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14–2 Human Chromosomes
Section 14–2
1 FOCUS
Objectives
A
human diploid cell contains more than 6 billion base pairs
of DNA. All of this DNA is neatly packed into the 46 chromosomes present in every diploid human cell. In its own way,
each of these chromosomes is like a library containing hundreds
or even thousands of books. Although biologists are many
decades away from mastering the contents of those books,
biology is now in the early stages of learning just how many
books there are and what they deal with.
You may be surprised to learn that genes make up only a
small part of chromosomes. In fact, only about 2 percent of the
DNA in your chromosomes functions as genes—that is, is
transcribed into RNA. Genes are scattered among long segments of DNA that do not code for RNA. The average human
gene consists of about 3000 base pairs, while the largest gene in
the human genome has more than 2 million base pairs!
Human Genes and Chromosomes
Chromosomes 21 and 22 are the smallest human autosomes.
Chromosome 22 contains approximately 43 million DNA
base pairs. Chromosome 21 contains roughly 32 million base
pairs. These chromosomes were the first two human chromosomes whose sequences were determined. Their structural
features seem to be representative of other human chromosomes.
Chromosome 22 contains as many as 545 different genes,
some of which are very important for health. Genetic disorders
on chromosome 22 include an allele that causes a form of
leukemia and another associated with neurofibromatosis, a
tumor-causing disease of the nervous system. However,
chromosome 22 also contains long stretches of repetitive DNA
that do not code for proteins. These long stretches of repetitive DNA are unstable sites where rearrangements can occur.
The structure of chromosome 21 is similar. It contains
about 225 genes, including one associated with amyotrophic
lateral sclerosis (ALS), also known as Lou Gehrig’s disease.
Chromosome 21 also has many regions with no genes at all.
As exploration of the larger human chromosomes continues,
molecular biologists may gradually learn more about how the
arrangements of genes on chromosomes affect gene expression
and development.
As you may recall, genes located close together on the same
chromosome are linked, meaning that they tend to be inherited
together. This is true for human genes. You also read earlier that
linked genes may be separated by crossing-over during meiosis;
this applies to human chromosomes as well.
Key Concepts
• Why are sex-linked disorders
more common in males than
in females?
• What is nondisjunction, and
what problems does it cause?
Vocabulary
sex-linked gene
nondisjunction
Reading Strategy:
Outlining Before you read,
use the headings of the section
to make an outline about
human chromosomes. As you
read, write a sentence under
each head to provide key
information.
Tim
r
• Teaching Resources, Lesson Plan 14–2,
Adapted Section Summary 14–2, Adapted
Sav14–2,
Worksheets 14–2, Section Summary
e
e
Worksheets 14–2, Section Review 14 –2,
Enrichment
• Reading and Study Workbook A, Section 14–2
• Adapted Reading and Study Workbook B,
Section 14–2
Vocabulary Preview
Write the word nondisjunction on the
board. Challenge students to identify
the prefixes in the word. Ask: What
does junction mean? (Joining together) Add the suffix dis-, and ask: What
does disjunction mean? (The act of
separating) Finally, challenge students
to infer the meaning of nondisjunction. (The act of not separating)
Reading Strategy
Encourage students to add the green
subheadings to their outlines and
write a sentence under each. Remind
students to include information in
their outlines that is presented in figure captions.
2 INSTRUCT
왖 Figure 14–11 Lou Gehrig died
at age 37 of ALS. ALS causes a progressive loss of muscle control due to
the destruction of nerves in the brain
and spinal cord.
SECTION RESOURCES
Print:
14.2.1 Identify characteristics of
human chromosomes.
14.2.2 Describe some sex-linked
disorders and explain why
they are more common in
males than in females.
14.2.3 Explain the process of
X-chromosome inactivation.
14.2.4 Summarize nondisjunction
and the problems it causes.
• Issues and Decision Making, Issues and
Decisions 7
Technology:
Human Genes and
Chromosomes
Build Science Skills
Calculating Have students reexamine the human karyotype on page
341. Ask: Which chromosomes are
the largest? (1 and 2) The smallest?
(18 through 22) Considering the
chromosome sizes, how many
bases might chromosome 1 have if
chromosome 22 has about 43 million bases? (About three times as
many, or 129 million)
• BioDetectives DVD, “Coming Home: A
Nation’s Pledge”
• iText, Section 14 –2
• Animated Biological Concepts DVD, 24
• Transparencies Plus, Section 14 –2
The Human Genome
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14–2 (continued)
Page 350
X Chromosome
Duchenne
muscular
dystrophy
Sex-Linked Genes
Use Visuals
Melanoma
Figure 14–13 Discuss the symbols
used in the Punnett square and relate
them to pedigree. Ask: Why is the
box for one son shaded? (He
expresses the trait for colorblindness.)
Would you expect the colorblind
son to have sons who are colorblind? (No, the son can pass only the
Y chromosome to his sons.) What is
the probability that the daughter
who is a carrier will have a colorblind child if she marries a man
with normal vision? (25%)
X-inactivation center
X-linked severe
combined
immunodeficiency
(SCID)
Colorblindness
Hemophilia
Y Chromosome
Testis-determining
factor
Build Science Skills
Using Models Challenge student
pairs to design a pedigree that traces
the inheritance of colorblindness in a
family over several generations.
Students can invent the family and
the affected individuals. Then, students should write three questions
about their pedigree and the inheritance of colorblindness. Have groups
exchange pedigrees and answer the
questions.
Sex-Linked Genes
왖
Figure 14–12 Genes on X and Y
chromosomes, such as those shown
in the diagrams, are called sex-linked
genes. Interpreting Graphics
Which chromosome carries more
genes?
Is there a special pattern of inheritance for genes located on the
X chromosome or the Y chromosome? The answer is yes. Because
these chromosomes determine sex, genes located on them are
said to be sex-linked genes. Many sex-linked genes are found
on the X chromosome, as shown in Figure 14–12. More than 100
sex-linked genetic disorders have now been mapped to the X
chromosome. The human Y chromosome is much smaller than
the X chromosome and appears to contain only a few genes.
Colorblindness Three human genes associated with color
vision are located on the X chromosome. In males, a defective
version of any one of these genes produces colorblindness, an
inability to distinguish certain colors. The most common form of
this disorder, red-green colorblindness, is found in about 1 in 10
males in the United States. Among females, however, colorblindness is rare—only about 1 female in 100 has colorblindness.
Why the difference?
Males have just one X chromosome. Thus, all
X-linked alleles are expressed in males, even if they are
recessive. In order for a recessive allele, such as the one for
colorblindness, to be expressed in females, there must be two
copies of the allele, one on each of the two X chromosomes. This
means that the recessive phenotype of a sex-linked genetic
disorder tends to be much more common among males than
among females. In addition, because men pass their X chromosomes along to their daughters, sex-linked genes move from
fathers to their daughters and may then show up in the sons of
those daughters, as shown in Figure 14–13.
Address Misconceptions
Students might think that colorblind
people see the world only in black
and white. Show students charts
used to diagnose colorblindness.
Explain that a colorblind person
either cannot see the object in the
pattern or might see a different
object. Help students realize that
people who are red-green colorblind
do see objects as blue or yellow or
shades of red; they cannot see
objects as green.
XCY
Figure 14–13
X-linked alleles are always
expressed in males, because males have only one X
chromosome. Males who receive the recessive Xc allele
all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles.
Normal
Colorblind vision
Mother
(carrier)
Male
Father
(normal vision)
XC
Y
XC
XCXC
Daughter
(normal vision)
XCY
Son
(normal vision)
Xc
XCXc
Daughter
(carrier)
XcY
Son
(colorblind)
XCXc
Female
UNIVERSAL ACCESS
Inclusion/Special Needs
Have students draw diagrams to show how chromosomal disorders can occur. Make sure students
realize that nondisjunction occurs during the formation of the egg cell or sperm cell. You might
also have students draw Punnett squares to show
the possible genotypes produced in a cross
between a normal sex cell and a cell in which
nondisjunction has occurred.
350
Chapter 14
Less Proficient Readers
For each blue heading in the section, have students write a sentence that describes the main
idea of that subsection. Encourage students to
write the sentences in their own words, after
reading the subsection and thinking about
what it is about. You might also encourage students to illustrate their main ideas, if they wish.
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How is colorblindness
transmitted?
Materials 2 plastic cups, 3 white beans, black
marker, red bean
Procedure
1. On a sheet of paper, draw a data table with the column headings “Trial,” “Colors,” “Sex of Individual,”
and “Number of X-Linked Alleles.” Draw 10 rows
under the headings and fill in the numbers 1
through 10 under “Trial.” Use the marker to label
one cup “father” and the other “mother.”
2. The white beans represent X chromosomes. Use
the marker to mark a dot on 1 white bean to
represent the X-linked allele for colorblindness.
Place this bean, plus 1 unmarked white bean, into
the cup labeled “mother.”
3. Mark a black dot on 1 more white bean. Place
this bean, plus 1 red bean, into the cup labeled
“father.” The red bean represents a Y chromosome.
4. Close your eyes and pick one bean from each cup
to represent how each parent contributes a sex
chromosome to a fertilized egg.
5. In your data table, record the color of each bean
and the sex of an individual who would carry this
pair of sex chromosomes. Also record how many
X-linked alleles the individual has. Put the beans
back in the cups they came from.
6. Determine whether the individual would have
colorblindness.
7. Repeat steps 4 to 6 for a total of 10 pairs of beans.
Analyze and Conclude
1. Drawing Conclusions How do the sex chromosomes keep the numbers of males and females
roughly equal?
2. Calculating Share your data with your classmates. Calculate the class totals for each table
column. How many females were colorblind? How
many males? How would you explain these results?
3. Using Models Evaluate the adequacy of your
model. How accurately does it represent the
transmission of colorblindness in a population?
Hemophilia Hemophilia is another example of a sex-linked
disorder. Two important genes carried on the X chromosome
help control blood clotting. A recessive allele in either of these
two genes may produce a disorder called hemophilia
(hee-moh-FIL-ee-uh). In hemophilia, a protein necessary for
normal blood clotting is missing. About 1 in 10,000 males is born
with a form of hemophilia. People with hemophilia can bleed to
death from minor cuts and may suffer internal bleeding from
bumps or bruises. Fortunately, hemophilia can be treated by
injections of normal clotting proteins, which are now produced
using recombinant DNA.
Duchenne Muscular Dystrophy Duchenne muscular
dystrophy (DIS-truh-fee) is a sex-linked disorder that results in
the progressive weakening and loss of skeletal muscle. In the
United States, one out of every 3000 males is born with this
condition. Duchenne muscular dystrophy is caused by a defective version of the gene that codes for a muscle protein.
Researchers in many laboratories are trying to find a way to
treat or cure this disorder, possibly by inserting a normal allele
into the muscle cells of Duchenne muscular dystrophy patients.
What causes Duchenne muscular dystrophy?
FACTS AND FIGURES
Hemophilia and royalty
The frequency of hemophilia was much higher
among the royal families of nineteenth-century
Europe than among the general population. This
was probably due to the fact that these families
often intermarried. Queen Victoria of England was
a carrier of the disease, as were two of her daughters. At one time, it was calculated that of
Victoria’s 69 descendants, 18 were either affected
males or female carriers, though none of these
individuals were British.
Objective Students will be able to
model how colorblindness is transmitted.
Skill Focus Using Models,
Calculating, Drawing
Conclusions
Materials 2 plastic cups, 3 white
beans, black marker, red bean
Time 20 minutes
Strategies
• After students read the procedure,
ask: Is either parent colorblind?
(Yes, the father) Is the mother
heterozygous or homozygous
for colorblindness?
(Heterozygous) Is she a carrier?
(Yes, she has one allele for colorblindness.)
• Remind students to keep their
eyes closed while picking the
beans so that they choose
randomly.
Expected Outcomes Students
will conclude that colorblindness
occurs more frequently in males
because they have only one copy of
the X chromosome.
Analyze and Conclude
1. There is a 50 : 50 chance that a
child will receive an X or Y chromosome from the father.
2. About 50% of the females will be
colorblind and about 50% of the
males will be colorblind. The mother
is heterozygous, so her sons have a
50% chance of inheriting the X
chromosome that carries the allele
for colorblindness. The father is colorblind, so the daughters have a
50% chance of inheriting X chromosomes from both parents that carry
the allele for colorblindness.
3. The model was accurate in
representing the randomness of
chromosome movement during
meiosis and gametes joining during
fertilization. It also accurately models the independence of each
fertilization event. However, in a real
population, the ratio of colorblind
to noncolorblind people will be
much lower, because the allele for
colorblindness is not present in 50%
of the people.
Answers to . . .
A defective gene that
codes for a muscle protein
Figure 14–12 The X chromosome
The Human Genome
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X-Chromosome Inactivation
14–2 (continued)
X-Chromosome
Inactivation
Build Science Skills
Observing Set up microscope
stations with slides of animal body
cells that have Barr bodies. Encourage
students to draw their observations,
labeling the cell cytoplasm, nucleus,
nucleoplasm, chromosomes, and Barr
bodies. Then, give students two
unknown slides and challenge them
to identify which slide came from a
female.
Chromosomal
Disorders
Use Visuals
Figure 14–15 Ask: What phase of
meiosis is illustrated by the first
cell? (Metaphase I) If necessary,
review meiosis so that students
remember that in meiosis I, homologous chromosomes separate to
produce a haploid cell and that in
meiosis II, chromosome copies (or
sister chromatids) separate. Ask:
What types of gametes are produced when nondisjunction
occurs? (Some gametes that have two
copies of the chromosome and other
gametes with no copies of it)
왖 Figure 14–14 This cat’s fur
color is controlled by a gene on the
X chromosome. Drawing
Conclusions Is the cat shown a
male or a female?
왔 Figure 14–15
Nondisjunction causes gametes to have
abnormal numbers of chromosomes. The result of nondisjunction
may be a chromosome disorder
such as Down syndrome.
Homologous
chromosomes
fail to separate.
Meiosis I:
Nondisjunction
Meiosis II
Females have two X chromosomes, but males have only one. If
just one X chromosome is enough for cells in males, how does
the cell “adjust” to the extra X chromosome in female cells? The
answer was discovered by the British geneticist Mary Lyon. In
female cells, one X chromosome is randomly switched off. That
turned-off chromosome forms a dense region in the nucleus
known as a Barr body. Barr bodies are generally not found in
males because their single X chromosome is still active.
The same process happens in other mammals. In cats, for
example, a gene that controls the color of coat spots is located on
the X chromosome. One X chromosome may have an allele for
orange spots and the other may have an allele for black spots.
In cells in some parts of the body, one X chromosome is switched
off. In other parts of the body, the other X chromosome is switched
off. As a result, the cat’s fur will have a mixture of orange and
black spots, as shown in Figure 14–14. Male cats, which have just
one X chromosome, can have spots of only one color. By the way,
this is one way to tell the sex of a cat. If the cat’s fur has three
colors—white with orange and black spots, for example—you can
almost be certain that it is female.
Chromosomal Disorders
Most of the time, the mechanisms that separate human
chromosomes in meiosis work very well, but every now
and then something goes wrong. The most common error
in meiosis occurs when homologous chromosomes fail to separate. This is known as nondisjunction, which means “not
coming apart.” Nondisjunction is illustrated in Figure 14–15.
If nondisjunction occurs, abnormal numbers of
chromosomes may find their way into gametes, and a
disorder of chromosome numbers may result.
Down Syndrome If two copies of an autosomal chromosome
fail to separate during meiosis, an individual may be born with
three copies of a chromosome. This is known as a trisomy,
meaning “three bodies.” The most common form of trisomy
involves three copies of chromosome 21 and is called Down
syndrome. Figure 14–16 shows a karyotype of a person with
Down syndrome. In the United States, approximately 1 baby in
800 is born with Down syndrome. Down syndrome produces
mild to severe mental retardation. It is also characterized by an
increased susceptibility to many diseases and a higher frequency of some birth defects.
Why should an extra copy of one chromosome cause so much
trouble? That is still not clear, and it is one of the reasons
scientists have worked so hard to learn the DNA sequence for
chromosome 21. Now that researchers know all of the genes on
the chromosome, they can begin experiments to find the exact
genes that cause problems when present in three copies.
HISTORY OF SCIENCE
Barr bodies
Barr bodies were named for Murray Barr, who
first observed them in the nerve cells of female
cats in 1949. It was not until the early 1960s
that Mary Lyon proposed that one X chromosome is randomly inactivated. In body cells, she
observed that one X chromosome replicated
352
Chapter 14
later than the other. The late-replicating X chromosome is inactivated when the embryo
implants in the uterine wall. All body cells have
the same activated and inactivated X chromosomes as the embryonic cell from which they
were derived.
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Build Science Skills
Sex Chromosome Disorders Disorders also occur among
the sex chromosomes. Two of these abnormalities are Turner’s
syndrome and Klinefelter’s syndrome.
In females, nondisjunction can lead to Turner’s syndrome. A
female with Turner’s syndrome usually inherits only one X
chromosome (karyotype 45,X). Women with Turner’s syndrome
are sterile, which means that they are unable to reproduce.
Their sex organs do not develop at puberty.
In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,XXY). The extra X chromosome interferes with meiosis
and usually prevents these individuals from reproducing. Cases of
Klinefelter’s syndrome have been found in which individuals were
XXXY or XXXXY. There have been no reported instances of babies
being born without an X chromosome, indicating that the X
chromosome contains genes that are vital for the survival and
development of an embryo.
These sex chromosome abnormalities point out the essential
role of the Y chromosome in male sex determination in humans.
The human Y chromosome contains a sex-determining region
that is necessary to produce male sexual development, and it
can do this even if several X chromosomes are present. However,
if this region of the Y chromosome is absent, the embryo develops as a female.
Figure 14–16 The karyotype on
the right is from a person with Down
syndrome. Down syndrome causes
mental retardation and various physical problems. People with Down
syndrome can, however, lead active,
happy lives. Observing Analyze
this karyotype. What characteristic
enables you to identify it as belonging to a person with Down syndrome? Is that person male or
female?
Key Concept Why are
sex-linked disorders more common in males than in females?
2.
Key Concept How does
nondisjunction cause chromosome number disorders?
3. List at least two examples of
human sex-linked disorders.
4. Describe two sex chromosome
disorders.
5. Critical Thinking Comparing
and Contrasting Distinguish
between sex-linked disorders and
sex chromosome disorders.
3 ASSESS
Evaluate Understanding
Have students construct a concept
map that summarizes the concepts
described in this section. Students
should include the Vocabulary terms
and Key Concepts in the map.
Reteach
Have students use Punnett squares to
model how sex-linked traits are transmitted from parents to offspring.
Challenge students to show how a
dominant sex-linked allele has a different pattern of inheritance from a
recessive sex-linked allele.
14–2 Section Assessment
1.
Drawing Conclusions From
patients with sex chromosomes disorders, physicians and geneticists have
been able to infer the functions of the
X and Y chromosomes to sex determination. Ask: Why do geneticists
believe that the X chromosome
contains genes that are vital for
survival? (Babies without an X chromosome have never been born.) Why is
the Y chromosome thought to
cause male sexual development? (In
the absence of a Y chromosome, the
embryo develops as a female.)
Explaining a Process
Write a paragraph explaining
the process of nondisjunction.
Hint: To organize your writing,
refer to Figure 14–15 and use
this diagram to create a
flowchart that shows the steps
in the process.
Paragraphs should describe in a
step-by-step process the failure of
one pair of homologous chromosomes to separate during
anaphase I or the failure of one
pair of chromatids to separate during anaphase II.
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 14–2.
14–2 Section Assessment
1. Males have just one X chromosome. Thus, all
X-linked alleles are expressed in males, even if
they are recessive.
2. Chromosomes fail to separate, causing
gametes to have abnormal numbers of chromosomes.
3. Answers include colorblindness, hemophilia,
and Duchenne muscular dystrophy.
4. A female with Turner’s syndrome has only
one X chromosome and is sterile. A male
with Klinefelter’s syndrome has one or more
extra X chromosomes and is usually sterile.
5. Sex-linked disorders are caused by alleles of
genes usually carried on the X chromosome.
Sex chromosome disorders are caused by
nondisjunction, or sex chromosomes failing
to separate correctly during meiosis.
Answers to . . .
Figure 14 –14 The cat is female.
Figure 14 –16 Since it has three
copies of chromosome 21, it is the
karyotype of a person with Down syndrome; the two X chromosomes
indicate a female.
The Human Genome
353
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Who Controls Your DNA?
Encourage one group of students to
learn what companies or agencies are
interested in knowing about an individual’s DNA. Students should find
out what the motives of these agencies and companies are. Have
another group of students learn
about medical records and who has
access to that information. A third
group of students can research how
DNA information has allegedly been
used to discriminate against individuals. Have each group present its
findings to the class. Then, have a
class discussion about the pros and
cons of keeping DNA information
private.
Research and Decide
1. Accept all reasonable answers.
Justified: Employers use DNA information as a record of employees’
identity, as in the case of the military.
Withhold: Individuals are concerned
that DNA information could be used
against them, causing them to lose
promotions or even their jobs.
2. Students will have different opinions, but all opinions should include
reasonable explanations.
3. Some students might think the
insurance company has a right to test
for cystic fibrosis so that it can decide
not to insure a family carrying the
allele to prevent a profit loss. Others
might think the insurance company
does not have the right to test for the
cystic fibrosis allele, because DNA
information is private and should not
be used to discriminate against an
individual.
Students can research the right to
control DNA information on the
site developed by authors Ken
Miller and Joe Levine.
354
Chapter 14
T
he U.S. Department of Defense requires that
soldiers submit DNA samples for a database
that could be used to identify soldiers’ remains. Two
Marines, Corporal John C. Mayfield and Corporal
Joseph Vlacovsky, refused. At their court martial,
the two Marines argued that DNA samples could
be examined for genes related to disease or even
behavior and, therefore, the database was an invasion of privacy. As a result of the concerns raised by
this case, the U.S. Department of Defense has
changed its policies. It now destroys DNA samples
upon request when an individual leaves military
service. Do people have a right to control their own
DNA samples?
The Viewpoints
Research and Decide
DNA Information Is Not Private
1. Analyzing the Viewpoints Learn more
about this issue by consulting library or
Internet resources. Then decide whether there
are any circumstances in which an employer
might be justified in demanding DNA samples
from its employees. Why might an employee
wish to withhold such samples?
2. Forming Your Opinion Should the control of
DNA databases be a matter of law, or should it
be a matter to be negotiated between people,
their employers, and insurance companies?
3. Persuasive Speaking Suppose you were a
doctor working as a consultant to a health insurance company. The insurance company is trying
to decide whether to test adults for cystic fibrosis alleles before agreeing to insure their families. What advice would you give to the company
about this?
As the court recognized, the U.S. Department of
Defense had good reasons for requiring that DNA
samples be taken and stored. Furthermore, DNA
sequences are no more private and personal than
fingerprints or photographs, which are taken by
private and government agencies all the time.
An employer has a right to take and keep such
information. Individuals should have no reason to
fear the abuse of such databases.
DNA Infomation Is Private and Personal
The use of DNA for personal identification by the
military may be justified. An individual’s genetic
information, however, is a private matter. A recent
study at Harvard and Stanford universities turned
up more than 200 cases of discrimination because
of genes individuals carried or were suspected of
carrying. Employers with DNA information might
use it to discriminate against workers who carry
genes they suspect might cause medical or behavioral problems. Individuals must have the right to
control their own DNA and to withhold samples
from such databases.
For: Links from
the authors
Visit: PHSchool.com
Web Code: cbe-4142