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Ch. 7 - Linkage and Chromosome Mapping in Eukaryotes
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
All textbooksKlug 10th EditionCh. 7 - Linkage and Chromosome Mapping in EukaryotesProblem 20b
Chapter 7, Problem 20b

In Drosophila, Dichaete (D) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the F1 progeny, with a Dichaete phenotype, were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table,
Table showing phenotypes and their numbers from a Dichaete and ebony, pink backcross in Drosophila flies.
What is the sequence and interlocus distance between these three genes?

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Step 1: Identify the parental and recombinant phenotypes from the table. The parental phenotypes are the most frequent classes, which are 'Dichaete' (401) and 'ebony, pink' (389). The recombinant phenotypes are the less frequent classes, such as 'Dichaete, ebony' (84), 'pink' (96), 'Dichaete, pink' (2), 'ebony' (3), 'Dichaete, ebony, pink' (12), and 'wild type' (13).
Step 2: Determine the gene order by analyzing double crossover classes. The double crossover phenotypes are the rarest classes, which are 'Dichaete, pink' (2) and 'ebony' (3). These indicate which gene is in the middle because double crossovers affect the middle gene. Compare these to the parental types to infer the gene order.
Step 3: Calculate the recombination frequencies between each pair of genes. Use the formula for recombination frequency (RF): \(\text{RF} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100\) Calculate RF for each gene pair by summing the appropriate recombinant classes that involve crossovers between those genes.
Step 4: Assign the gene order based on the double crossover analysis and recombination frequencies. The gene in the middle will be the one involved in the double crossover classes. The two outer genes will have recombination frequencies that add up approximately to the total distance between the outermost genes.
Step 5: Calculate the interlocus distances (map units) between the genes using the recombination frequencies. The distances between genes are expressed in centiMorgans (cM), where 1% recombination frequency equals 1 cM. Use the recombination frequencies calculated to find the distances between each pair of genes.

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Key Concepts

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

Genetic Linkage and Recombination

Genetic linkage refers to genes located close together on the same chromosome that tend to be inherited together. Recombination occurs during meiosis when crossing over between homologous chromosomes can separate linked genes, producing new allele combinations. The frequency of recombination between two genes is proportional to their physical distance on the chromosome, measured in map units or centimorgans (cM).
Recommended video:
Guided course
07:33
Chi Square and Linkage

Three-Point Test Cross Mapping

A three-point test cross involves crossing an individual heterozygous for three genes with a homozygous recessive individual to analyze recombination frequencies. By examining the phenotypes and their numbers in the progeny, one can determine the gene order and calculate the distances between genes. Double crossover events are key to identifying gene order and refining map distances.
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Mapping Overview

Interpreting Phenotypic Classes in Linkage Analysis

Phenotypic classes in a backcross reflect parental and recombinant genotypes. The most frequent classes represent parental (non-recombinant) types, while less frequent classes indicate single or double crossovers. Identifying these classes allows calculation of recombination frequencies and helps deduce gene order and map distances.
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Chi Square and Linkage
Related Practice
Textbook Question

A female of genotype

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 8 different genotypes will be produced? Assuming the order a–b–c and the allele arrangement previously shown, what is the map distance between these loci?

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Textbook Question

In a certain plant, fruit is either red or yellow, and fruit shape is either oval or long. Red and oval are the dominant traits. Two plants, both heterozygous for these traits, were testcrossed, with the following results.

Determine the location of the genes relative to one another and the genotypes of the two parental plants.

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Textbook Question

In Drosophila, Dichaete (D) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the F₁ progeny, with a Dichaete phenotype, were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table.

Diagram this cross, showing the genotypes of the parents and offspring of both crosses.

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Textbook Question

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table.

If these three genes were all assorting independently, how many genotypic and phenotypic classes would result in the offspring, and in what proportion, assuming simple dominance and recessiveness in each gene pair?

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Textbook Question

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table.

Answer part (a) again, assuming the three genes are so tightly linked on a single chromosome that no crossover gametes were recovered in the sample of offspring.

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Textbook Question

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table.

What can you conclude from the actual data about the location of the three genes in relation to one another?

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