The rediscovery of Mendel’s laws of heredity in 1900 launched a new era in the experimental study of heredity. Around the world, biologists began designing new research programs to study evolution using the new techniques of Mendelian breeding analysis. Because the universities were not well equipped to accommodate the needs of genetics, new private research institutes were created In the United States, Britain, and Germany devoted to the experimental study of heredity. Women were present in these institutes from the beginning. This paper explores the ways in which gender impacted the day-to-day work carried on in genetics laboratories, including interpersonal relationships, the organization of scientific labor, scientific communication, and the overall output of research programs. In focusing on the social, intellectual, and scientific dynamics of mixed-gender laboratories, I argue that the inclusion of women in the new experimental institutes in genetics between 1900 and 1940 ushered in significant changes in the organization of scientific research.