Figure 4. Crossover patterning promotes proper disjunction.

(A) In wild-type flies, crossovers (COs) are produced almost exclusively by the Class I pathway. Entry into this pathway requires Blm helicase activity. Interference and the centromere effect impact which intermediates will be designated to become crossovers. Repair intermediates not selected to become crossovers are repaired into noncrossovers (NCOs). On chromosome 4, crossover patterning processes prevent all recombination intermediates from earning the crossover designation, so all are repaired as noncrossovers (indicated by red 4). Highly regulated crossover patterning ensures that each bivalent receives at least one crossover designation, leading to crossover assurance. Assurance is not absolute in Drosophila, so some achiasmate bivalents remain; however, the achiasmate segregation pathway ensures accurate disjunction of these.

(B) In Blm mutant flies, the Class I pathway is not populated, so all DSBs are repaired by the backup Class II pathway. Because the Class II pathway lacks crossover patterning, every DSB has a fixed probability of become a crossover, regardless of genomic location. Crossover assurance is absent, leading to an elevation in achiasmate bivalents. The achiasmate segregation system cannot compensate for the high number of E₀ bivalents, so nondisjunction is elevated.