Final answer:
Crossing-over occurs in prophase I of meiosis I, allowing for the exchange of genetic material between homologous chromosomes and contributing to genetic diversity in offspring.
Step-by-step explanation:
Crossing-over occurs during prophase I of meiosis I. This process is the exchange of genetic material between non-sister chromatids of homologous chromosomes and is essential for producing genetically unique offspring due to the new combinations of alleles formed on the chromosomes. This crucial event in sexual reproduction takes place when the homologous chromosomes pair up and align gene-for-gene, forming a tetrad structure. Recombination of genes during crossing-over increases genetic variation which is a key aspect of evolutionary adaptation.