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Ch. 5 - Genetic Linkage and Mapping in Eukaryotes
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 5 - Genetic Linkage and Mapping in EukaryotesProblem 7
Chapter 5, Problem 7

Genes A, B, and C are linked on a chromosome and found in the order A–B–C. Genes A and B recombine with a frequency of 8%, and genes B and C recombine at a frequency of 24%. For the cross a⁺b⁺c/abc⁺ × abc/abc, predict the frequency of progeny genotypes. Assume interference is zero.

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Understand the problem: Genes A, B, and C are linked on the same chromosome, and their recombination frequencies are given. The recombination frequency between A and B is 8% (0.08), and between B and C is 24% (0.24). The goal is to predict the frequency of progeny genotypes for the given cross, assuming no interference (i.e., double crossovers occur at the expected frequency).
Determine the parental and recombinant gametes: The parental gametes are a⁺b⁺c and abc⁺, as these are the original combinations of alleles. Recombinant gametes arise from crossing over between A and B, B and C, or both (double crossover).
Calculate the frequency of single crossovers: The frequency of single crossovers between A and B is 8% (0.08), and between B and C is 24% (0.24). These frequencies represent the proportion of gametes that will result from crossing over in these regions.
Account for double crossovers: Double crossovers involve recombination in both the A–B and B–C regions. Since interference is zero, the frequency of double crossovers is the product of the single crossover frequencies: (0.08) × (0.24). This represents the proportion of gametes that result from double crossovers.
Distribute the frequencies among the progeny genotypes: Use the recombination frequencies to calculate the proportion of each progeny genotype. Parental genotypes will have the highest frequencies, single crossover genotypes will have intermediate frequencies, and double crossover genotypes will have the lowest frequencies. Ensure that the total frequency sums to 1 (100%).

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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 is due to their physical proximity, which reduces the likelihood of recombination events occurring between them. Understanding linkage is crucial for predicting the inheritance patterns of linked genes, as it affects the ratios of offspring genotypes.
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Chi Square and Linkage

Recombination Frequency

Recombination frequency is a measure of the likelihood that two genes will be separated during meiosis due to crossing over. It is expressed as a percentage, indicating the proportion of recombinant offspring produced. In the given question, the recombination frequencies between genes A and B (8%) and between genes B and C (24%) are essential for calculating the expected genotype frequencies in the progeny.
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Interference

Interference is a phenomenon in genetics where the occurrence of one crossover event affects the likelihood of another crossover happening nearby. When interference is zero, as stated in the question, it implies that crossovers occur independently, allowing for straightforward calculations of expected genotype frequencies based on the recombination frequencies. This simplifies the analysis of linked genes in genetic crosses.
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Related Practice
Textbook Question

In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.


Do you expect any of these gene pair(s) to assort independently? Explain your reasoning.

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Textbook Question

In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.


A wild-type female fruit fly with the genotype y⁺w⁺f/ywf⁺ is crossed to a male fruit fly that has yellow body, white eye, and forked bristles. Predict the frequency of each progeny phenotype class produced by this mating.

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Textbook Question

In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.


Explain how each of the predicted progeny classes is produced.

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Textbook Question

Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.


Draw two possible genetic maps for these three genes, and identify the recombination frequencies predicted for each map.

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Textbook Question

Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.


Assuming that organisms with any desired genotype are available, propose a genetic cross whose result could be used to determine which of the proposed genetic maps is correct.

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Textbook Question

Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.


The test cross Ae/aE x ae/ae  indicates the genes recombine with a frequency of 28%. If 1000 progeny are produced by this test cross, determine the number of progeny in each outcome class.

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