Final answer:
Crossing-over during meiosis increases genetic variation by exchanging DNA between non-sister chromatids of homologous chromosomes, leading to gametes with unique combinations of maternal and paternal genes. This process, along with independent assortment and random fertilization, contributes to the genetic diversity within a population.
Step-by-step explanation:
Crossing-over is a crucial process during meiosis which significantly increases genetic variation. During prophase I of meiosis, homologous chromosomes pair up and form tetrads, which consist of four chromatids. Crossover events occur between these non-sister chromatids, allowing for reciprocal exchange of equivalent DNA segments between a maternal chromosome and a paternal chromosome.
This recombination results in gametes that carry unique combinations of genes, with each chromatid potentially having a mix of maternal and paternal DNA. Such exchange can happen multiple times and at almost any position along the chromosomes, which ensures that different cells undergoing meiosis produce gametes with different combinations of genes. This variation is the cornerstone of genetic diversity in sexually reproducing populations, providing the raw material for evolution and adaptation.
Therefore, crossing-over, along with independent assortment and random fertilization, contributes to the extensive genetic variation observed among individuals within a population. The randomness in the alignment of recombined chromosomes during metaphase, combined with the effects of crossing-over, is largely responsible for the genetic uniqueness in offspring.