Centric pairing and crossing-over in Drosophila melanogaster.

This cyto-genetic disproportionality has fostered the widespread inference that the centric and medial regions of the visible chromosome have a differential organization that influences their ability to undergo exchange. Such a view is in harmony with the concept that the centric regions are genetically “inert,” which has developed out of their heterochromatic properties. MATHER, however, has demonstrated (MATHER 1939) that centric heterochromatin undergoes crossingover more frequently when attached medially, and that proximity to the centromere itself is a deterrent to exchange. This is in agreement with BEADLE’S earlier observation (BEADLE 1932) that distal material attached near a centromere by translocation undergoes a reduction in exchanges, and with BEADLE’S interpretation of a “centromere effect” on crossing-over. A special case of the disparity of physical and genetic length is chromosome 4 of D. melanogaster. This is a small chromosome, often called a “dot chromosome” because of its appearance in somatic metaphase. Nonetheless, its cytological length is such that compared with regions of equal size in other chromosomes it should produce in the neighborhood of one to five percent crossovers. Actually, however, crossing-over in this pair of chromosomes almost never occurs in diploids.