Final answer:
Crossing over, a process that contributes to genetic diversity, occurs during prophase I of meiosis I when homologous chromosomes pair up and exchange genetic material.
Step-by-step explanation:
Crossing over normally takes place during meiosis I. During prophase I, homologous chromosomes pair up and exchange segments of their genetic material to form recombinant chromosomes. This process increases genetic variation, because the resulting chromatids are no longer identical after crossing over. As a result, when the chromosomes proceed to meiosis II and separate, the daughter cells may receive chromosomes with recombinant alleles. Therefore, crossing over contributes to the genetic diversity of offspring.