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Chapter 7: Problem 5
Why is a 50 percent recovery of single-crossover products the upper limit, even when crossing over always occurs between two linked genes?
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Review the Chapter Concepts list on p. \(121 .\) Most of these center on the process of crossing over between linked genes. Write a short essay that discusses how crossing over can be detected and how the resultant data provide the basis of chromosome mapping.
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?
Are sister chromatid exchanges effective in producing genetic variability in an individual? in the offspring of individuals?
In Drosophila, the two mutations Stubble bristles (Sb) and curled wings ( $c u$ ) are linked on chromosome III. Sb is a dominant gene that is lethal in a homozygous state, and \(c u\) is a recessive gene. If a female of the genotype \\[ \frac{S b}{+} \\] is to be mated to detect recombinants among her offspring, what male genotype would you choose as her mate?
A female of genotype \\[ \frac{a}{+++} \\] produces 100 meiotic tetrads. Of these, 68 show no crossover events. Of the remaining 32,20 show a crossover between \(a\) and \(b\), 10 show a crossover between \(b\) and \(c,\) and 2 show a double crossover between \(a\) and \(b\) and between \(b\) and \(c .\) Of the 400 gametes produced, how many of each of the eight different genotypes will be produced? Assuming the order \(a-b-c\) and the allele arrangement shown above, what is the map distance between these loci?
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