The Distribution of X-Ray Induced Crossovers From Curly Inversion Heterozygotes of Drosophila Melanogaster Females

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Abstract
Virgin females having the genotype S Pfd / Cy lt3 L4 and heterozygous for inversions involving the regions of Cy and L4 were exposed to 2250 r units of X-rays, and testcrossed to lt/lt males. Ten transfers of each mated female were made, the first 7 transfers being made every other day and the last 3 at longer intervals. Fertility was reduced by irradiation, but recombination values were increased throughout the 31 days of egg-laying, with a peak at 4-8 days after treatment. When the offspring from each of several females were considered separately, deviations from the mean total recombination value of 4.73 [plus or minus] 0.35% ranged from 0 to 12 + %. There were wide variations in proportions of region 2 (Cy-lt3) to region 3 (lt3-L4) crossovers, and inequalities in complementary crossover classes within a region. There was a tendency for the occurrence of clusters of recombinants involving the same region. All these deviations could be explained by oogonial crossing over or by a localized weakening of a chromatid in an oogonial cell, which might favor meiotic crossing over in this region after several mitotic divisions had intervened. The inequalities in complementary cross over classes could be explained by cell lethals if it was assumed that oogonial crossing over caused some of the cells to become homozygous for S, Pfd, Cy or L4, and that the homozygous condition was lethal to the cell.