In sociology, in the natural sciences, and in mainstream American society, there is general agreement that “sex” is biological. For sociologists, biological sex remains separate and distinct from gender, gender identity, or sexuality, and we use genetic sex (XX and XY chromosomes) as a foil for arguments that gender is different – historically, culturally, and socially constructed. I re-examine the professional discourse surrounding the foundational 1905 “sex chromosome” discoveries to reveal that (1) the empirical basis for the chromosomal determination of sex depended on specific concepts of sex, and was far from conclusive, that (2) the powerful validity of the chromosomal theory of heredity left the causality of chromosome/sex relationship unexamined, and that (2) the “sex” that early genetics researchers correlated with the X/Y chromosome combinations was heavily inflected by contemporary gender norms. I argue that sociologists must reconsider their use of the “settled fact” that the X and Y chromosomes determine sex, particularly when using sex as a contrasting concept to gender, and that the history of the chromosome discoveries implies the need to re-set for the boundaries between sociological and biological explanations.