Crossing over refers to...

• A. The separation of sister chromatids during mitosis.
• B. The replication of chromosomes prior to cell division.
• C. The exchange of genetic material between homologous chromosomes during meiosis, contributing to genetic variation.
• D. The fusion of two gametes during fertilization.

Answer: (C)

Crossing over is the exchange of genetic material between homologous chromosomes during meiosis, specifically in prophase I when homologs pair up and form a tetrad. The non-sister chromatids of these homologs swap segments at points called chiasmata, producing chromatids with new combinations of alleles. This recombination creates genetic variation in gametes beyond what independent assortment provides. It does not occur during mitosis, where sister chromatids separate, and it isn’t simply DNA replication or fertilization. So this description best captures the process and its role in generating genetic diversity.