[McClung and gender determination: from wrong to right]

Focusing primarily on McClung's hypothesis linking the extra chromosome (X) to gender determination, the article studies some contributions derived from the initial stages in development of the chromosome theory of gender determination. The arguments of McClung and others are analyzed, along with the reaction of the era's scientific community. Although McClung's hypothesis was wrong, it triggered a series of research studies that eventually led to clarification of the question. Even mistaken hypotheses, when adopted for investigation and testing, can contribute to scientific development.