Side-by-side pairing of the XY bivalent in spermatocytes and the ubiquity of the H-Y locus.

The pairing mechanism of the XY bivalent, the possibility of crossing-over between X and Y chromosomes during meiotic prophase, and the location of the H-Y locus are of interest with regard to genetic control mechanisms, male gametogenesis, and testicular organization. A whole-mount electron microscope technique has permitted the study of a large number of mouse and hamster spermatocytes to evaluate the spatial relationship of sex chromosomes and autosomes. X and Y chromosomes showed a transient, extensive side-by-side pairing segment along most of the length of the Y chromosome. This extensive pairing segment may cause genetic exchange between X and Y chromosomes. The finding of a small unpaired paracentromeric region of the Y chromosome could be related to a locus of totally sex-linked gene(s) that determine the development of the testis from the undifferentiated embryonic gonad.