The "X-Y crossover model" described in this paper postulates the (1) the pairing observed between the X and the Y chromosome at zygotene is a consequence of genetic homology, (2) there is a single obligatory crossover between the X and Y pacing segments, and (3) the segment of the X which pairs with the Y is protected from subsequent inactivation. Genes distal to the proposed crossover ("pseudoautosomal genes") will appear to be autosomally inherited because they will be transmitted to both male and female offspring. Some criteria for identifying pseudoautosomal genes are outlined. The existence of a single obligatory crossover between the X and Y of the mouse is strongly supported by a recent demonstration that the sex-reversing mutation Sxr, which is passed equally to XX and XY offspring by male carriers, is transmitted on the sex chromosomes. Pseudoautosomally inherited genes may also be responsible for XX sex reversal in goats and familial XX sex reversal in man.