Final answer:
Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes that occurs during prophase I of meiosis I. It leads to new combinations of genes on each chromosome.
Step-by-step explanation:
Crossing-over occurs during prophase I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Homologous chromosomes form pairs during prophase I of meiosis I, and crossing-over can occur during this pairing. The exchange of genetic material between homologous chromosomes results in new combinations of genes on each chromosome.
During crossing-over, the chromatids of homologous chromosomes break and exchange genetic material, creating unique combinations of genes. This process is also known as recombination and occurs when the homologous chromosomes are closely paired during meiosis I.