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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 21b
Chapter 7, Problem 21b

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.
Table showing progeny genotypes and counts from a testcross of AaBbCc with aabbcc, highlighting linked gene inheritance.
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.

Verified step by step guidance
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Step 1: Understand the testcross setup. The organism with genotype AaBbCc is crossed with a triply recessive organism aabbcc. The progeny genotypes reflect the gametes produced by the AaBbCc parent combined with the aabbcc parent's gametes.
Step 2: Recognize that if the three genes are tightly linked with no crossover, only the parental (non-recombinant) gametes will be produced by the AaBbCc parent. This means the progeny will only show genotypes corresponding to the original parental haplotypes.
Step 3: Identify the parental haplotypes from the AaBbCc parent. Since the parent is heterozygous at all three loci, the two parental haplotypes are the combinations of alleles found together on each chromosome. For example, one chromosome might carry ABC and the other abc.
Step 4: Determine which progeny genotypes correspond to these parental haplotypes. Only progeny with genotypes matching these haplotypes (combined with the recessive aabbcc parent) will be observed if no crossover occurs.
Step 5: Conclude that the progeny genotypes that do not match the parental haplotypes represent recombinant types and would not be present if no crossover occurred. Therefore, only the progeny with genotypes corresponding to the parental haplotypes should be counted.

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Key Concepts

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

Testcross and Its Purpose

A testcross involves crossing an organism with a dominant phenotype but unknown genotype with a homozygous recessive organism. This helps reveal the genotype of the dominant parent by analyzing the phenotypes and genotypes of the offspring, especially useful for studying linked genes and recombination.
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Purpose of X Inactivation

Genetic Linkage and Recombination

Genetic linkage occurs when genes are located close together on the same chromosome and tend to be inherited together. Recombination through crossing over can separate linked genes, producing new allele combinations. If no crossover occurs, only parental allele combinations appear in offspring.
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Chi Square and Linkage

Interpreting Progeny Ratios in Linked Genes

Progeny genotype frequencies from a testcross can indicate linkage and crossover events. Equal numbers of parental types and fewer recombinants suggest linkage with some crossing over. If no recombinants are observed, all offspring genotypes correspond to parental haplotypes, indicating very tight linkage.
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Related Practice
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

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.

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

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Textbook Question

Based on our discussion of the potential inaccuracy of mapping, would you revise your answer to Problem 22? If so, how?

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