Final answer:
Crossover occurs during meiosis when non-sister chromatids of homologous chromosomes exchange genetic material, leading to new combinations of genes. This process is known as recombination and creates genetic variation.
Step-by-step explanation:
Crossover occurs during meiosis when non-sister chromatids of homologous chromosomes exchange genetic material. This process is also known as recombination and leads to new combinations of genes on each chromosome. During prophase I of meiosis I, homologous chromosomes form pairs and crossing-over can occur. It results in the reciprocal exchange of DNA between maternal and paternal chromosomes, creating genetic variation.