Crossing over may occur during prophase I of meiosis. (a) A pair of replicated homologous chromosomes. (b) Chromatids of the homologous chromosomes form a tetrad. The chromatids are crossed in two places. The chromatids may break at the points of crossing and become fused to the opposite chromosome, resulting in crossing over. (c) Genetic material is exchanged following crossing over of the chromatids.
