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Ch. 14 - Translation and Proteins
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 14 - Translation and ProteinsProblem 34
Chapter 14, Problem 34

Deep in a previously unexplored South American rain forest, a plant species was discovered with true-breeding varieties whose flowers were pink, rose, orange, or purple. A very astute plant geneticist made a single cross, carried to the F₂ generation, as shown:
P₁: purple × pink
F₁: all purple
F₂: 27/64 purple 16/64 pink 12/64 rose 9/64 orange
Based solely on these data, he proposed both a mode of inheritance for flower pigmentation and a biochemical pathway for the synthesis of these pigments. Carefully study the data. Create a hypothesis of your own to explain the mode of inheritance. Then propose a biochemical pathway consistent with your hypothesis. How could you test the hypothesis by making other crosses?

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Analyze the F₁ generation: Since all F₁ offspring are purple, this suggests that purple is dominant over pink. This indicates that the mode of inheritance involves dominance relationships between alleles.
Examine the F₂ generation ratios: The phenotypic ratios in the F₂ generation (27/64 purple, 16/64 pink, 12/64 rose, 9/64 orange) suggest a dihybrid cross with two genes involved. The total number of offspring (64) aligns with a 4x4 Punnett square, indicating two independently assorting genes.
Propose a genetic hypothesis: Assume two genes (A and B) control flower color. Purple may result from the presence of at least one dominant allele at both loci (A_B_), pink may result from homozygosity for the recessive allele at one locus (aaB_), rose may result from heterozygosity at both loci (AaBb), and orange may result from homozygosity for the recessive allele at both loci (aabb).
Propose a biochemical pathway: Gene A could encode an enzyme that produces a precursor pigment, and Gene B could encode an enzyme that modifies this precursor to produce the final pigment. For example, the pathway might be: colorless precursor → (enzyme A) → intermediate pigment → (enzyme B) → final pigment. The absence of either enzyme could lead to different intermediate colors.
Design a test cross: To test the hypothesis, perform a test cross between F₁ individuals (AaBb) and homozygous recessive individuals (aabb). Analyze the phenotypic ratios in the offspring. If the hypothesis is correct, the offspring should display a 1:1:1:1 ratio of purple, pink, rose, and orange flowers, consistent with independent assortment and the proposed genetic model.

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Key Concepts

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

Mendelian Inheritance

Mendelian inheritance refers to the principles of heredity established by Gregor Mendel, which describe how traits are passed from parents to offspring through dominant and recessive alleles. In this case, the dominance of the purple flower trait over pink suggests a simple Mendelian inheritance pattern, where the purple allele is dominant and pink is recessive. Understanding these principles is crucial for hypothesizing the genetic basis of the observed flower colors.
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Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. The F₂ generation data shows a phenotypic ratio of 27:16:12:9 for purple, pink, rose, and orange flowers, respectively. Analyzing these ratios can help infer the number of genes involved and their interactions, which is essential for formulating a hypothesis about the inheritance pattern and the underlying genetic mechanisms.
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Biochemical Pathways

Biochemical pathways are sequences of chemical reactions occurring within a cell that lead to the synthesis of specific compounds, such as pigments. In the context of flower pigmentation, understanding the biochemical pathway for pigment synthesis can help explain the variation in flower colors. By proposing a pathway based on the observed phenotypes, one can identify key enzymes and intermediates, which can be tested through genetic crosses or molecular analysis to validate the hypothesis.
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Related Practice
Textbook Question

How would the results vary in cross (a) of Problem 32 if genes A and B were linked with no crossing over between them? How would the results of cross (a) vary if genes A and B were linked and 20 map units (mu) apart?

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Textbook Question

The flow of genetic information from DNA to protein is mediated by messenger RNA. If you introduce short DNA strands (called antisense oligonucleotides) that are complementary to mRNAs, hydrogen bonding may occur and 'label' the DNA/RNA hybrid for ribonuclease-H degradation of the RNA. One study [Lloyd et al. (2001). Nucl. Acids Res. 29:3664–3673] compared the effect of different-length antisense oligonucleotides upon ribonuclease-H–mediated degradation of tumor necrosis factor (TNFα) mRNA. TNFα exhibits antitumor and pro-inflammatory activities. The following graph indicates the efficacy of various-sized antisense oligonucleotides in causing ribonuclease-H cleavage. Describe how antisense oligonucleotides interrupt the flow of genetic information in a cell.

Textbook Question

The flow of genetic information from DNA to protein is mediated by messenger RNA. If you introduce short DNA strands (called antisense oligonucleotides) that are complementary to mRNAs, hydrogen bonding may occur and 'label' the DNA/RNA hybrid for ribonuclease-H degradation of the RNA. One study [Lloyd et al. (2001). Nucl. Acids Res. 29:3664–3673] compared the effect of different-length antisense oligonucleotides upon ribonuclease-H–mediated degradation of tumor necrosis factor (TNFα) mRNA. TNFα exhibits antitumor and pro-inflammatory activities. The following graph indicates the efficacy of various-sized antisense oligonucleotides in causing ribonuclease-H cleavage. What general conclusion can be drawn from the graph?

Textbook Question

Many antibiotics are effective as drugs to fight off bacterial infections because they inhibit protein synthesis in bacterial cells. Using the information provided in the following table that highlights several antibiotics and their mode of action, discuss which phase of translation is inhibited: initiation, elongation, or termination. What other components of the translational machinery could be targeted to inhibit bacterial protein synthesis?

Textbook Question

Three independently assorting genes (A, B, and C) are known to control the following biochemical pathway that provides the basis for flower color in a hypothetical plant:

Three homozygous recessive mutations are also known, each of which interrupts a different one of these steps. Determine the phenotypic results in the F1 and F2 generations resulting from the P1 crosses of true-breeding plants listed here:

colorless (aaBBCC) × green (AABBcc)

Textbook Question

Three independently assorting genes (A, B, and C) are known to control the following biochemical pathway that provides the basis for flower color in a hypothetical plant:

Three homozygous recessive mutations are also known, each of which interrupts a different one of these steps. Determine the phenotypic results in the F1 and F2 generations resulting from the P1 crosses of true-breeding plants listed here:

yellow (AAbbCC) × green (AABBcc)