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Ch. 5 - Chromosome Mapping in Eukaryotes
All textbooksKlug 12th EditionCh. 5 - Chromosome Mapping in EukaryotesProblem 9
Chapter 5, Problem 9

The genes dumpy (dp), clot (cl), and apterous (ap) are linked on chromosome II of Drosophila. In a series of two-point mapping crosses, the following genetic distances were determined. What is the sequence of the three genes? dp–ap 42 dp–cl 3 ap–cl 39

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1
Identify the genetic distances between each pair of genes: dp-ap is 42, dp-cl is 3, and ap-cl is 39.
Recognize that the smallest genetic distance (dp-cl = 3) suggests that dp and cl are closest to each other on the chromosome.
Consider the larger distances: dp-ap is 42 and ap-cl is 39. These suggest that ap is further away from both dp and cl.
Arrange the genes in a sequence that reflects these distances: since dp and cl are closest, they should be adjacent, with ap being further away.
Conclude that the gene sequence is dp-cl-ap, as this arrangement satisfies all given genetic distances.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This phenomenon occurs because linked genes are less likely to be separated by recombination events. Understanding linkage is crucial for determining the relative positions of genes on a chromosome, as it influences the outcomes of genetic crosses.
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Chi Square and Linkage

Two-Point Mapping

Two-point mapping is a method used to determine the distance between two genes on a chromosome based on the frequency of recombination between them. The genetic distance is expressed in centimorgans (cM), where 1 cM corresponds to a 1% chance of recombination occurring. This technique helps in constructing genetic maps and inferring the order of genes based on their recombination frequencies.
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Recombination Frequency

Recombination frequency is the measure of how often recombination occurs between two genes during meiosis, affecting the inheritance patterns of traits. It is calculated as the number of recombinant offspring divided by the total number of offspring, expressed as a percentage. This frequency is essential for determining the relative positions of genes on a chromosome and helps in constructing genetic maps.
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Related Practice
Textbook Question
Why are double-crossover events expected less frequently than single-crossover events?
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Textbook Question
What is the proposed basis for positive interference?
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Textbook Question
What two essential criteria must be met in order to execute a successful mapping cross?
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Textbook Question
Colored aleurone in the kernels of corn is due to the dominant allele R. The recessive allele r, when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, Y and y. The dominant Y allele results in green color, whereas the homozygous presence of the recessive y allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained: colored, green 88 colored, yellow 12 colorless, green 8 colorless, yellow 92 Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.
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Textbook Question
In the cross shown here, involving two linked genes, ebony (e) and claret (ca), in Drosophila, where crossing over does not occur in males, offspring were produced in a 2 + : 1 ca : 1 e phenotypic ratio: These genes are 30 units apart on chromosome III. What did crossing over in the female contribute to these phenotypes?

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Textbook Question
In a series of two-point mapping crosses involving five genes located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed: pr–adp 29% pr–vg 13 pr–c 21 pr–b 6 adp–b 35 adp–c 8 adp–vg. 16 vg–b. 19 vg–c 8 c–b. 27 Given that the adp gene is near the end of chromosome II (locus 83), construct a map of these genes.
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