Ch. 4 - Extensions of Mendelian Genetics

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

Ch. 4 - Extensions of Mendelian Genetics

Problem 20a

Chapter 4, Problem 20a

In a plant, a tall variety was crossed with a dwarf variety. All F₁ plants were tall. When F₁xF₁ plants were interbred, 9/16 of the F₂ were tall and 7/16 were dwarf. Explain the inheritance of height by indicating the number of gene pairs involved and by designating which genotypes yield tall and which yield dwarf. (Use dashes where appropriate.)

Verified step by step guidance

Step 1: Recognize that the inheritance pattern suggests a dihybrid cross, as the phenotypic ratio (9:7) deviates from the classic Mendelian monohybrid ratio (3:1). This indicates that two gene pairs are involved in determining plant height.

Step 2: Assign symbols to the alleles. Let 'A' and 'B' represent dominant alleles for height, and 'a' and 'b' represent recessive alleles. The tall phenotype requires at least one dominant allele from each gene pair (A-B-), while the dwarf phenotype occurs when either gene pair is homozygous recessive (aa-- or --bb).

Step 3: Write the genotypes for the F₁ generation. Since all F₁ plants are tall, they must be heterozygous for both gene pairs (AaBb). This ensures that they carry one dominant allele for each gene pair, resulting in the tall phenotype.

Step 4: Perform a Punnett square for the F₁ x F₁ cross (AaBb x AaBb). This involves combining the alleles from each parent to determine the genotypes of the F₂ generation. The possible genotypes are: AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBB, aaBb, and aabb.

Step 5: Determine the phenotypes of the F₂ generation based on the genotypes. The tall phenotype (A-B-) includes genotypes with at least one dominant allele for each gene pair (AABB, AABb, AaBB, AaBb), which accounts for 9/16 of the offspring. The dwarf phenotype includes genotypes with homozygous recessive alleles for at least one gene pair (AAbb, Aabb, aaBB, aaBb, aabb), which accounts for 7/16 of the offspring.

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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 case, the tall variety is dominant over the dwarf variety, meaning that the presence of at least one dominant allele results in a tall phenotype. This principle helps explain the observed phenotypic ratios in the offspring.

Gene Pairs and Alleles

In this scenario, height in plants is determined by a pair of alleles, where one allele is dominant (T for tall) and the other is recessive (t for dwarf). The genotypes can be TT (homozygous tall), Tt (heterozygous tall), or tt (homozygous dwarf). The combination of these alleles during reproduction leads to the phenotypic ratios observed in the F₂ generation.

Phenotypic Ratios

Phenotypic ratios describe the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this case, the F₂ generation exhibits a 9:7 ratio of tall to dwarf plants, indicating that 9 out of 16 plants are tall (TT or Tt) and 7 out of 16 are dwarf (tt). This ratio reflects the inheritance pattern and the interaction of the alleles involved.

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Related Practice

Textbook Question

Given the inheritance pattern of coat color in rats described in Problem 17, predict the genotype and phenotype of the parents who produced the following offspring:

3/8 black: 4/8 albino: 1/8 cream

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Textbook Question

In a species of the cat family, eye color can be gray, blue, green, or brown, and each trait is true breeding. In separate crosses involving homozygous parents, the following data were obtained:

How many genes are involved? Define gene symbols and indicate which genotypes yield each phenotype.

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Textbook Question

In a species of the cat family, eye color can be gray, blue, green, or brown, and each trait is true breeding. In separate crosses involving homozygous parents, the following data were obtained:

In a cross between a gray-eyed cat and one of unknown genotype and phenotype, the F₁ generation was not observed. However, the F₂ resulted in the same F₂ ratio as in cross C. Determine the genotypes and phenotypes of the unknown P₁ and F₁ cats.

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Textbook Question

In a plant, a tall variety was crossed with a dwarf variety. All F₁ plants were tall. When F₁xF₁ plants were interbred, 9/16 of the F₂ were tall and 7/16 were dwarf. What proportion of the F₂ plants will be true breeding if self-fertilized? List these genotypes.

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Textbook Question

In a unique species of plants, flowers may be yellow, blue, red, or mauve. All colors may be true breeding. If plants with blue flowers are crossed with red-flowered plants, all F₁ plants have yellow flowers. When these produced an F₂ generation, the following ratio was observed:

9/16 yellow: 3/16 blue: 3/16 red: 1/16 mauve

Describe the inheritance of flower color by defining gene symbols and designating which genotypes give rise to each of the four phenotypes.

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Textbook Question

9/16 yellow: 3/16 blue: 3/16 red: 1/16 mauve

In still another cross using true-breeding parents, yellow-flowered plants are crossed with mauve-flowered plants. Again, all F₁ plants had yellow flowers, and the F₂ showed a 9:3:3:1 ratio, as just shown. Determine the F₁ and F₂ results of a cross between true-breeding red and true-breeding mauve-flowered plants.

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