Final answer:
In sexual reproduction, two gametes fuse during syngamy, followed by meiosis where crossing over occurs during prophase I, leading to genetically unique offspring due to DNA recombination.
Step-by-step explanation:
Two compatible nuclei fuse and undergo meiosis and crossing over within the gametes during sexual reproduction. This crucial process occurs during prophase I of meiosis, where homologous chromosomes pair up and exchange segments of DNA, leading to genetic variation. The fusion of two gametes, in a process known as syngamy, results in a diploid cell with combined genetic information from both parents. Subsequent steps of meiosis, including metaphase I and telophase II, contribute to further genetic diversity due to random assortment of homologs and recombination of maternal and paternal chromosome segments.
During prophase I, the homologous chromosomes perform crossing-over, exchanging DNA and creating recombinant chromosomes that carry genetic material from both parents. This recombination is responsible for producing genetically unique haploid cells following cytokinesis in telophase II. The end result is the formation of gametes that possess a mix of maternal and paternal alleles, adding to the overall genetic variability seen in sexually reproducing populations.