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Crossing Over When homologous chromosomes undergo synapsis (pairing), two chromosomes (thus, four chromatids) are involved. While in this paired condition, nonsister chromatids (chromatids from different chromosomes) may break and rejoin, but in the process exchange the broken pieces (cross over) (Fig. 3-12). Remember that one of the chromosomes in the pair came from Mom and one came from Dad, so they are not genetically identical.