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Ch. 25 - Quantitative Genetics and Multifactorial Traits
All textbooksKlug 12th EditionCh. 25 - Quantitative Genetics and Multifactorial TraitsProblem 21
Chapter 24, Problem 21

A 3-inch plant was crossed with a 15-inch plant, and all F₁ plants were 9 inches. The F₂ plants exhibited a 'normal distribution,' with heights of 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 inches.

What will be the outcome if the F₁ plants are testcrossed with plants that are homozygous for all nonadditive alleles?

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1
span>Identify the genetic basis of the trait: The problem suggests a quantitative trait, as indicated by the normal distribution of plant heights in the F₂ generation. This implies polygenic inheritance, where multiple genes contribute to the phenotype.</span
span>Understand the F₁ generation: The F₁ plants are all 9 inches tall, which is the average of the parental heights (3 inches and 15 inches). This suggests that the F₁ plants are heterozygous for the alleles contributing to plant height.</span
span>Define the testcross: A testcross involves crossing the F₁ plants with a plant that is homozygous for all nonadditive alleles. In this context, nonadditive alleles are those that do not contribute to the increase in plant height.</span
span>Predict the outcome of the testcross: Since the testcross involves crossing heterozygous F₁ plants with homozygous recessive plants, the offspring will exhibit a range of phenotypes. The distribution of these phenotypes will depend on the number of contributing alleles inherited from the F₁ plants.</span
span>Consider the phenotypic distribution: The offspring of the testcross will likely show a range of heights, but the average height will be lower than that of the F₁ generation, as the homozygous recessive parent contributes no alleles that increase height.</span

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Key Concepts

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

Quantitative Traits

Quantitative traits are characteristics that are influenced by multiple genes and exhibit a continuous range of phenotypes, such as height. In this case, the plant heights show a normal distribution, indicating that multiple alleles contribute to the trait, resulting in a variety of heights in the F₂ generation.
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Testcross

A testcross is a genetic cross between an individual with an unknown genotype and a homozygous recessive individual. This method helps determine the genotype of the unknown individual by analyzing the phenotypes of the offspring. In this scenario, crossing the F₁ plants with homozygous nonadditive alleles will reveal the genetic contributions of the F₁ plants.

Additive vs. Nonadditive Alleles

Additive alleles contribute to the phenotype in a cumulative manner, while nonadditive alleles do not follow this pattern and can mask the effects of other alleles. In the context of the question, crossing F₁ plants with homozygous nonadditive alleles will help clarify how these alleles influence the resulting plant heights and the overall distribution of traits in the offspring.
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Related Practice
Textbook Question

Many traits of economic or medical significance are determined by quantitative trait loci (QTLs) in which many genes, usually scattered throughout the genome, contribute to expression.

What general procedures are used to identify such loci?

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Textbook Question

Many traits of economic or medical significance are determined by quantitative trait loci (QTLs) in which many genes, usually scattered throughout the genome, contribute to expression.

What is meant by the term cosegregate in the context of QTL mapping? Why are markers such as RFLPs, SNPs, and microsatellites often used in QTL mapping?

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Textbook Question

A 3-inch plant was crossed with a 15-inch plant, and all F₁ plants were 9 inches. The F₂ plants exhibited a 'normal distribution,' with heights of 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 inches.

What ratio will constitute the 'normal distribution' in the F₂?

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Textbook Question

In a cross between a strain of large guinea pigs and a strain of small guinea pigs, the F₁ are phenotypically uniform, with an average size about intermediate between that of the two parental strains. Among 1014 F₂ individuals, 3 are about the same size as the small parental strain and 5 are about the same size as the large parental strain. How many gene pairs are involved in the inheritance of size in these strains of guinea pigs?

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Textbook Question

Type A1B brachydactyly (short middle phalanges) is a genetically determined trait that maps to the short arm of chromosome 5 in humans. If you classify individuals as either having or not having brachydactyly, the trait appears to follow a single-locus, incompletely dominant pattern of inheritance. However, if one examines the fingers and toes of affected individuals, one sees a range of expression from extremely short to only slightly short. What might cause such variation in the expression of brachydactyly?

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Textbook Question

In a series of crosses between two true-breeding strains of peaches, the F₁ generation was uniform, producing 30-g peaches. The F₂ fruit mass ranges from 38 to 22 g at intervals of 2 g. Using these data, determine the number of polygenic loci involved in the inheritance of peach mass.

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