Final answer:
Sexual reproduction enhances genetic variation through crossing-over during prophase I of meiosis, independent assortment of chromosomes during metaphase I, and the randomness of fertilization, creating over 64 trillion possible genetic combinations in humans.
Step-by-step explanation:
Genetic variation is essential for the survival of species, and several mechanisms during sexual reproduction contribute to this diversity. Crossing-over occurs during prophase I of meiosis, where there is an exchange of genetic material between non-sister chromatids of homologous chromosomes. This recombination creates new combinations of genes.
Independent assortment refers to how chromosomes line up independently of each other during metaphase I of meiosis. This process creates over 8 million possible chromosome combinations in humans, as each chromosome inherited from a parent can assort into any gamete. When we consider that both male and female gametes undergo this process, the potential for variation is immense.
Random fertilization increases genetic variation further since any one of those 8 million combinations in a sperm can fertilize any one of the 8 million combinations in an egg. This randomness results in more than 64 trillion possible genetic combinations for offspring. The harmony of crossing-over, independent assortment, and random fertilization ensures that each individual created by sexual reproduction has a unique genetic makeup.