Question

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?

Short answer

Answer: The relationship between recombination frequency and gene distance states that the frequency of recombination between two genes is proportional to the distance between them. This means that as the distance between genes increases, the likelihood of crossovers occurring between them also increases. By measuring the recombination frequency (percentage of recombinant offspring), it is possible to map genes' relative positions on a chromosome and construct a genetic map based on this data.

Step-by-step solution

(a) Recombination frequency and gene distance

: It was established experimentally by observing the relationship between the crossover events and the genetic recombination of offspring. Sturtevant, a student of Thomas Hunt Morgan, proposed that the frequency of recombination between two genes is proportional to the distance between them. The greater the distance between two genes on a chromosome, the higher the likelihood of crossovers occurring between them. By measuring the recombination frequency (percentage of recombinant offspring), it is possible to map genes' relative positions on a chromosome and construct a genetic map from the data.

(b) Determining linked genes on a single chromosome

: The presence of specific genes on a single chromosome, rather than on separate chromosomes, is determined by observing non-random inheritance patterns of different characteristics and susceptibility to crossing over. If two genes are linked, they will be inherited together more frequently and have a lower recombination frequency than expected for unlinked genes. These observations can be further substantiated by constructing a genetic map that demonstrates the relative positions of these genes on a single chromosome.

(c) Evidence for physical exchange between chromatids during crossing over

: Crossing over was demonstrated to result from a physical exchange between chromatids by conducting experiments involving chromosome cytology (the study of chromosomes and their structure, function, and behavior). Scientists observed chromosomes during meiosis under a microscope and noticed that non-sister chromatids appeared to undergo a process of physical exchange. It was later confirmed by genetic evidence showing the direct correlation between these observed physical exchanges and the production of recombinant offspring, which is a result of crossing over between chromatids.

(d) Sister chromatids recombination during mitosis

: Generally, sister chromatids do not undergo recombination during mitosis, as the purpose of mitotic divisions is to generate two identical daughter cells. Recombination occurs during meiosis (a type of cell division responsible for the production of gametes) to create genetic diversity among offspring. However, occasional instances of mitotic recombination are observed between sister chromatids in somatic cells. These rare events could be detected by monitoring the changes in the genetic makeup of the cells after mitosis. Such detection is essential in research where new genetic combinations generated by genomic alterations may be studied for their role in cell diversification or disease progression.