Genetics of inbred Drosophila melanogaster. VI. Crossing-over in secondary non-disjunction exceptionals.

Abstract

Although Bridges (1916, Genetics 1: 1–52 and 107–183) found recombinants among exceptional XXY females in tests of secondary non-disjunction, he misinterpreted their origin and claimed that true secondary non-disjunction resulted in XX eggs with non-crossover chromosomes. Preliminary tests of inter-strain crosses of inbred lines indicated different frequencies of recombinant exceptional females in reciprocal crosses. Large scale tests confirmed this observation. However, they indicated that, as judged by recombination in the most distal end (y-w interval), there was presumably nearly the same frequency of chromosome pairing in XX females as in exceptional females from XXY mothers. In the more proximal regions there was a decreasing proportion of recombinants among exceptional females as compared to regular females. The decrease was more pronounced in one direction of crosses than in the reciprocal indicating a maternal influence. The use of inbred lines made it possible to compare the frequencies of recombinant X-chromosomes in females of primary and secondary non-disjunctional origin in the same genetic and cytoplasmic background. It was shown that the absolute frequency of recombinants was much higher in secondary non-disjunction than in primary. Some complex phenodeviants among the exceptional females revealed that their recombination could occur between euchromatic parts of the X-chromosomes and between X and either Y-arm indicating trivalent formation, however, of type different from that suggested by Cooper (1948, Proc. Nat. Acad. Sci. 34,5: 179–187).