Figure 3.

Use of the pathway that generates class I crossovers requires Blm. In wild type flies, crossover patterning processes contribute to designating which DSBs become crossovers, resulting in observance of the centromere effect, interference, and assurance, as well as the absence of crossovers on 4. In a Blm mutant, crossovers arise from a backup pathway and are not patterned, resulting in a random distribution of COs and NCOs across the genome, including on chromosome 4.