A Remote Coincidence

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Abstract
From a re-examination of Weinstein''s forty-year old data on recombination along the total length of the X-chromosome in Drosophila melanogaster, it is concluded that the number of chiasmata is greatly restricted, and that chiasmata are not located at random. Over 90 percent of the recombination was attributable to the effects of single chiasma bivalents and less than 0.5 percent to bivalents with triple chiasmata. Each type of bivalent produced chiasmata in rather definite modal positions. A single chiasma tended to be located toward the mid-region of the chromosome. When double or triple chiasmata were formed, the proximal one was located closer to the centromere, and the others were crowded toward the distal end of the chromosome. Consequently an increase in total crossover frequency (accomplished by a switch from single to double chiasmata bivalents) should be realized chiefly through increased recombination in the centric and distal regions of the chromosome. It is pointed out that this pattern of chiasma location may have rather far-reaching consequences on genetic theory and, in particular, on interpretations of interference, relative map lengths, compensatory crossing over both within and between chromosomes, temperature effects, and the apparent complexity of pseudo-alleles.