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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In Drosophila, a cross was made between females expressing the three X-linked recessive traits, scute bristles \((s c),\) sable body \((s)\) and vermilion eyes ( \(v\) ), and wild-type males. All females were wild type in the \(F_{1},\) while all males expressed all three mutant traits. The cross was carried to the \(\mathrm{F}_{2}\) generation and 1000 offspring were counted, with the results shown in the following table. No determination of sex was made in the \(\mathrm{F}_{2}\) data. (a) Using proper nomenclature, determine the genotypes of the \(P_{1}\) and \(F_{1}\) parents. (b) Determine the sequence of the three genes and the map distance between them. (c) Are there more or fewer double crossovers than expected? (d) Calculate the coefficient of coincidence; does this represent positive or negative interference?
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?
The genes dumpy wings \((d p),\) clot eyes \((c l),\) and apterous wings \((a p)\) are linked on chromosome II of Drosophila. In a series of twopoint mapping crosses, the genetic distances shown below were determined. What is the sequence of the three genes?
Why does more crossing over occur between two distantly linked genes than between two genes that are very close together on the same chromosome?
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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