Abstract
1. The two independent inversions In (X) Sp and In (X) Sd found in the X-chromosome of D. virilis, when existing together in the heterozygous state, increase the crossing over in the terminal regions of the third and fifth chromosomes and that in the proximal region of the third chromosome.
2. They also increase the crossing over in the terminal region of the X-chromosome itself.
3. The distal inversion existing singly also increases the crossing over in the terminal region of the third chromosome, but not that in the terminal region of the X-chromosome.
4. The proximal inversion existing singly has no effect on the crossing over in the terminal region of either the third or the X-chromosome.
5. The degree in which an inversion affects the crossing over in other chromosome or in other region of the same chromosome, depends on the degree of suppression caused by the inversion of the crossing over in its adjacent region.
6. The effect is of the same nature as that of high temperature or of X-ray radiation: an increase in one region of the chromosome is always accompanied by a decrease in the other. This ‘competitive’ (Mather) or ‘compensatory’ (Kikkawa) relation is due to the existence of the upper limit in the number of chiasmata formed in the same nucleus.
