Fertile females produced by inactivation of an X chromosome of ‘sex-reversed’ mice

Abstract

Sex-reversed (Sxr) is a dominant mutation that confers maleness on X/X Sxr mice¹. Although its mode of inheritance is typically autosomal, all attempts to map Sxr to an autosomal chromosome have failed². P. Burgoyne (personal communication) suggested that if Sxr were located at the end of the sex chromosomes, distal to a postulated obligatory cross-over in the X–Y pairing segment, it would show such an apparent autosomal pattern of inheritance. To test whether Sxr is located on one of the X chromosomes, and is perhaps therefore subject to X-chromosome inactivation, we have examined mice heterozygous for an X-autosome translocation, T(X; 16)H (ref. 3), such that the normal X chromosome postulated to carry Sxr is preferentially inactive. Independently, Singh and Jones⁴ have now shown by in situ hybridization that Sxr consists of a duplicated section of Y chromosome material that is indeed translocated to the distal terminus of one X chromatid during male meiosis^5,6. Our results show that when the X chromosome carrying Sxr is preferentially inactivated, fertile T(X; 16)H/X Sxr females can be produced, owing to an associated inactivation of the male-determining Sxr sequences.