Some of Drosophila Melanogaster

LL present theories of crossing over tacitly assume that events in one tetA rad (at least as far as crossing over is concerned) are independent of those occurring in the other tetrads of the cell. Yet as long ago as 1919 STURTEVANT suspected that the presence of a heterozygous inversion in one chromosome pair increases crossing over in a non-homologous pair of chromosomes. The effect was definitely established by SCHULTz and REDFIELD (MORGAN, BRIDGES) and SCHULTZ 1932, 1933)) who showed that in Drosophila melanogaster heterozygous inversions in the first and/or second chromosomes increase crossing over in the third chromosome and that heterozygous inversions in the first and/or third chromosomes increase crossing over in the second chromosome, and by GLASS (1933) who found that an inversion in the second chromosome increased crossing over in the third chromosome (this observation was independent of that of SCHULTZ and REDPIELD). Subsequently STEINBERG (1936) showed that crossing over was increased in the first chromosome in the presence of heterozygous inversions in the second and/or third chromosomes. These experiments all involved an increase in crossing over in a tetrad which was not itself heterozygous for an inversion. STURTEVANT’S original observation was on a tetrad which was itself heterozygous for an inversion, although the increase in crossing over took place in an uninverted segment. He found that crossing over between the second chromosome mutants purple and curved, which was reduced to about one percent in the presence of the inversions carried in the second chromosome of the Nova Scotia stock, was increased to about 2 0 percent (that is, approximately the standard value) in the simultaneous presence of an inversion in the third chromosome. STURTEVANT’S observations were supported by GLASS’S report in 1933 that crossing over within the Plum inversion in the second chromosome was increased in the presence of an heterozygous inversion in the third chromosome. SIDEROW, SOKOLOW, and TROFIMOW (1936) in making use of GLASS’S observation for other purposes showed that heterozygous inversions in the second and third chromosomes increase crossing over in the X chromosome when it also is heterozygous for an inversion. These data are of particular interest because the regions of the X chromosome studied by these authors are similar to those studied by STEINBERG (1936) and may be compared with them. They will be discussed in this connection in a later section of this paper. The observations on Drosophila melanogaster were confirmed for Drosophila pseudoobscura by MACKNIGHT