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Ch. 25 - Quantitative Genetics and Multifactorial Traits
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 25 - Quantitative Genetics and Multifactorial TraitsProblem 6a
Chapter 25, Problem 6a

An inbred strain of plants has a mean height of 24 cm. A second strain of the same species from a different geographic region also has a mean height of 24 cm. When plants from the two strains are crossed together, the F₁ plants are the same height as the parent plants. However, the F₂ generation shows a wide range of heights; the majority are like the P₁ and F₁ plants, but approximately 4 of 1000 are only 12 cm high and about 4 of 1000 are 36 cm high.
What mode of inheritance is occurring here?

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1
Step 1: Identify the parental (P) generation phenotype and the F₁ generation phenotype. Both parental strains have a mean height of 24 cm, and the F₁ plants also have the same height of 24 cm, indicating no dominance or recessiveness in the F₁ generation phenotype.
Step 2: Observe the F₂ generation phenotypic distribution. Most F₂ plants resemble the parents and F₁ (24 cm), but a small fraction (about 4 in 1000) show extreme phenotypes of 12 cm and 36 cm, which are shorter and taller than the parents, respectively.
Step 3: Consider the pattern of inheritance that explains the presence of intermediate F₁ phenotype and extreme phenotypes in the F₂. The appearance of phenotypes beyond the parental range in the F₂ suggests incomplete dominance or additive gene action rather than simple dominance.
Step 4: Recognize that the extreme phenotypes (12 cm and 36 cm) likely represent homozygous genotypes for alleles that decrease or increase height, while the intermediate 24 cm phenotype represents heterozygotes with additive effects of both alleles.
Step 5: Conclude that the mode of inheritance is incomplete dominance or additive gene action, where heterozygotes have an intermediate phenotype and homozygotes have phenotypes at either extreme.

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Key Concepts

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

Quantitative Inheritance

Quantitative inheritance involves traits controlled by multiple genes, each contributing a small effect, resulting in continuous variation in the phenotype. Unlike simple Mendelian traits, these traits show a range of phenotypes, such as height, rather than discrete categories.
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Organelle Inheritance

Polygenic Inheritance and Additive Gene Effects

Polygenic inheritance occurs when several genes influence a single trait, with their effects adding up to determine the phenotype. This explains the continuous distribution of traits and the appearance of extreme phenotypes in the F₂ generation due to different gene combinations.
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Maternal Effect

Segregation and Recombination in F₂ Generation

In the F₂ generation, alleles segregate and recombine, producing a wider range of phenotypes than in the parents or F₁. This leads to some individuals showing extreme traits (e.g., 12 cm or 36 cm height) due to inheriting more or fewer additive alleles influencing the trait.
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Recombination after Single Strand Breaks
Related Practice
Textbook Question

A dark-red strain and a white strain of wheat are crossed and produce an intermediate, medium-red F₁. When the F₁ plants are interbred, an F₂ generation is produced in a ratio of 1 dark-red: 4 medium-dark-red: 6 medium-red: 4 light-red: 1 white. Further crosses reveal that the dark-red and white F₂ plants are true breeding

Predict the outcome of the F1 and F2 generations in a cross between a true-breeding medium-red plant and a white plant.

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Textbook Question

Height in humans depends on the additive action of genes. Assume that this trait is controlled by the four loci R, S, T, and U and that environmental effects are negligible. Instead of additive versus nonadditive alleles, assume that additive and partially additive alleles exist. Additive alleles contribute two units, and partially additive alleles contribute one unit to height.

Can two individuals of moderate height produce offspring that are much taller or shorter than either parent? If so, how?

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Textbook Question

Height in humans depends on the additive action of genes. Assume that this trait is controlled by the four loci R, S, T, and U and that environmental effects are negligible. Instead of additive versus nonadditive alleles, assume that additive and partially additive alleles exist. Additive alleles contribute two units, and partially additive alleles contribute one unit to height.

If an individual with the minimum height specified by these genes marries an individual of intermediate or moderate height, will any of their children be taller than the tall parent? Why or why not?

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Textbook Question

An inbred strain of plants has a mean height of 24 cm. A second strain of the same species from a different geographic region also has a mean height of 24 cm. When plants from the two strains are crossed together, the F₁ plants are the same height as the parent plants. However, the F₂ generation shows a wide range of heights; the majority are like the P₁ and F₁ plants, but approximately 4 of 1000 are only 12 cm high and about 4 of 1000 are 36 cm high.

How many gene pairs are involved?

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Textbook Question

An inbred strain of plants has a mean height of 24 cm. A second strain of the same species from a different geographic region also has a mean height of 24 cm. When plants from the two strains are crossed together, the F₁ plants are the same height as the parent plants. However, the F₂ generation shows a wide range of heights; the majority are like the P₁ and F₁ plants, but approximately 4 of 1000 are only 12 cm high and about 4 of 1000 are 36 cm high.

How much does each gene contribute to plant height?

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Textbook Question

An inbred strain of plants has a mean height of 24 cm. A second strain of the same species from a different geographic region also has a mean height of 24 cm. When plants from the two strains are crossed together, the F₁ plants are the same height as the parent plants. However, the F₂ generation shows a wide range of heights; the majority are like the P₁ and F₁ plants, but approximately 4 of 1000 are only 12 cm high and about 4 of 1000 are 36 cm high.

Indicate one possible set of genotypes for the original P₁ parents and the F₁ plants that could account for these results.

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