A Cytogenetic Study of Crossing over in Inversion Heterozygotes of Drosophila Melanogaster.

HE recombinant off spring of Drosophila melanogaster females having free X T chromosomnls heterozygous for a long inversion are limited to even multiple crossover types. But from inversion heterozygotes in which the two X chromosomes are attached at their centromeric ends (= tandem metacentric compound X chromosome), single crossovers are frequently recovered. The synaptic configurations of these two structural heterozygotes are compared in Figure 1 . Although this difference in recovery of single crossovers could be explained by supposing the products of single exchange in free X inversion heterozygotes to be inviable, there is no demonstrable lethality of zygotes produced by such females. The resolution of this apparent paradox was presented by STURTEVANT and BEADLE (1936) in their classic paper on the mechanics of crossing over in inversion heterozygotes; they proposed that single exchanges do occur in free X inversion heterozygotes, that the products of such exchange are invariably excluded from the functional egg nucleus, and that excluston is accomplished by directed orientation of noncrossover chromatids towards the terminal products of the second meiotic division. The cytological and genetic consequences of various kinds of exchange in free X inversion heterozygotes and in tandem metacentric compound X chromosomes