How was it established experimentally that the frequency of recombination (crossing over) between two genes is related to the distance between them along the chromosome?

How do we know that specific genes are linked on a single chromosome, in contrast to being located on separate chromosomes?

How do we know that crossing over results from a physical exchange between chromatids?

How do we know that sister chromatids undergo recombination during mitosis?

Write a short essay that discusses how crossing over can be detected and how the resultant data provide the basis of chromosome mapping.

Describe the cytological observation that suggests that crossing over occurs during the first meiotic prophase.