RECOMBINATION BETWEEN THE X-CHROMOSOME AND Y-CHROMOSOME AND THE SXR REGION OF THE MOUSE

The Sxr (sex-reversed) region that carries a copy of the mouse Y chromosomal testis-determining gene can be attached to the distal end of either the Y or the X chromosome. During male meiosis, Sxr recombined freely between the X and Y chromosomes, with an estimated recombination frequency not significantly different from 50% in either direction. During female meiosis, Sxr recombined freely between the X chromosome to which it was attached and an X-autosome translocation. A male mouse carrying the original Sxr(a) region on its Y chromosome, and the shorter Sxr(b) variant on the X, also showed 50% recombination between the sex chromosomes. Evidence of unequal crossing-over between the two Sxr regions was obtained: using five markers deleted from Sxr(b), 3 variant Sxr regions were detected in 159 progeny (1.9 %). Four other variants (one from the original cross and three from later generations) were presumed to have been derived from illegitimate pairing and crossing-over between Sxr(b) and the homologous region on the short arm of the Y chromosome. The generation of new variants throws light on the arrangement of gene loci and other markers within the short arm of the mouse Y chromosome.