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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 20a
Chapter 7, Problem 20a

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 numbers of Drosophila offspring from a backcross involving Dichaete, ebony, and pink mutations.
Diagram this cross, showing the genotypes of the parents and offspring of both crosses.

Verified step by step guidance
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Step 1: Define the alleles and their dominance relationships. Dichaete (D) is a dominant mutation on chromosome III and lethal when homozygous (D/D lethal). ebony (e) and pink (p) are recessive mutations on the same chromosome. The wild-type alleles are d (for Dichaete), E (for ebony), and P (for pink).
Step 2: Determine the genotypes of the parental flies in the initial cross. The Dichaete stock flies must be heterozygous for Dichaete (D/d) and carry wild-type alleles for ebony and pink (E/P), since ebony and pink mutations are recessive and not expressed in this stock. The ebony, pink flies are homozygous recessive for both ebony and pink (e/e, p/p) and wild-type for Dichaete (d/d).
Step 3: Write the genotype of the F₁ progeny from the initial cross. Since D is dominant and lethal when homozygous, the F₁ flies with the Dichaete phenotype are heterozygous for Dichaete (D/d) and heterozygous for ebony and pink (E/e, P/p).
Step 4: Diagram the backcross between the F₁ heterozygous flies (D/d, E/e, P/p) and the homozygous recessive ebony, pink flies (d/d, e/e, p/p). List all possible gametes from each parent considering the three loci and then combine them to predict offspring genotypes.
Step 5: Use the phenotypic data from the backcross table to assign phenotypes to genotypes. For example, flies with Dichaete phenotype carry at least one D allele, ebony phenotype requires homozygous e/e, and pink phenotype requires homozygous p/p. This will help in confirming the parental genotypes and the arrangement of alleles on the chromosomes.

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Key Concepts

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

Linkage and Recombination

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 creates new allele combinations. The frequency of recombinant offspring helps map gene distances and order on chromosomes.
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Chi Square and Linkage

Dominant and Recessive Mutations

Dominant mutations express their phenotype when present in one copy, while recessive mutations require two copies to show their effect. In this question, Dichaete is dominant and lethal when homozygous, whereas ebony and pink are recessive mutations, influencing how genotypes translate into phenotypes in crosses.
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Variations on Dominance

Backcross and Genetic Cross Diagrams

A backcross involves crossing a hybrid organism with one of its parents or a genetically similar individual to analyze inheritance patterns. Diagramming crosses with genotypes and phenotypes clarifies allele segregation, linkage, and recombination events, essential for interpreting offspring ratios and mapping genes.
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Related Practice
Textbook Question

In Drosophila, two mutations, Stubble (Sb) and curled (cu), are linked on chromosome III. Stubble is a dominant gene that is lethal in a homozygous state, and curled is a recessive gene. If a female of the genotype

is to be mated to detect recombinants among her offspring, what male genotype would you choose as a mate?

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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,

What is the sequence and interlocus distance between these three genes?

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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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