Factors affecting recognition and disjunction of chromosomes at distributive pairing in female Drosophila melanogaster. II. The effect of a second arm.

The behavior of heterozygously inverted X chromosomes that were members of the distributive pool at least 70% of the time was studied when the other pool members were either two free 4's or one compound 4. The X's were structurally modified by additions or deletions of heterochromatin, so that the two homologues differed in both size and configuration or in size alone. In the noncompetitive situation, with two free 4's, recognition between the X's remained high despite the modifications, and primary X nondisjunction was low. In the competitive situation, with the compound 4, distributive nondisjunction of the X's increased approximately two orders of magnitude, and trivalent formation was indicated. Disjunction from the trivalent varied with X size and configuration. When both X's were acrocentric, the smaller X directed the larger X and the very small (see PDF) to the same pole; when the larger X carried a second arm, it assumed the directing role; when the size ratio of the smaller, one-armed X to the larger, two-armed X became less than approximately 5/9, the smaller X again directed the other two.