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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 21b
Chapter 25, Problem 21b

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
Step 1: Understand the genetic basis of the problem. The F₁ plants are intermediate in height (9 inches), suggesting that the trait is controlled by multiple genes with additive effects. The normal distribution in the F₂ generation indicates polygenic inheritance, where multiple loci contribute to the phenotype.
Step 2: Define the testcross scenario. A testcross involves crossing the heterozygous F₁ plants with homozygous recessive individuals (plants homozygous for all nonadditive alleles). This allows us to analyze the segregation of alleles and their contribution to the phenotype.
Step 3: Determine the genetic makeup of the F₁ plants. Since the F₁ plants are intermediate in height, they are heterozygous at all loci contributing to plant height. Each locus has additive alleles (contributing to height) and nonadditive alleles (not contributing to height).
Step 4: Predict the segregation pattern. In the testcross, each heterozygous locus in the F₁ plants will segregate into two types of gametes: one carrying the additive allele and the other carrying the nonadditive allele. The homozygous recessive testcross partner will only contribute nonadditive alleles.
Step 5: Analyze the phenotypic outcomes. The offspring of the testcross will exhibit a range of heights based on the number of additive alleles inherited from the F₁ parent. Since the testcross partner contributes no additive alleles, the phenotypic distribution will reflect the segregation of additive alleles from the F₁ parent.

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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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