Download Chapter 26: Quantitative Genetics

Chapter 26: Quantitative Genetics
Student Learning Objectives
Upon completion of this chapter you should be able to:
1.
2.
3.
4.
5.
Understand the concept of a quantitative trait.
Use statistical tools to analyze variations within groups of individuals.
Understand the concepts of polygenic inheritance and quantitative trait loci.
Understand the principles of heritability.
Distinguish between the genetic and environmental factors that affect quantitative
traits.
6. Understand the effects of selective breeding and its relationship to heritability.
26.1 Overview of Quantitative Traits
Overview
This chapter examines complex traits, which are determined by several genes and are
influenced by environmental factors. Because they can be described numerically, many of the
complex traits are also termed quantitative traits. As shown in Table 26.1, quantitative traits can
be categorized as anatomical, physiological, or behavioral. Moreover, many human diseases
exhibit characteristics and inheritance patterns analogous to those of quantitative traits. Three of
the leading causes of deaths worldwide—heart disease, cancer, and diabetes—are considered
quantitative traits.
Most of the traits we have discussed so far in the textbook fall into discrete categories.
For example, pea plants might have wrinkled or smooth seeds, and fruit flies may have red or
white eyes. These types of traits are called discontinuous traits. In contrast, quantitative traits
show a continuum of phenotypic variation within a group of individuals. Indeed, they often show
a normal distribution (Figure 26.1).
Outline of Key Terms
Quantitative genetics
Quantitative traits
Complex traits
Continuous traits
Meristic traits
Threshold traits
Discontinuous traits
Frequency distributuion
Normal distribution
Focal Points


Types of quantitative traits (Table 26.1)
Normal distribution of a quantitative trait (Figure 26.1)
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Exercises and Problems
For questions 1 to 5, match the term with its appropriate definition.
_____ 1. Frequency distribution
_____ 2. Quantitative traits
_____ 3. Biometric field
_____ 4. Normal distribution curve
_____ 5. Discontinuous traits
a.
b.
c.
d.
e.
Traits that can be placed into specific phenotypic categories.
A distribution is divided into convenient, discrete units for analysis.
A distribution that varies in a symmetric manner around an average value.
Traits that demonstrate a continuum of phenotypic variation.
The study of the variation of quantitative traits within groups of individuals.
For questions 6 to 8, complete the sentences with the correct word or phrase.
6. Quantitative traits, such as height and weight, that do not fall into discrete categories are
termed ________ traits
7. Quantitative traits, such as bristle number in Drosophila, that can be counted and expressed
into whole numbers are called ________ traits.
8. Certain diseases, like diabetes, are inherited quantitatively due to the contribution of many
genes, but the traits themselves are expressed qualitatively. These types of diseases are called
________ traits.
26.2 Statistical Methods for Evaluating Quantitative Traits
Overview
This section introduces some important statistical methods that are used in the study of
quantitative genetics. Although most genetics students have a background in statistics, it is
important that you review the descriptions of mean, variance, standard deviation, and correlation
coefficient before proceeding into the next sections. In addition, be sure that you understand the
variables in the formulas and can apply them as needed.
Outline of Key Terms
Biometric field
Mean
Variance
Standard deviation
Correlation coefficient (r)
Covariance
Regression analysis
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Focal Points



Mathematical explanations of mean, variance, and standard deviation (pages 656-657)
The relationship between standard deviation and normal distribution (Figure 26.2)
Calculation and analysis of correlation coefficient (pages 657-659)
Exercises and Problems
For questions 1 to 6, choose the correct term that matches the statistical statement provided.
_____ 1.
How two factors vary in relation to one another.
_____ 2.
The sum of all of the values in the group divided by the number of individuals.
_____ 3.
A measure of how much one variable will change in response to a second variable.
_____ 4.
The sum of the squared deviations from the mean divided by the degrees of freedom.
_____ 5.
The degree of variation between two variables in a group.
_____ 6.
The square root of the variance.
a.
b.
c.
d.
e.
f.
variance
standard deviation
covariance
correlation coefficient
regression analysis
mean
26.3 Polygenic Inheritance
Overview
Many quantitative traits can be explained by the fact that more than one gene is
contributing to the phenotype. This is called polygenic inheritance and is the focus of this section
of the chapter. First, notice how phenotypes based on polygenic mechanisms vary from
monogenic mechanisms (Figure 26.4). The section then focuses on quantitative trait loci (QTLs),
which are chromosomal regions harboring genes that affect the outcome of quantitative traits. As
you proceed through this section, examine how geneticists map these quantitative trait loci.
Outline of Key Terms
Polygenic inheritance
Quantitative trait loci (QTL)
QTL mapping
Focal Points


Continuous variation and polygenic inheritance (Figures 26.3 and 26.4)
QTL mapping via molecular markers (Figure 26.5)
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Exercises and Problems
For questions 1 to 4, complete the statements below with the correct word or phrase.
1. An association between genetically determined phenotypes and molecular markers is called
_________.
2. When assessing a QTL, a ________ is often performed to produce F2 generation individuals
that differ with regard to their combinations of the parental chromosomes.
3. A trait that is governed by two or more genes is called __________.
4. The pigmentation of the hulls in wheat is an example of __________.
5. You are experimenting with pigmentation in the hull of bread wheat, Triticum aestivum. You
cross a plant with genotype R1r1r2r2 to one that is R1r1R2R2. What is the genotypic and
phenotypic ratio in the offspring?
26.4 Heritability
Overview
The term heritability refers to the amount of phenotypic variation within a group of
individuals that is due to genetic variation. The role of the environment in determining the
phenotype of the organism has been introduced several times in the text. This fourth section of
the chapter discusses how scientists analyze the genetic and environmental components
contributing to the observed phenotypes of quantitative traits. As with the previous sections,
familiarize yourself with the terms and formulas associated with the study of heritability. For
example, note the differences between the phenomena of genotype-environment interaction and
genotype-environment association (pages 665-666), and narrow-sense and broad-sense
heritability (pages 666-667).
Outline of Key Terms
Heritability
Inbreeding
Monomorphic
Genotype-environment interaction
Genotype-environment association
Broad-sense heritability
Narrow-sense heritability
Focal Points



Formulas associated with heritability studies (pages 665-667)
A schematic example of genotype-environment interaction (Figure 26.6)
Heritability of human fingerprint patterns (pages 667-669)
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Exercises and Problems
Complete the paragraphs below with the correct word or phrase.
The term heritability refers to the amount of (1) ________ variation that is due to
(2) ________ variation. If all of the variation is due to environmental effects, the heritability
would have a value of (3) ________. When the environmental effects on phenotype differ
according to genotype, this phenomenon is called a (4) ________, and is denoted by the symbol
(5) ________. When certain genotypes are preferentially found in particular environments, this
phenomenon is called (6) ________, and is denoted by the symbol (7) ________.
Variance due to genetics is denoted by the symbol VG. Geneticists usually subdivide VG
into three different genetic categories: VA is the variance due to the (8) ________ effects of
alleles; VD is the variance due to the effects of alleles that follow a (9) ________ pattern of
inheritance; and VI is the variance due to the effects of alleles that interact in an (10) ________
manner. The heritability of a trait that takes into account different types of genetic variation is
called (11) ________. The heritability of a trait due to the additive effects of alleles is called
(12) ________.
26.5 Selective Breeding
Overview
Selective breeding, or artificial selection, refers to programs and procedures designed to
modify phenotypes in commercially important plants and animals. The dramatic effect selective
breeding has on quantitative traits in various organisms is shown in Figures 26.9 and 26.10. The
section also examines how selective breeding in a genetically diverse population can modify a
trait in a desired direction until a selection limit is reached (Refer to Figure 26.11). The section
ends with a discussion of how selective breeding provides a way to estimate heritability.
Outline of Key Terms
Selective breeding
Artificial selection
Selection limit
Realized heritability
Focal Points



Some common breeds of dogs obtained by selective breeding (Figure 26.9)
Crop plants developed by selective breeding of the wild mustard plant (Figure 26.10)
Selective breeding for a quantitative trait (Figure 26.11)
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Exercises and Problems
Complete the sentences below with the correct word or phrase.
1. Selective breeding is also termed _________.
2. After several generations of selective breeding, a ________ may be reached; at this point
selective breeding is no longer effective.
3. Using selective breeding experiments, the response to selection is a common way to estimate
the ________ in a starting population.
4. The heritability measured during selective breeding experiments is referred to as ________.
Chapter Quiz
1. Which of the following provides a measure of the amount of variation within the group?
a. mean
b. variance
c. standard deviation
d. regression
e. correlation coefficient.
2. Which term describes the variation of a trait in a symmetric manner around an average value?
a. biometric
b. frequency distribution
c. mean
d. normal distribution
3. The square root of the variance is called the ________.
a. standard deviation
b. mean
c. covariance
d. correlation coefficient
4. This measures the strength of the association between two variables.
a. mean
b. variance
c. standard deviation
d. regression
e. correlation coefficient
5. A plant with white flowers is crossed to one with red flowers. The F1 is then self-crossed. In
the F2 generation, only 2/126 of the offspring had white flowers. How many genes are involved?
a. 1
b. 2
c. 3
d. 4
e. 5
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6. Mendel’s work on pea plants described which of the following?
a. inbreeding
b. continuous traits
c. heritability
d. discontinuous traits
7. The selection of traits that provide economic importance is an example of ________.
a. natural selection
b. inbreeding
c. artificial selection
d. heritability
8. All of the following are quantitative traits in humans EXCEPT
a. weight.
b. intelligence.
c. diabetes.
d. eye color.
e. heart disease.
9. Which of the following term is defined as “a measure of the variation in a phenotype due to
genetic factors”?
a. narrow-sense heritability
b. broad-sense heritability
c. realized heritabilty
d. variance
e. biometry
10. A trait that is the same in 99% of the members of a species is said to be ________.
a. quantitative
b. discontinuous
c. polymorphic
d. monomorphic
Answer Key for Study Guide Questions
This answer key provides the answers to the exercises and chapter quiz for this chapter. Answers
in parentheses ( ) represent possible alternate answers to a problem, while answers marked with
an asterisk (*) indicate that the response to the question may vary.
26.1
1. b
2. d
3. e
4. c
5. a
6. quantitative
7. meristic
8. threshold
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26.2
1. d
2. f
3. e
4. a
5. c
6. b
26.3
1. QTL mapping
2. backcross
3. polygenic
4. polygenic inheritance
5. Genotypic ratio is: 1/4 R1R1R2r2 : 2/4 R1r1R2r2 : 1/4 r1r1R2r2
Phenotypic ratio: 1/4 medium red : 2/4 intermediate red : 1/4 light red
26.4
1. phenotypic
2. genetic
3. 0 (zero)
4. genotype-environment interaction
5. VGXE
6. genotype-environment association
7. VG↔E
8. additive
9. dominant/recessive
10. epistatic
11. broad-sense heritability
12. narrow-sense heritability
26.5
1. artificial selection
2. selection limit
3. narrow-sense heritability
4. realized heritability
Quiz
1. c
2. d
3. a
4. e
5. c
6. d
7. c
8. d
9. b
10. d
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