This paper will address the collaborative networks and the gendered organization of the scientific work at the first Unit on Human Genetics of the Mexican Institute for Social Security. There, women and men had different tasks, duties and authority according to their gender and individual and professional skills. I will focus on physician Susana Kofman, who specialized in cytogenetics with Jérôme Lejeune and Jean de Grouchy in France, and physician Leonor Buentello, who graduated in virus genetics in Germany. This narrative (...) intends to return them to the forefront of the history of cytogenetics and to illustrate the contribution of women to scientific developments when research on human genetics was becoming a medical domain for diagnosis at an international level. (shrink)