Final answer:
Inhibition of DNA synthesis after S phase interferes with the homologous chromosome pairing and crossover events necessary for genetic recombination during meiosis, leading to a reduction in recombination in the meiotic products.
Step-by-step explanation:
When you apply a drug that specifically inhibits DNA synthesis to yeast cells after S phase but before prophase of meiosis, and you observe a drastic reduction in recombination, the explanation lies in the timing and mechanisms of meiosis. DNA synthesis is crucial for the pairing of homologous chromosomes and the exchange of genetic material between them during prophase I of meiosis. Without DNA synthesis, the physical basis for recombination, which involves the creation of chiasmata between homologous chromosomes, is compromised. Therefore, by inhibiting DNA synthesis, the drug prevents proper pairing and the necessary crossover events that lead to genetic recombination in meiotic products.