Final answer:
Crossing-over occurs during prophase I of meiosis, when homologous chromosomes line up in pairs and exchange genetic material. This process creates unique combinations of genes.
Step-by-step explanation:
Crossing-over occurs during prophase I of meiosis. It is the exchange of genetic material between non-sister chromatids of homologous chromosomes.
During prophase I, homologous chromosomes line up in pairs, gene-for-gene, forming a configuration called a tetrad.
Some material from two chromatids switch chromosomes, creating unique combinations of genes.