Textbook Question
What two essential criteria must be met in order to execute a successful mapping cross?
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Ch. 5 - Chromosome Mapping in Eukaryotes
Klug12th EditionConcepts of Genetics ISBN: 9780135564776
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Table of contents
- Ch. 1 - Introduction to Genetics17
- Ch. 2 - Mitosis and Meiosis36
- Ch. 3 - Mendelian Genetics51
- Ch. 4 - Extensions of Mendelian Genetics74
- Ch. 5 - Chromosome Mapping in Eukaryotes51
- Ch. 6 - Genetic Analysis and Mapping in Bacteria and Bacteriophages46
- Ch. 7 - Sex Determination and Sex Chromosomes33
- Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement40
- Ch. 9 - Extranuclear Inheritance31
- Ch. 10 - DNA Structure and Analysis43
- Ch. 11 - DNA Replication and Recombination44
- Ch. 12 - DNA Organization in Chromosomes30
- Ch. 13 - The Genetic Code and Transcription54
- Ch. 14 - Translation and Proteins47
- Ch. 15 - Gene Mutation, DNA Repair, and Transposition41
- Ch. 16 - Regulation of Gene Expression in Bacteria29
- Ch. 17 - Transcriptional Regulation in Eukaryotes41
- Ch. 18 - Post-transcriptional Regulation in Eukaryotes33
- Ch. 19 - Epigenetics33
- Ch. 20 - Recombinant DNA Technology44
- Ch. 21 - Genomic Analysis36
- Ch. 22 - Applications of Genetic Engineering and Biotechnology36
- Ch. 23 - Developmental Genetics37
- Ch. 24 - Cancer Genetics34
- Ch. 25 - Quantitative Genetics and Multifactorial Traits54
- Ch. 26 - Population and Evolutionary Genetics45
Chapter 5, Problem 10
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.
Verified step by step guidance1
Identify the phenotypes and genotypes of the testcross parent: since it is homozygous recessive for both traits, its genotype is rr yy, producing colorless aleurone and yellow plant color.
Assign symbols to the unknown plant's genotype: since it shows dominant phenotypes, it must have at least one R allele and one Y allele. Let the unknown genotype be Rr Yy, but we need to determine the arrangement (coupling or repulsion) of these alleles on homologous chromosomes.
Analyze the progeny phenotypes and their numbers: the four classes are colored green (88), colored yellow (12), colorless green (8), and colorless yellow (92). The large numbers correspond to parental types, and the small numbers correspond to recombinant types, indicating linkage between the R and Y genes.
Determine which phenotypes correspond to parental and recombinant types: the two most frequent classes (colored green and colorless yellow) represent parental combinations, so the unknown plant's alleles are arranged in coupling phase as R Y / r y or in repulsion phase as R y / r Y. The data suggest coupling because colored green and colorless yellow are most frequent.
Conclude the exact genotype and allele arrangement of the unknown plant: it is heterozygous for both genes with alleles in coupling phase, meaning one homolog carries R and Y together, and the other homolog carries r and y together, i.e., genotype R Y / r y.
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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 individual with an unknown genotype to a homozygous recessive individual to reveal the unknown genotype based on progeny phenotypes. It helps determine whether the unknown parent is homozygous or heterozygous for the traits by analyzing the ratio of offspring phenotypes.
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00:52
Purpose of X Inactivation
Linked Genes and Recombination
Genes located close together on the same chromosome tend to be inherited together, a phenomenon called linkage. Recombination during meiosis can separate linked alleles, producing recombinant offspring. The frequency of recombinant phenotypes helps map gene order and determine allele arrangement on homologous chromosomes.
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07:56Sex-Linked Genes
Dominance and Phenotypic Expression
Dominant alleles mask the expression of recessive alleles in heterozygotes, determining the phenotype. In this question, the dominant R allele produces colored aleurone, and dominant Y produces green plants, while recessive homozygotes (rr or yy) show colorless aleurone or yellow plants, respectively.
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Related Practice
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:
In another set of experiments, a sixth gene, d, was tested against b and pr:
Predict the results of two-point mapping between d and c, d and vg, and d and adp.
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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:
Given that the adp gene is near the end of chromosome II (locus 83), construct a map of these genes.
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Textbook Question
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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Textbook Question
What is the proposed basis for positive interference?
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