In this chapter, we focused on linkage, chromosomal mapping, and many associated phenomena. In the process, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) 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? (b) How do we know that specific genes are linked on a single chromosome, in contrast to being located on separate chromosomes? (c) How do we know that crossing over results from a physical exchange between chromatids? (d) How do we know that sister chromatids undergo recombination during mitosis?

a) The relationship between recombination frequency and the distance between two genes was established through experimental observations by Thomas Hunt Morgan. He found that the frequency of recombination is proportional to the distance between the genes on a chromosome, leading to the development of the concept of "map units" or "centimorgans." b) To identify specific genes linked on a single chromosome, researchers analyze inheritance patterns through multiple generations and calculate the recombination frequency between genes. A lower frequency indicates genes are likely to be linked and located close to each other on the same chromosome. c) Barbara McClintock and Harriet Creighton provided evidence that crossing over results from a physical exchange between chromatids. By studying maize plants with visible cytological markers on chromosomes, they observed offspring with markers from both parents, suggesting that segments of chromatids were exchanged during meiosis. d) Recombination between sister chromatids during mitosis, known as "sister chromatid exchange" (SCE), was observed under a microscope in the late 1960s and early 1970s. SCE occurs during mitosis and allows for the repair of damaged DNA, maintaining genetic stability.