Recombination at the bar locus in an inverted attached-X system in Drosophila melanogaster.
Recombination at the Bar locus in Drosophila melanogaster was investigated in an inverted attached-X system which enhanced the frequency of homozygosis for the Bar region. Females among the progeny of homozygous B mothers were searched for changes of B to BB and to B(+). Marker genes were followed and exceptional half-tetrads were analyzed in regard to two hypotheses: that of exchange between obliquely synapsed members of the duplication, which is associated with exchange of outside markers, and that of intrachromosomal exchange, which does not involve recombination of markers.-Recombinant exceptions of B(+) /BB genotype, carrying the outside marker combinations predicted on the hypothesis of exchange between obliquely synapsed duplication members, were encountered repeatedly. It is established that B(+) and BB strands are reciprocal products of the same event.-Twelve nonrecombinant exceptional strands were isolated; ten of these were B(+) and two were BB. Only one of the nonrecombinant half-tetrads offered the opportunity to test the prediction of reciprocity of the intrachromosomal event. Analysis showed the exceptional female to be of the constitution BB/B, a type not expected on the hypothesis. While it could have arisen through some kind of copy error in the repair of a chromatid break, a valid test of the hypothesis of intrachromosomal exchange must rest on the isolation and analysis of more cases of the appropriate exceptional genotype.-In several cases Bar changes were found to be associated with aberrations; all but one of these involved spontaneous, cytologically identifiable deletions.
D000818:Animals; D002869:Chromosome Aberrations; D003434:Crossing Over, Genetic; D004331:Drosophila melanogaster; D005123:Eye; D005260:Female; D005838:Genotype; D008297:Male; D011995:Recombination, Genetic; D012730:Sex Chromosomes