Diagram of the simulations of sister chromatid exchange (SCE) and loop deletions. (A) The rDNA loci were modeled as strings of symbols with uninserted units represented by 0, R1-inserted by 1, R2-inserted by 2, and double-inserted by 3. The outcome of recombination was determined by two parameters: the offset size (an integer between 1 and 4, 1 and 8, or 1 and 16 in the various simulations) and the crossover location (random in all simulations). (B) Loop deletions occur when part of a chromatid forms a loop and a crossover generates an extrachromosomal circle that is lost.