), from plants to mice and from mice to human chromosomes. Tumor chromosomes were created and recreated in mice bodies by Eva Klein and Georg Klein as ascites—fluid—tumors, whereas human tumors were transplanted from the bodies of cancer patients into mice by Helene W. Toolan. The cultures of cytogenetics and microscopic observation were therefore opened up, from agricultural and botanical research to the clinical laboratory. I suggest it was research on tumors and cancer cells that led to a method for obtaining clear slides, thereby providing evidence of the new number of forty-six human chromosomes, as presented by Tjio and Levan in 1956. Along this research trajectory, the knowledge and practices of cytogenetics were medicalized—a medicalization that situated both cancer research and cytogenetics at the origins of biomedicine.

In 1949, Canadian anatomist Murray Barr announced the discovery of a peculiar entity in the cell nucleus that was present in females and absent in males. The identity of this entity remained uncertain for a decade even though Barr hypothesized a relationship between it and the sex chromosomes and called it the “sex chromatin.” This hypothesis inspired the development of the chromatin into a technology that could indicate “chromosomal” or “genetic” sex, which supposedly established male and female sex difference as a binary and fundamental characteristic of humans and other animals at conception. Barr collaborated with other researchers and potential patients who applied the sex chromatin test, hoping that it could identify the “true” sex of intersexuals, homosexuals, and transsexuals. Ironically, the application of the test to intersexuals would lead to a revision of the identity of the sex chromatin itself. The history of the sex chromatin illuminates how the significance and essence of this laboratory object evolved with its use as a clinical and research tool. Researchers had hoped that the test would sort the intersex into just two categories, male and female. Instead, the sex chromatin helped to multiply categories of the intersex, distinguished them from inverts, underpinned psychosocial gender as a new dimension of sex difference, and in the process had its own identity refashioned. Today, we call it the Barr body and its story reminds us of the power and limit of biotechnologies to determine who we are.

This paper explores the research and administrative efforts of Ernest Brown Babcock, head of the Division of Genetics in the College of Agriculture at the University of California, Berkeley, the first academic unit so named in the United States. It explores the rationale for his choice of "model organism," the development——and transformation——of his ambitious genetics research program centering on the weedy plant genus named Crepis (commonly known as the hawkbeard), along with examining in detail the historical development of the understanding of genetic mechanisms of evolutionary change in plants leading to the period of the evolutionary synthesis. Chosen initially as the plant counterpart of Thomas Hunt Morgan's Drosophila melanogaster , the genus Crepis instead came to serve as the counterpart of Theodosius Dobzhansky's Drosophila pseudoobscura , leading the way in plant evolutionary genetics, and eventually providing the first comprehensive systematic treatise of any genus that was part of the movement known as biosystematics, or the "new" systematics. The paper also suggests a historical rethinking of the application of the terms model organism, research program, and experimental system in the history of biology.