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An unstable long tandem duplication which includes the white locus twice, marked with wsp in the left and w17g in the right locus, when kept in males has been found to produce red-eyed sons which have lost the long duplication and with it the wsp and w17g mutants. Such exceptions were produced also when w17g had been exchanged for wa. Stocks originating from these exceptions are unstable, producing: 1) zeste males, also unstable, 2) w- deletions, stable, 3) transpositions of the white locus to sites in other chromosomes. The instability is interpreted as the effect of an IS element, within or adjacent to the white locus, which is supposed to retain a duplication of the proximal zeste interacting part of this locus. According to the orientation of the IS element the duplicated part can be active or inactive, giving a zeste or red eye phenotype. The frequency of exceptional offspring after X-ray treatment of the red and zeste unstable stocks have been compared to stable stocks with corresponding genotypes.

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