Final answer:
Barbara McClintock and Harriet Creighton provided the visual proof of chromosome crossover during meiosis in corn plants, and Curt Stern demonstrated the same process in Drosophila. They observed the exchange of chromosome segments by crossing flies, observing the offspring, and visualizing their chromosomes. This discovery showed that homologous chromosomes regularly exchange segments in meiosis.
Step-by-step explanation:
In 1931, Barbara McClintock and Harriet Creighton demonstrated the crossover of homologous chromosomes in corn plants. Weeks later, Curt Stern demonstrated microscopically homologous recombination in Drosophila. Stern observed several X-linked phenotypes that were associated with a structurally unusual and dissimilar X chromosome pair in which one X was missing a small terminal segment, and the other X was fused to a piece of the Y chromosome. By crossing flies, observing their offspring, and then visualizing the offspring's chromosomes, Stern demonstrated that every time the offspring allele combination deviated from either of the parental combinations, there was a corresponding exchange of an X chromosome segment. Using mutant flies with structurally distinct X chromosomes was the key to observing the products of recombination because DNA sequencing and other molecular tools were not yet available. We now know that homologous chromosomes regularly exchange segments in meiosis by reciprocally breaking and rejoining their DNA at precise locations.