Induced Crossing-Over Variation in the X-Chromosome of Drosophila

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Abstract
Preliminary data are reported on the effect of x-rays (54 H) on crossing-over in the X-chromosome. These indicate strongly that in the extreme "left" end of the chromosome the treatment decreases crossing-over frequency, in the middle it leaves it relatively unchanged, and in the "right" end it increases it. The results tend to clarify the apparent contradictions in Mavor''s findings and to show the effect of x-rays on crossing-over in the X-chromosomes really to be similar to that previously observed by the author in the case of autosomes, when chromosome regions are compared which correspond in their relation to the point of spindle-fiber attachment. Regions near the latter point have crossing-over frequency increased by x-rays and some other non-genetic agents; regions further away have it unaffected or decreased. As this rule holds both in cases in which the fiber causes the chromosomes to be bent (as in the autosomes having median attachment) and in cases in which the chromosomes remain relatively straight (X-chromosomes, with terminal attachment), the peculiar crossing-over effects in the neighborhood of the spindle fiber cannot be caused by the bent condition of the chromosomes there but must be referred to some other peculiarity resulting from the fiber attachment.

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