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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.

RECOMBINATION AT THE BAR LOCUS IN AN INVERTED ATTACHED-X SYSTEM IN DROSOPHILA MELANOGASTER