Final answer:
Crossing-over during prophase I of meiosis I involves exchanging genetic material between non-sister chromatids of homologous chromosomes, creating recombinant chromosomes with mixed maternal and paternal alleles, and significantly contributes to genetic variation.
Step-by-step explanation:
Homologous Recombination and Crossing-Over
Crossing-over is a critical process during prophase I of meiosis I, where there is an exchange of genetic material between non-sister chromatids of homologous chromosomes. This event occurs when homologous chromosomes line up gene-for-gene, forming a tetrad, and swap segments of DNA at various points along their length. The result of this are recombinant chromosomes, which possess a mix of maternal and paternal alleles, different from those found in the original chromosomes.
First documented by Frans Janssen in 1909 through his observation of chiasmata, the phenomenon is essential for increasing genetic variation among offspring. The process can occur multiple times within the same pair of homologous chromosomes, thereby fueling the potential for over 64 trillion genetically unique human combinations. Crucially, this mechanism ensures that daughter cells produced during meiosis have varied combinations of genes inherited from both parents.