In three inbred lines of Drosophila melanogaster X-ray induced X-chromosome mutations were isolated. After at least nine generations of backcrossing to the inbred lines the mutants were used as markers for recombination tests in a near-homozygous background (intra-strain crosses) and in heterozygotes (inter-strain crosses). Short distal (y-w), median (sn-lz) and proximal (r-f) regions agreed with standard map distances in all combinations. Brood analysis over a two- or three-week period showed only a small variation in sub-proximal regions (v-g-f), while in the distal region (y-rb-cx) there were highly significant differences between the neighbouring regions y-rb and rb-cx. In later broods the recombination increased considerably in the y-rb region. It was shown with reciprocal crosses that the cytoplasm affected the level of increase of recombination.
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