Ch. 3 - Cell Division and Chromosome Heredity

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 3 - Cell Division and Chromosome Heredity

Problem 19b

Chapter 3, Problem 19b

Four eye-color mutants in Drosophila—apricot, brown, carnation, and purple—are inherited as recessive traits. Red is the dominant wild-type color of fruit-fly eyes. Eight crosses (A through H) are made between parents from pure-breeding lines.

Predict F₂ phenotype ratios of crosses A, B, D, and G.

Verified step by step guidance

Understand the problem: The question involves predicting F₂ phenotype ratios for specific crosses (A, B, D, and G) in Drosophila eye-color mutants. These mutants are inherited as recessive traits, and red is the dominant wild-type eye color. The F₂ generation results from crossing F₁ individuals, which are heterozygous for the traits.

Step 1: Identify the genetic basis of each mutant. Each mutant (apricot, brown, carnation, and purple) is caused by a recessive allele. Let’s assign symbols to represent the alleles: A (apricot), B (brown), C (carnation), and P (purple). The wild-type alleles are dominant and can be represented as A⁺, B⁺, C⁺, and P⁺.

Step 2: Determine the genotypes of the F₁ generation for each cross. For example, if cross A involves apricot and brown mutants, the F₁ individuals will be heterozygous for both traits (A⁺A and B⁺B). Similarly, analyze the genotypes for crosses B, D, and G based on the mutants involved.

Step 3: Use a Punnett square to predict the F₂ generation. For each cross, set up a dihybrid Punnett square if two traits are involved (e.g., apricot and brown in cross A). The F₂ phenotype ratios can be determined by combining the probabilities of inheriting each allele. For example, the classic dihybrid ratio for two independent traits is 9:3:3:1 (9 wild-type, 3 with one mutant trait, 3 with the other mutant trait, and 1 with both mutant traits).

Step 4: Consider linkage or epistasis if applicable. If the genes for the traits are linked (on the same chromosome) or if one gene affects the expression of another (epistasis), the expected ratios may deviate from the standard Mendelian ratios. Analyze the problem for any such indications and adjust the predictions accordingly.

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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 include the concepts of dominant and recessive traits. In this context, the eye color traits in Drosophila demonstrate these principles, where red is the dominant trait and the mutants (apricot, brown, carnation, and purple) are recessive. Understanding how these traits segregate during gamete formation is essential for predicting phenotype ratios in offspring.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from genetic crosses. In the case of Drosophila eye color, the ratios can be predicted based on the genotypes of the parents and the dominance of traits. For example, a typical monohybrid cross involving one dominant and one recessive trait would yield a 3:1 ratio in the F₂ generation, which is crucial for analyzing the results of the specified crosses.

Pure-Breeding Lines

Pure-breeding lines consist of organisms that consistently produce offspring with a specific phenotype when crossed among themselves. In the context of the question, the pure-breeding lines for eye color mutants ensure that the traits are homozygous, allowing for predictable inheritance patterns. This concept is vital for understanding the outcomes of the crosses A, B, D, and G, as it establishes the genetic background from which the F₂ phenotypes will arise.

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Textbook Question

In cats, tortoiseshell coat color appears in females. A tortoiseshell coat has patches of dark brown fur and patches of orange fur that each in total cover about half the body but have a unique pattern in each female. Male cats can be either dark brown or orange, but a male cat with tortoiseshell coat is rarely produced. Two sample crosses between males and females from pure-breeding lines produced the tortoiseshell females shown.

Cross I P: dark brown male × orange female

F₁: orange males and tortoiseshell females

Cross II P: orange male × dark brown female

F₁: dark brown males and tortoiseshell females

The genetics service of a large veterinary hospital gets referrals for three or four male tortoiseshell cats every year. These cats are invariably sterile and have underdeveloped testes. How are these tortoiseshell male cats produced? Why do you think they are sterile?

Textbook Question

The gene causing Coffin–Lowry syndrome (OMIM 303600) was recently identified and mapped on the human X chromosome. Coffin–Lowry syndrome is a rare disorder affecting brain morphology and development. It also produces skeletal and growth abnormalities, as well as abnormalities of motor control. Coffin–Lowry syndrome affects males who inherit a mutation of the X-linked gene. Most carrier females show no symptoms of the disease but a few carriers do. These carrier females are always less severely affected than males. Offer an explanation for this finding.

Textbook Question

Four eye-color mutants in Drosophila—apricot, brown, carnation, and purple—are inherited as recessive traits. Red is the dominant wild-type color of fruit-fly eyes. Eight crosses (A through H) are made between parents from pure-breeding lines.

Which of these eye-color mutants are X-linked recessive and which are autosomal recessive? Explain how you distinguish X-linked from autosomal heredity.

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Textbook Question

For each pedigree shown,

Identify which simple pattern of hereditary transmission (autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive) is most likely to have occurred. Give genotypes for individuals involved in transmitting the trait.

Textbook Question

For each pedigree shown,

Determine which other pattern(s) of transmission is/are possible. For each possible mode of transmission, specify the genotypes necessary for transmission to occur.

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Textbook Question

For each pedigree shown,

Identify which pattern(s) of transmission is/are impossible. Specify why transmission is impossible.

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