The wood lemming, Myopus schisticolor, possesses a unique sex determining system comprising both XX and XY females. Normal female development in the presence of XY is guaranteed by a mutation on the X, apparently associated with a structural rearrangement in Xp. This mutation inactivates the testis-inducing and male-determining factor on the Y and distinguishes X* from X, and X*Y females from XY males. Normal fertility of X*Y females is ensured by a mitotic (double) nondisjunction mechanism which, at an early fetal stage, eliminates the Y from the germ line and replaces it by a copy of the X*.
Numerical sex chromosome aberrations are not infrequent and the trisomics XXY and X*XY are relatively common. XXY individuals are sterile males with severe suppression of spermatogenesis. Among X*XY animals, both males and females, as well as a true lateral hermaphrodite have been observed. Primary deficiency of germ cells, impairment of spermatogenesis and sterility are characteristic traits of the X*XY males, whereas X*XY females have normal oogenesis and are fertile. Both these extremes (except female fertility) coexist in the true hermaphrodite described in the present study. These apparently contradictory observations are explainable under the assumption that X* and X in X*XY individuals are inactivated non-randomly or that the cells are distributed unequally. Inactivation of the X or X* determines whether or not the H-Y antigen will be expressed. When comparing conditions in Myopus and in man, an additional assumption has to be made in relation to the gene(s) involved in sex determination, located in Xp:In Myopus they do not escape inactivation, whereas in man they have been claimed to remain active.