Abstract
c(3)G is a gene in Drosophila melanogaster defined by two independently isolated mutants on the third chromosome. When homozygous in females, the mutants—c(3)G17 or c(3)G68—result in the elimination of meiotic crossing over and a great increase in nondisjunction at the first meiotic division. The gametic frequency of X-, second-, or third-chromosome nondisjunction is approximately .3 in c(3)G17, and .4 in c(3)G68; for the fourth chromosome, the frequency is .2 in c(3)G17 and .3 in c(3)G68. These values are at least two hundred fold greater than for spontaneous nondisjunction, though not high enough to indicate that chromosomes are distributed at random to the first meiotic division poles. Chromosomes loss is inferred from an excess of nullo-exceptional over diplo-exceptional ova. Loss is more frequent in c(3)G68. If c(3)G females mate at low temperature, crossing over is still absent, but nondisjunction is decreased. c(3)G17 is more temperature sensitive than c(3)G68.—Nonhomologous chromosomes tend to undergo nondisjunction in the same meiotic cells in c(3)G. Moreover, there is substantial nonhomologous pairing involving the larger chromosomes of the genome, inferred from the tendency for nonhomologs to disjoin from each other. Nonhomologous segregation is not observed between chromosome 4 and any other chromosome. c(3)G68 exhibits more nonhomologous segregation than does c(3)G17, and, for either allele, the degree of nonhomologous segregation is directly proportional to the similarity in length of the two nonhomologs being considered. The degree of nonhomologous segregation is increased at low temperature.—Heterozygosity for inversions tends to increase c(3)G-mediated nondisjunction, and to alter the patterns of nonhomologous segregations. The effects are observed even if the inversion does not disrupt centromeric heterochromatin, and even though the inversions do not change the lengths of the chromosomes involved. In XXY females, c(3)G17 shows more separation of the two X’s from the Y chromo-some than does c(3)G68. Fourth-chromosome nondisjunction is increased by the presence of a Y chromosome in both kinds of mutant females. But in XXY;c(3)G females which are also heterozygous for an X inversion, frequencies of fourth-chromosome nondisjunction are little different from those in XX; c(3)G females, while the degrees of XX-from-Y disjunction are increased.—The chromosome behavior of the two alleles of c(3)G is readily rationalized by a model which assumes that c(3)G+ controls a stage of meiosis prior to synapsis and crossing over. If exchange is directly disrupted in c(3)G homozygotes, disjunctional consequences should be the same in c(3)G17 and c(3)G68. They are not. If, however, c(3)G+ controls a precondition to crossing over—such as the association of homologous and nonhomologous chromosomes—then the two alleles could each abolish crossing over, but lead to different amounts and patterns of nondisjunction.