This essay examines how the X became the “female chromosome” and how the association of the X with femaleness influences research questions, models, and descriptive language in human sex chromosome research. I trace how the X is gendered female in scientific and popular discourse; document the contingent technical, material, and ideological factors that led to the feminization of the X during the first decades of sex chromosome research; and track the introduction of the “female chromosome” into human genetics at midcentury. In the second part of the essay, I document the continuing influence of the feminization of the X on genetic research, exemplified by “X chromosome mosaicism” theories of female biology and behavior.History of Scienc
