Final answer:
Genetic variation in humans arises from independent assortment, crossing over during meiosis, and random fertilization, resulting in over 8 million ways chromosomes can arrange themselves and more than 64 trillion genetically unique potential children from each human couple.
Step-by-step explanation:
Genetic variation results from the combined effects of independent assortment, crossing over during meiosis, and random fertilization. In humans, the process of meiosis is critical, resulting in a possible 2²³ or over 8 million different ways to arrange chromosomes during metaphase I. Moreover, the additional genetic recombination through crossing over among non-sister chromatids generates unique combinations of genes, which further contributes to genetic diversity. This means that one set of 23 chromosomes from the mother and one set from the father combine to produce immense genetic variation among offspring.
Crossing over, one of the first sources of genetic variation produced by meiosis, involves a reciprocal exchange of equivalent DNA between maternal and paternal chromosomes. This results in gametes carrying unique combinations of DNA, which did not exist before the crossover. Together with independent assortment, in which each chromosome of a pair can segregate into any gamete independently of the other pairs, and random fertilization, between a sperm and an egg each having over 8 million chromosome combinations, immense genetic diversity arises. Consequently, each human couple has the potential to produce more than 64 trillion genetically unique children.