Final answer:
Crossing-over occurs during prophase I of meiosis, where homologous chromosomes exchange segments of their genetic material, resulting in new combinations of genes on each chromosome.
Step-by-step explanation:
Crossing-over occurs during prophase I of meiosis, where homologous chromosomes exchange segments of their genetic material, resulting in new combinations of genes on each chromosome. This process, also known as recombination, increases genetic diversity among gametes by creating unique combinations of genes. Crossing-over is an important mechanism for genetic variation and is essential for the normal segregation of chromosomes during meiosis.