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.

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