Final answer:
The rate of crossing-over in human genes is generally low due to their physical distance from each other on the chromosomes.
Step-by-step explanation:
Crossing-over, which is the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs during prophase I of meiosis. In humans, the rate of crossing-over is generally low in genes due to their physical distance from each other on the chromosomes. When two genes are located close together on the same chromosome, the chance of a crossover event happening between them is slim. As the distance between two genes increases, the probability of crossovers between them increases, resulting in more variation.