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Humans possess twenty-two pairs of autosomal chromosomes and one pair of sex chromosomes—X and Y for males, X and X for females. Today it is well established that the Y carries a critical genetic switch for male sex determination. The X, however, has no parallel relationship to fe- maleness. Female sexual development is directed by hormones acting in concert with genes carried by many chromosomes and is not localized to the X. Indeed, the X is arguably more important to male biology, given the large number of X-linked diseases to which men are uniquely exposed. Despite this, researchers attribute feminine behavior to the X itself and assume that female genes and traits are located on it. Researchers look to the X to explain sex differences and female quirks and weaknesses and have argued that men are superior because they possess one fewer X than females.
The X chromosome offers a poignant example of how the gendering of objects of biological study can shape scientific knowledge.
The female X has its roots in early sex chromosome science, which assumed for half a century—until the 1950s, when the Y was confirmed as the carrier of the sex-determining locus—that the X was female deter- mining in humans. In the first part of what follows, I document the contingent technical, material, and ideological factors that led to the fem- inization of the X during the first decades of sex chromosome research and track the introduction of the “female chromosome” into human ge- netics at midcentury. In the second part, I demonstrate the continuing influence of the historical feminization of the X on genetic research, ex- emplified by “X chromosome mosaicism” theories of female biology, be- havior, and disease. Focusing on the case of X-mosaicism theories of the higher incidence of autoimmunity in women, I show how the assumption that the X is the female chromosome operates to sustain and cohere hy- potheses of dubious empirical merit in research areas urgently relevant to women’s health.