Polygenic Traits Many traits are controlled by two or more genes and are called polygenic traits. Each gene of a polygenic trait often has two or more alleles. As a result, a single polygenic trait often has many possible genotypes and even more different phenotypes. Often those phenotypes are not clearly distinct from one another.

Height in humans is one example of a polygenic trait. Height varies from very short to very tall and everywhere in between. You can sample phenotypic variation in this trait by measuring the height of all the students in your class. You can then calculate average height for this group. Many students will be just a little taller or shorter than average. Some, however, will be very tall or very short. If you graph the number of individuals of each height, you may get a graph similar to the one in Figure 17–5. The symmetrical bell-like shape of this curve is typical of polygenic traits. A bell-shaped curve is also called a normal distribution.

The diagram illustrates the distribution of phenotypes when many genes contributed to the trait.

FIGURE 17–5 A Range of Phenotypes The graph above shows the distribution of phenotypes that would be expected for a trait if many genes contributed to the trait. The photograph shows the actual distribution of heights in a group of young men. Interpret Graphs What does the shape of the graph indicate about height in humans?

d

17.1 Assessment

Review Key Concepts

    1. Review Define the terms gene pool and allele frequency.

    2. Explain In genetic terms, what indicates that evolution is occurring in a population?

    3. Predict Suppose a dominant allele causes a plant disease that usually kills the plant before it can reproduce. Over time, what would probably happen to the frequency of that allele in the population?

    1. Review List three sources of genetic variation.

    2. Explain How does genetic recombination result in genetic variation?

    3. Relate Cause and Effect Why does sexual reproduction provide more opportunities for genetic variation than asexual reproduction?

    1. Review What is a single-gene trait? What is a polygenic trait?

    2. Explain How does the range of phenotypes for single-gene traits differ from the range for polygenic traits?

    3. Infer A black guinea pig and a white guinea pig mate and have offspring. All the offspring are black. Is the trait of coat color probably a single-gene trait or a polygenic trait? Explain.

WRITE ABOUT SCIENCE

Explanation

  1. Explain how mutations are important in the process of biological evolution. (Hint: How does mutation affect genetic variation?)

End ofPage 486

Table of Contents

Miller & Levine Biology
About the Authors
Changing the World of Biology Education
Contents
Diversity of Life: A Visual Guide DOL•1–DOL•64
Labs and Activities
Features
UNIT 1 The Nature of Life
1 The Science of Biology
2 The Chemistry of Life
Unit Project
UNIT 2 Ecology
3 The Biosphere
4 Ecosystems and Communities
5 Populations
6 Humans in the Biosphere
Unit Project
UNIT 3 Cells
7 Cell Structure and Function
8 Photosynthesis
9 Cellular Respiration and Fermentation
10 Cell Growth and Division
Unit Project
UNIT 4 Genetics
11 Introduction to Genetics
12 DNA
13 RNA and Protein Synthesis
14 Human Heredity
15 Genetic Engineering
Unit Project
UNIT 5 Evolution
16 Darwin's Theory of Evolution
17 Evolution of Populations
18 Classification
19 History of Life
Unit Project
UNIT 6 From Microorganisms to Plants
20 Viruses and Prokaryotes
21 Protists and Fungi
22 Introduction to Plants
23 Plant Structure and Function
24 Plant Reproduction and Response
Unit Project
UNIT 7 Animals
25 Introduction to Animals
26 Animal Evolution and Diversity
27 Animal Systems I
28 Animal Systems II
29 Animal Behavior
Unit Project
UNIT 8 The Human Body
30 Digestive and Excretory Systems
31 Nervous System
32 Skeletal, Muscular, and Integumentary Systems
33 Circulatory and Respiratory Systems
34 Endocrine and Reproductive Systems
35 Immune System and Disease
Unit Project
A Visual Guide to The Diversity of Life
HOW TO USE THIS GUIDE
THE TREE OF LIFE
Bacteria
Protists
Fungi
Plants
Animals
Appendices
Appendix A Science Skills
Appendix B Lab Skills
Appendix C Technology & Design
Appendix D Math Skills
Appendix E Periodic Table
Glossary
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q, R
S
T
U
V
W
X, Y, Z
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Credits
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