Download Chapter 8: Variation in Chromosome Structure and Number

Chapter 8: Variation in Chromosome Structure and Number
Student Learning Objectives
Upon completion of this chapter you should be able to:
1. Know the principles and terminology associated with variations in chromosome
structure.
2. Know the principles and terminology associated with variations in chromosome
number.
3. Recognize the effects of chromosomal changes on the phenotype of the organism
and/or its offspring.
4. Understand the processes of mitotic nondisjunction, meiotic nondisjunction, and
chromosome loss.
5. Understand the experimental techniques that can be used to produce changes in
chromosome number.
8.1 Microscopic Examination of Eukaryotic Chromosomes
Overview
In earlier chapters we dealt with the topic of allelic variation. In this chapter we focus on
chromosome variation, which comes in two main types: variations in structure and variations in
number. These larger alterations may affect the expression of many genes simultaneously,
thereby influencing phenotypes. The first section introduces us to scientists called cytogeneticists,
who study the structure and function of chromosomes.
To analyze the chromosomal composition of a species, the chromosomes in actively
dividing cells are examined microscopically. Cytogeneticists have various ways to classify and
identify chromosomes. The three most commonly used features are: 1) size; 2) centromere
location; and 3) banding patterns (Refer to Figure 8.1).
Outline of Key Terms
Karyotype
G Bands
Genetic variation
Allelic variation
Cytogeneticist
Chromosome types
Metacentric
Submetacentric
Acrocentric
Telocentric
Metacentric
Focal Points

Features of normal chromosomes (Figure 8.1)
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Exercises and Problems
For questions 1 to 4, match the chromosome type to its correct description.
a. submetacentric
b. telocentric
c. metacentric
d. acrocentric
_____ 1. The centromere is at one end
_____ 2. The centromere is closer to the end than to the middle.
_____ 3. The centromere is near the middle.
_____ 4. The centromere is slightly off center.
8.2 Changes in Chromosome Structure: An Overview
Overview
Variations in chromosome structure can occur in many ways. In some cases, the total
amount of genetic material within a single chromosome can be increased or decreased
significantly. In other cases, the genetic material in one or more chromosomes may be rearranged
without affecting the total amount of material. This section provides an introduction to the four
main types of chromosomal structural changes: deletion, duplication, inversion, and translocation.
Outline of Key Terms
Translocation
Simple translocation
Reciprocal translocation
Deletion
Deficiency
Duplication
Inversion
Focal Points

Types of changes in chromosome structure (Figure 8.2)
Exercises and Problems
For questions 1 to 4, complete the sentence with the most appropriate term(s):
1. A(n) ______ involves a change in direction of the DNA material along a single chromosome.
2. A(n) ______ occurs when a segment of a chromosome is missing.
3. In a(n) ______ two different types of chromosomes exchange pieces.
4. In a(n) ______ a single piece of chromosome is attached to another chromosome.
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8.3 Deletions and Duplications
Overview
In this and the next section we will take a closer look at changes in the structure of a
single chromosome (or sometimes two), and how this may influence the expression of genes and
the phenotype of the organism. Typically, student difficulties with this chapter rest primarily in
the terminology associated with each form of variation. One of the best mechanisms of studying
this material is to draw your own examples of each form of variation, noticing the
loss/gain/change of genetic material on the chromosomes.
The first two forms of chromosome variation, deletions and duplications, involve changes
in the total amount of genetic material within a chromosome. In general, deletions are more
harmful than duplications. Some deletions are associated with human genetic disorders such as
cri-du-chat syndrome (Figure 8.4). Note that deletions and duplications may occur simultaneously
due to a misaligned crossing over event (Figure 8.5). While the duplication may have an effect on
the phenotype of the organism, it can also have important evolutionary consequences. Gene
duplications may form gene families, such as the globin gene family in humans (Figure 8.7).
Gene families provide a species with a set of closely related proteins that have slight variations in
function. Other consequences of duplications include copy number variations. These refer to a
type of structural variation in which a segment of DNA that is 1 Kilobasepair or more in length
exhibits copy number differences in members of the same species (Figure 8.8).
Outline of Key Terms
Duplication
Gene duplication
Gene family
Homologous genes
Paralogs
Repetitive sequences
Copy number variations
Segmental duplication
Deletion
Terminal deletion
Interstitial deletion
Nonallelic homologous recombination
Focal Points



Production of terminal and interstitial deletions (Figure 8.3)
Nonallelic homologous recombination (Figure 8.5)
Gene duplication and the evolution of paralogs (Figure 8.6)
Exercises and Problems
For questions 1 to 4, complete the sentence with the most appropriate term(s):
1. A deletion that occurs in the middle of a chromosome is termed _________.
2. CNV, or _________, are fairly common structural variations in members of the same species.
3. Homologous genes in a single species are called _________ and constitute a gene family.
4. The globin gene family includes genes for the proteins _________ and _________.
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For questions 5 to 11, use the following key:
a. statement applies to deletions only
b. statement applies to duplications only
c. statement applies to both deletions and duplications
d. statement applies to neither deletions nor duplications
_____ 5.
Responsible for the formation of gene families
_____ 6.
The cause of Angelman syndrome and Prader-Willi syndrome
_____ 7.
Includes pericentric and paracentric types
_____ 8.
The cause of cri-du-chat syndrome
_____ 9.
The cause of Charcot-Marie-Tooth disease
_____ 10. May arise from nonallelic homologous recombination
_____ 11. Alters the total amount of genetic material
8.4 Inversions and Translocations
Overview
This section examines inversions and translocations, which are chromosomal
rearrangements. Both tend to be more difficult to visualize than the duplication/deletions
previously presented. However, the text and figures do a good job at elucidating these complex
chromosomal changes.
Inversions can be divided into two types, pericentric and paracentric, based on the
presence or absence of the centromere in the inverted segment (Figure 8.9). Translocations may
be caused by different mechanisms, including chromosome breakage and subsequent repair, and
nonhomologous crossing over (Figure 8.11). A particular type of translocation is termed
Robertsonian. It involves an exchange between a short arm and a long arm of two acrocentric
chromosomes (Figure 8.12). Familial Down syndrome is due to such an event (See Figure 8.13).
Two other key figures you need to focus on are 8.10 and 8.14. Notice how the chromosomes align
during meiosis. Pay special attention to the effects of these forms of variation on the production
of gametes. Both of these may result in reduced fertility for an organism due to the loss of
gametes because of chromosomal abnormalities.
Outline of Key Terms
Telomeres
Translocation
Simple translocation
Unbalanced translocation
Reciprocal translocation
Balanced translocation
Robertsonian translocation
Translocation cross
Semisterility
Inversion
Pericentric inversion
Paracentric inversion
Position effect
Inversion heterozygote
Inversion loop
Dicentric chromosome
Dicentric bridge
Acentric fragment
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Focal Points





Types of inversions (Figure 8.9)
Inversion loops (Figure 8.10)
Causes of translocations (Figure 8.11)
Transmission of familial Down syndrome (Figure 8.13)
Translocation crosses (Figure 8.14)
Exercises and Problems
For questions 1 to 5, complete the sentence with the most appropriate term(s):
1. A piece of chromosome without a centromere is termed an __________.
2. _________ translocations are reciprocal translocations that do not alter the total amount of
genetic material.
3. In a _________ inversion, the centromere lies outside the inverted region of a chromosome.
4. Familial Down syndrome is caused by a ___________ between chromosomes 14 and 21.
5. A _________ refers to a change in phenotype that occurs when the location of a genes changes
from one chromosomal site to another.
For questions 6 to 10, assume that two non-homologous chromosomes have the following
combination of genes:
-----------o-------------A B C D E F G
------------o--------------Q R S
T U V
For each combination indicated below, state the form of structural variation (duplication,
deletion, inversion, and translocation) that would have to occur to produce the sequence shown.
The ----o---- indicates the location of the centromere. Note that some answers may require more
than one process.
_____ 6. ------------- o ---------------A B C
D G F E
_____ 7. ------------- o ---------------A B C
D U V
------------- o ---------------Q R S
T E F G
_____ 8. ------------- o ---------------A B C
D F G
_____ 9. ------------- o ---------------A B C
D E
------------- o ---------------Q R S
T U VG F
_____ 10. --------------- o ---------------A B A BC D E FG
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For questions 11 to 16, use the following key:
a. statement applies to inversions only
b. statement applies to translocations only
c. statement applies to both inversions and translocations
d. statement applies to neither inversions nor translocations
_____ 11. In some cases, the chromosomes form a cross pattern during meiotic pairing
_____ 12. Can result in the formation of a chromosome with a dicentric bridge
_____ 13. Usually caused by modifications to the telomere of the chromosome
_____ 14. Causes homologous chromosomes to form a loop during meiotic pairing
_____ 15. Caused by a crossover between homologous chromosomes
_____ 16. Involves a chromosomal rearrangement
8.5 Changes in Chromosome Number: An Overview
Overview
As we saw in the three previous sections, chromosome structure can be altered in a
variety of ways. Likewise, the total number of chromosomes can vary. This section introduces
these numerical changes, which can be divided into two main groups: 1) Euploidy, or variations
in the number of sets of chromosomes; and 2) aneuploidy, which is variation in the number of
particular chromosomes within a set. Refer to Figure 8.15 for a good overview.
Outline of Key Terms
Euploidy
Polyploid
Triploid
Tetraploid
Aneuploidy
Trisomic
Monosomic
Focal Points

Types of variations in chromosome number (Figure 8.15)
Exercises and Problems
In the garden pea, Pisum sativum, 2n = 14. How many chromosomes would each of the following
have?
_____ 1. A diploid cell
_____ 2. A triploid cell
_____ 3. A tetraploid cell
_____ 4. A trisomic cell
_____ 5. A monosomic cell
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8.6 Variation in the Number of Chromosomes Within A Set: Aneuploidy
Overview
Changes in chromosome number also have an effect on gene expression and the
phenotype of the organism. Once again, the most common problems with the next two sections
occurs in the terminology. However, the terminology for variations in chromosome number is
actually easy to understand if you take the time to examine the term for patterns. Before
proceeding into this section, study the material in Figure 8.15 carefully.
Aneuploidy involves an alteration in a number that is not an exact multiple of a
chromosome set. These chromosome changes may be represented algebraically. For a diploid
organism (2n), examples of aneuploidy include trisomy (2n + 1) and monosomy (2n –1).
One of the more important concepts of this section is how these conditions relate to gene
expression. Previously in the text the concept of proteins being responsible for phenotypes was
introduced. We have observed how many organisms are very careful to regulate levels of gene
expression when the chromosome number is not the same in the sexes. Therefore, any change in
chromosome number should also have an effect on the phenotype. This is basically due to an
imbalance in gene products (Refer to Figure 8.16). The section also takes a look at the various
aneuploid conditions in humans (Table 18.1). It focuses on Down syndrome, which is the most
famous of these conditions. Note how the incidence of Down syndrome births increases with
maternal age (Figure 8.17).
Outline of Key Terms
Nondisjunction
Focal Points



Imbalance of gene products (Figure 8.16)
Aneuploid conditions in humans (pages 175-176, Table 8.1)
The incidence of Down syndrome births according to the age of the mother (Figure 8.17)
Exercises and Problems
For questions 1 to 6, complete the sentence with the most appropriate term(s):
1. In humans, a single set of chromosomes contains about ________ to ________ different genes.
2. __________ refers to the failure of chromosomes to separate normally during cell division.
3. An individual with triple X syndrome has a total of _______ chromosomes in each cell.
4. Klinefelter syndrome only affects _______.
5. _________ syndrome may also be termed monosomy X.
6. Incidence of Down syndrome increases with the mom’s age, because as the woman ages, her
primary oocytes have been arrested in _________ of meiosis for a progressively longer period
of time.
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For each of the following conditions in humans, indicate whether the condition is due to a change
in the number of autosomal or sex chromosomes, and the genotype of the individual.
7. Jacobs Syndrome
8. Klinefelter Syndrome
9. Turner Syndrome
10. Down Syndrome
11. Patau Syndrome
12. Edwards Syndrome
8.7 Variation in the Number of Sets of Chromosomes
Overview
Euploidy involves changes in the number of sets of chromosomes. This phenomenon is
rare in animals. Indeed, it is not tolerated at all in mammals. However, there are examples of
naturally occurring variations in euploidy. These include the haplodiploidy system of bees and
ants, where males are haploid and females are diploid. Note that some tissues in an animal may
exhibit polyploidy. An example is the polytene chromosomes found in the salivary cells of
Drosophila (Refer to Figure 8.19).
Polyploidy is common in plants, and has found many advantages in agriculture. Many of
the foods and grains we eat are produced from polyploidy plants. These tend to be larger and
more robust than their diploid counterparts (See Figure 8.20b). Also note that polyploid plants
having an odd number of chromosome sets usually cannot reproduce sexually. This is due to the
unequal separation of homologous chromosomes during anaphase I of meiosis (Figure 8.21).
Outline of Key Terms
Haplodiploid
Endopolyploidy
Polytene chromosomes
Chromocenter
Focal Points



Polytene chromosomes in Drosophila (Figure 8.19)
Examples of polyploid plants (Figure 8.20)
Schematic representation of anaphase I of meiosis in a triploid organism containing three
sets of four chromosomes (Figure 8.21)
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Exercises and Problems
Complete the following sentences with the most appropriate word or phrase:
Variations in euploidy occur naturally in a few animal species. These include bees, which
are an example of a haplodiploid species. Male bees, also called drones, are (1) ________. They
are produced from (2) _______ eggs. In contrast, female bees are produced from (3) ________
eggs and are thus (4) ________.
Variations in euploidy can also occur in certain tissues within an animal. The occurrence
of polyploid tissues or cells in an organism that is otherwise diploid is known as (5) ________.
For example, human (6) ________ cells contain nuclei that can be triploid, tetraploid, and even
octoploid. An unusual example occurs in (7) ________ and other insects. In the salivary glands of
these animals, several rounds of repeated replication without cellular division produce a bundle of
sister chromatids that lie side by side. This bundle is called a (8) ________. The central point
where the chromosomes aggregate is called the (9) ________.
For each of the following, indicate the algebraic formula for the chromosome number in the
organism.
________ 10. Tetraploid
________ 11. Octaploid
________ 12. Trisomic
________ 13. Hexaploid
________ 14. Monosomic
________ 15. Haploid
8.8 Mechanisms that Produce Variation in Chromosome Number
Overview
The last section of this chapter examines the mechanisms by which variations in
chromosome number may occur. These mechanisms can be divided into two general classes:
those that naturally occur as a result of cell division, and those that are used by researchers to
artificially alter the chromosome number. You should recognize that regardless of which occurs,
the outcome is the same since the result is a cell that produces abnormal levels of protein.
Naturally, variations in chromosome number may occur as a result of nondisjunction.
While this usually occurs during meiotic cell division (Figure 8.22), it may also happen during
mitosis (Figure 8.23). Notice the difference between these two forms.
This section of the chapter also presents the concept of alloploidy, in which an organism
contains chromosomes from two different species. This introduces a whole new level to the
terminology from the previous section, since an organism may have multiple sets of
chromosomes, but not from the same species. One of the more interesting aspects of chance in
chromosome number is the influence of alloploidy on sterility. The final part of this section takes
a look at an experimental procedure used to promote polyploidy. These include the use of the
drug colchicine, which causes complete nondisjunction (Refer to Figure 8.28).
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Outline of Key Terms
Autopolyploidy
Alloploidy
Homeologous chromosomes
Allodiploidy
Allopolyploidy
Allotetraploidy
Nondisjunction
Mitotic nondisjunction
Meiotic nondisjunction
Complete nondisjunction
Mosaicism
Bilateral gynandromorphy
Focal Points



Nondisjuntion during meiosis (Figure 8.22)
Nondisjunction and chromosome loss during mitosis (Figure 8.23)
A comparison of autopolyploidy, alloploidy, and allopolyploidy (Figure 8.25)
Exercises and Problems
For questions 1 to 6, use the following key:
a. statement applies to mitotic nondisjunction only
b. statement applies to meiotic nondisjunction only
c. statement applies to both forms of nondisjunction
d. statement applies to neither form of nondisjunction
_____ 1. Results in a mosaic pattern of chromosome number in the organism.
_____ 2. May result in an organism called a bilateral gynandromorph.
_____ 3. Involves movement of two sister chromatids to the same pole.
_____ 4. Produces a normal cell and a monosomic cell.
_____ 5. By complete nondisjunction, this may produce a gamete without chromosomes.
_____ 6. Occurs in the somatic cells of the organism.
For questions 7 to 11, match the term with its correct definition.
_____ 7. Autopolyploidy
_____ 8. Allodiploid
_____ 9. Allotetraploid
_____ 10. Homeologous
_____ 11. Alloploid
a. An organism containing chromosome sets from more than one species.
b. An organism that has two complete sets of chromosomes from two different species.
c. An increase in the number of chromosome sets in a single species.
d. Evolutionarily related chromosomes from different species.
e. An organism that has one complete chromosome set from two different species.
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Chapter Quiz
1. Which of the following represents an organism with two complete sets of chromosomes from
two different species?
a. autotetraploid
b. allotetraploid
c. allodiploid
d. tetrasomy
e. none of the above
2. Which of the following is NOT an example of an autosomal aneuploidy?
a. Down syndrome
b. Turner syndrome
c. Edwards syndrome
d. Patau syndrome
e. Choose this answer if all of the above are autosomal aneuploidies
3. A chromosome has the gene sequence A B C D E F G • H I (where • = the centromere)
Which of the following is an example of a pericentric inversion?
a. A D C B E F G • H I
b. A B C D E H • G F I
c. A B F G • H I
d. A B C D E F G • H I F G • H I
e. A B C D G F E • H I
4. Which of the following indicates the chromosome number of an individual with non-familial
Down syndrome?
a. 2n –1
b. 3n
c. 2n + 1
d. n + 2
5. Cri-du-chat syndrome is a result of a(n)______________.
a. inversion
b. translocation
c. deletion
d. duplication
6. Liver cells in humans exhibit which of the following?
a. alloploidy
b. endopolyploidy
c. monoploidy
d. aneuploidy
7. A bilateral gynandromorph is the result of ________.
a. mitotic nondisjunction
b. meiotic nondisjunction
c. chromosome loss
d. reciprocal translocation
e. paracentric inversion
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8. Which of the following individuals will have the MOST trouble in producing functional
gametes during meiosis?
a. triploid with 30 total chromosomes
b. tetraploid with 48 total chromosomes
c. tetraploid with 60 total chromosomes
d. diploid with 46 total chromosomes
e. diploid with 8 total chromosomes
9. The loss of the telomere on a chromosome tends to favor which of the following?
a. inversions
b. deletions
c. duplications
d. translocations
e. nondisjunction
10. The house mouse Mus musculus has a diploid chromosome number of 40. Suppose that the
first meiotic division of a germ cell is normal, but a single chromosome in one of the two
daughter cells undergoes non-disjunction in meiosis II. How many chromosomes would be
present in each of the four gametes that result from that meiosis?
a. 12, 12, 8, 8
b. 10, 10, 12, 8
c. 22, 20, 18, 16
d. 21, 21, 19, 19
e. 20, 20, 21, 19
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.
8.1
1. b
2. d
3. c
4. a
8.3
1. interstitial
2. copy number variations
3. paralogs
4. hemoglobin and myoglobin
5. b
6. a
7. d
8. a
9. b
10. c
11. c
8.2
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1. inversion
2. deletion (deficiency)
3. reciprocal translocation
4. simple translocation
8.4
1. acentric fragment
2. balanced
3. paracentric
4. Robertsonian translocation
5. position effect
6. paracentric inversion
7. reciprocal translocation
8. interstitial deficiency
9. simple translocation and paracentric inversion of F G
10. duplication
11. b
12. a
13. b
14. a
15. d
16. c
8.5
1. 14
2. 21
3. 28
4. 15
5. 13
8.6
1. 20,000 to 25,000
2. nondisjunction
3. 47
4. males
5. Turner
6. prophase I
7. sex, XYY
8. sex, XXY
9. sex, XO
10. autosomal, trisomy 21
11. autosomal, trisomy 13
12. autosomal, trisomy 18
8.7
1. haploid
2. unfertilized
3. fertilized
4. diploid
5. endopolyploidy
6. liver
7. Drosophila
8. polytene chromosome
9. chromocenter
10. 4n
11. 8n
12. 2n + 1
13. 6n
14. 2n - 1
15. n
8.8
1. a
2. a
3. c
4. d
5. b
6. a
7. c
8. e
9. b
10. d
11. a
1. b
2. b
3. b
4. c
5. c
6. b
7. a
8. a
9. d
10. e
Quiz
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