Final answer:
Crossing over during meiosis I allows for the exchange of genetic material between non-sister chromatids of homologous chromosomes. If crossing over occurred in every meiotic cell, we could determine whether the two genes are on the same chromosome or different chromosomes based on the observed recombination frequency.
Step-by-step explanation:
Crossing over occurs during prophase I of meiosis I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This process results in new combinations of genes on each chromosome. Therefore, if crossing over occurred in every meiotic cell, we could determine that the two genes are on the same chromosome because they would exhibit a recombination frequency between 0 percent and 50 percent. This indicates that crossover occurs sometimes.