An exchange of portions of chromatids between homologous chromosomes. As the chromosomes begin to move apart at the end of the first prophase of meiosis, they remain in contact at a number of points (see chiasma). At these points the chromatids break and rejoin in such a way that sections are exchanged (see illustration). Crossing over thus alters the pattern of genes in the chromosomes. See recombination.

Crossing over at two chiasmata in a pair of homologous chromosomes