Final answer:
Crossing over during prophase I of meiosis generates new alleles by exchanging segments of DNA between homologous chromosomes, while independent assortment in metaphase I ensures random distribution of maternal and paternal chromosomes to gametes. Together, with random fertilization, these mechanisms result in a high potential for genetic variation in sexual reproduction.
Step-by-step explanation:
Genetic Variation Through Crossing Over and Independent Assortment
Crossing over and independent assortment are two mechanisms that occur during meiosis which lead to the production of gametes with new combinations of alleles, contributing to genetic variation in offspring. During prophase I of meiosis, homologous chromosomes pair up and exchange segments of DNA in a process known as crossing over. This recombination creates new combinations of genes on each chromosome. Independent assortment occurs later, in metaphase I, when chromosomes line up randomly at the cell equator before being separated. This ensures that each gamete receives a random mix of maternal and paternal chromosomes, resulting in a vast number of potential allele combinations. Finally, random fertilization further increases genetic diversity, as any sperm can fuse with any egg, each with their own unique genetic makeup.
The combination of these processes allows for sexual reproduction to produce a substantial degree of genetic variation. For humans, the number of possible chromosome combinations in gametes due to independent assortment alone is 223, or approximately 8.4 million different combinations, without considering the additional variation introduced by crossing over and random fertilization.