Textbook Question
The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.
What amount of DNA is contained in each nucleus at the end of the S phase?
Ch. 3 - Cell Division and Chromosome Heredity
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172
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Table of contents
- Ch. 1 - The Molecular Basis of Heredity, Variation, and Evolution64
- Ch. 2 - Transmission Genetics144
- Ch. 3 - Cell Division and Chromosome Heredity81
- Ch. 4 - Gene Interaction87
- Ch. 5 - Genetic Linkage and Mapping in Eukaryotes103
- Ch. 6 - Genetic Analysis and Mapping in Bacteria and Bacteriophages73
- Ch. 7 - DNA Structure and Replication71
- Ch. 8 - Molecular Biology of Transcription and RNA Processing79
- Ch. 9 - The Molecular Biology of Translation110
- Ch. 10 - Eukaryotic Chromosome Abnormalities and Molecular Organization100
- Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination86
- Ch. 12 - Regulation of Gene Expression in Bacteria and Bacteriophage90
- Ch. 13 - Regulation of Gene Expression in Eukaryotes38
- Ch. 14 - Analysis of Gene Function via Forward Genetics and Reverse Genetics72
- Ch. 15 - Recombinant DNA Technology and Its Applications69
- Ch. 16 - Genomics: Genetics from a Whole-Genome Perspective49
- Ch. 17 - Organelle Inheritance and the Evolution of Organelle Genomes27
- Ch. 18 - Developmental Genetics43
- Ch. 19 - Genetic Analysis of Quantitative Traits66
- Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels105
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
Chapter 3, Problem 6
An organism has alleles R₁ and R₂ on one pair of homologous chromosomes, and it has alleles T₁ and T₂ on another pair. Diagram these pairs of homologs at the end of metaphase I, the end of telophase I, and the end of telophase II, and show how meiosis in this organism produces gametes in expected Mendelian proportions. Assume no crossover between homologous chromosomes.
Verified step by step guidance1
Step 1: Understand the genetic setup. The organism has two pairs of homologous chromosomes: one pair with alleles R₁ and R₂, and another pair with alleles T₁ and T₂. Each homologous chromosome carries one allele of each gene.
Step 2: Diagram the chromosomes at the end of metaphase I. At this stage, homologous chromosomes are paired and aligned at the metaphase plate. You should show two pairs of homologs: one pair with R₁ and R₂ aligned together, and the other pair with T₁ and T₂ aligned together. Each chromosome consists of two sister chromatids.
Step 3: Diagram the chromosomes at the end of telophase I. After the first meiotic division, homologous chromosomes separate into two different cells. Each cell will have one chromosome from each pair (either R₁ or R₂, and either T₁ or T₂), but still with sister chromatids attached.
Step 4: Diagram the chromosomes at the end of telophase II. The second meiotic division separates sister chromatids. Each of the four resulting cells (gametes) will have one chromatid from each chromosome pair, carrying either R₁ or R₂ and either T₁ or T₂.
Step 5: Explain Mendelian proportions. Since there is no crossover, the alleles segregate independently according to Mendel's laws. The gametes will have combinations R₁T₁, R₁T₂, R₂T₁, and R₂T₂ in equal proportions (1:1:1:1), reflecting independent assortment of the two gene pairs.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Homologous Chromosomes and Alleles
Homologous chromosomes are pairs of chromosomes, one from each parent, that carry the same genes but may have different alleles. In this question, alleles R₁ and R₂ are on one homologous pair, and T₁ and T₂ on another. Understanding their pairing and segregation during meiosis is essential to track how genetic variation is passed to gametes.
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New Alleles and Migration
Stages of Meiosis I and II
Meiosis consists of two divisions: Meiosis I separates homologous chromosomes, and Meiosis II separates sister chromatids. Key stages include metaphase I (homologs aligned), telophase I (homologs separated), and telophase II (sister chromatids separated). Recognizing chromosome arrangement at these stages helps visualize allele distribution in gametes.
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Mendelian Inheritance and Gamete Formation
Mendelian inheritance predicts that alleles segregate independently and assort into gametes in specific ratios. Without crossover, alleles on different chromosome pairs segregate independently, producing gametes with combinations of R₁ or R₂ and T₁ or T₂ in expected Mendelian proportions (1:1:1:1). This principle explains genetic variation in offspring.
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Related Practice
Textbook Question
The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.
Explain why a somatic cell of Geneticus introductus has the same number of chromosomes and the same amount of DNA at the beginning of mitotic prophase as one of these cells does at the beginning of prophase I of meiosis.
Textbook Question
The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.
Complete the following table by entering the number of chromosomes and amount of DNA present per cell at the end of each stage listed.
Textbook Question
Explain how the behavior of homologous chromosomes in meiosis parallels Mendel's law of segregation for autosomal alleles D and d. During which stage of M phase do these two alleles segregate from one another?
Textbook Question
Suppose crossover occurs between the homologous chromosomes in the previous problem. At what stage of M phase do alleles D and d segregate?
Textbook Question
Alleles A and a are on one pair of autosomes, and alleles B and b are on a separate pair of autosomes. Does crossover between one pair of homologs affect the expected proportions of gamete genotypes? Why or why not? Does crossover between both pairs of chromosomes affect the expected gamete proportions? Why or why not?