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Section 4
Section 4
Heredity
Heredity
Objectives
䊳
Explain how genetic
information passes from
one generation to the
next.
䊳
Identify the causes of
genetic disorders.
䊳
Compare the role of
genes, environment, and
behavior in affecting a
person’s risk for disease.
Objectives
Before class begins, write the objectives
on the board. Have students copy the
objectives into their notebooks at the
start of class.
1. Focus
Warm-Up Quick Quiz
Quick Quiz Take a brief self-inventory of some of your physical
traits, or characteristics.
1
Vocabulary
•
•
•
•
heredity
chromosome
gene
genetic disorder
2
Do you have a
widow’s peak or
a smooth hairline?
Widow’s Peak
Smooth
Free
Attached
Do you have free
or attached earlobes?
Use the
clickers
to survey student responses.
Call on a few volunteers to share
their answers to the writing activity.
Reiterate any correct ideas they state
about dominant traits. Then ask: Why
do different people have different
forms of the same trait? (They inherit
different forms from their parents.)
Have the class brainstorm other
physical traits that people inherit
from their parents. (Sample answers:
hair color, eye color, nose shape,
body shape)
A widow’s peak and free earlobes are examples
of dominant traits. What do you think a “dominant trait” is?
The Basic Rules of Heredity
When a baby is born, people may say, “She looks just like her father,” or
“He has his grandmother’s ears.” Think about how children resemble their
parents, grandparents, and other relatives. Their eye color, the shape of
their ears, their height—these traits are determined in part from the
genetic information they inherit from their parents. Heredity is the passing on, or transmission, of biological traits from parent to child. People
are similar to each of their parents in some ways but different from their
parents in other ways. What determines the combination of traits that are
passed on?
Teaching Transparency W64
Chromosomes To answer these questions, you must first learn about
chromosomes. Chromosomes (KROH muh sohmz) are tiny structures
found within cells that carry information about the characteristics you will
inherit. Most of the cells in your body contain 23 pairs of chromosomes—
46 chromosomes in all. However, sex cells—sperm or eggs—contain half
this number, or 23 chromosomes. When a sperm and egg unite, the fertilized egg ends up with 46 chromosomes—23 from each parent.
478
Sensitive Issues
Heredity may be a sensitive issue for
students who live with adoptive or
foster parents. Avoid asking students
to talk about traits they share with
their biological parents or other biological relatives. Give them the option
of using hypothetical examples or
other families they know.
478
Chapter 18
Chapter 18
I!
FY
Cellular Differentiation
All the cells in the human body, except sex
cells, are the result of repeated mitotic divisions that began with the single cell that
was produced at fertilization. Thus, all
the body’s cells contain the same genes.
As development proceeds, cells become
specialized in structure and function. Cells
become different, despite having the same
genes, because only certain genes are
“turned on” in each cell. Which genes are
turned on is due to a complex combination
of genetic and environmental factors.
2. Teach
L3 EL Reading/Note Taking 18-4
L2 Adapted Reading/Note Taking 18-4
The Basic Rules of Heredity
L2 Teacher Demo
Your looks are
determined in part by the traits
you inherit.
FIGURE 12
Genes Every chromosome in your body is made up of many genes. A
gene is a section of a chromosome that determines or affects a characteristic, or trait. Like the chromosomes that contain them, genes come in
pairs. Since a sex cell contains only one half of each chromosome pair, it
also has only one half of each gene pair. Once a sperm fertilizes an egg,
however, the fertilized egg contains two copies of the gene for each trait—
one from the father and one from the mother. Hereditary information
passes from one generation to the next through genes contained on the
two sets of chromosomes that a person receives from their parents.
EL Building Vocabulary
Have students find the terms dominant
and recessive in an English language
dictionary. Ask students to explain the
different meanings (in both general
usage and in a genetics context) of each
term. Then, have them use the terms in
sentences that reflect the different
meanings.
Dominant and Recessive Traits Suppose a father has one trait
and the mother has another. Which trait will their child have? The answer
depends on the makeup of the pair of genes that the child inherits.
Consider earlobe shape. Earlobes can be either free or attached. There
are two forms, or versions, of the gene for earlobe shape. One form of the
gene carries information for free earlobes—the dominant trait. A dominant trait is one that appears in an offspring whenever its gene is present.
The other form of the gene carries information for attached earlobes—the
recessive trait. A recessive trait appears in an offspring only when the
dominant form of the gene is not present.
You need two copies of the recessive form of the gene to have attached
earlobes. You need just one dominant form of the gene to have free earlobes. Receiving a dominant form of the gene from both parents will also
result in free earlobes.
Note that the rules of heredity for most traits, such as height and eye
color, are more complex than those for earlobe shape. This is because
many different genes plus factors other than genetics affect most traits.
Connect to
YOUR LIFE
Do you get dimples in your cheeks when you
smile? Smile dimples are a dominant trait.
On the board, sketch a sperm and an
egg, with 23 chromosomes each, and a
cell with 23 pairs of chromosomes, or 46
chromosomes in all. (You can use tick
marks to represent the chromosomes.)
Use the sketches to explain how fertilization results in a cell with 23 pairs of
chromosomes.
L3 Cooperative Learning
For: Updates on genetic
disorders
Visit: www.scilinks.org/health
Web Code: ctn-6184
Reproduction and Heredity 479
Ask students who know about Punnett
squares to make and explain a Punnett
square for a cross between an Ee mother
and an Ee father (Ee ⫻ Ee), where E
represents the dominant gene for free
earlobes and e represents the recessive
gene for attached earlobes. Ask: What
kind of earlobes do the parents have?
(free) What possible genetic combinations can the offspring have? (EE, Ee,
and ee) Which offspring will have free
earlobes? (EE and Ee) Which offspring
will have attached earlobes? (ee)
Connect to Students may answer “yes”
YOUR LIFE or “no.” If students are
unsure whether they have dimples
when they smile, they can ask a
classmate.
L3 Content Update
L4 Gifted and Talented
Ask interested students to find a family
tree that shows the inheritance of a trait
through several generations of a real or
hypothetical family. (Such trees are available
online and in genetics textbooks.) Have
some students find a family tree showing a
recessive trait and others find one with a
dominant trait. Ask students to use their
family trees to explain to the class how the
traits have been passed from generation to
generation. Challenge other students to
decide which of the traits are recessive
and which are dominant, based on their
inheritance patterns.
Use the Web Code to
access up-to-date information about
genetic disorders. Have students complete the Web activity.
Reproduction and Heredity
479
Chapter 18, Section 4
Genetic Disorders
Heredity and Disease
L2 Visual Learning: Figure 13
Teaching Transparency 54
Guide students in comparing and contrasting the genetic disorders listed in
the figure. Ask: Which disorder is the
only one listed in the table that is
controlled by a dominant gene?
(Huntington’s disease) Which disorder
is the only one caused by abnormal
chromosomes? (Down syndrome)
Which disorders cause problems with
the blood? (sickle cell disease and
hemophilia) Which disorders cause
brain damage or mental retardation?
(Tay-Sachs disease, PKU, and Down
syndrome) Call on a student to answer
the caption question.
Caption Answer Duchenne muscular
dystrophy and hemophilia
Disorder
Type of Disorder
Effect on the Body
Sickle
cell
disease
Recessive disorder
High number of red blood cells have an abnormal
sickle shape; blood cells clump and block small
blood vessels, causing severe pain and weakness
Tay-Sachs disease
Recessive disorder
Lack of important chemical in the brain results
in brain damage and death in childhood
Cystic fibrosis
Recessive disorder
Mucus in lungs becomes thick and sticky,
trapping bacteria that cause infections and lung
damage; mucus also affects pancreas
Phenylketonuria
(PKU)
Recessive disorder
Body cannot break down phenylalanine,
a chemical found in food; causes brain damage
if not diagnosed and treated early
Duchenne muscular
dystrophy
Recessive disorder that
primarily affects males
Lack of important protein needed for muscle
function leads to loss of muscle control
Hemophilia
Recessive disorder that
primarily affects males
Blood does not clot properly, leading to internal
bleeding that can damage the joints
Huntington’s disease
Dominant disorder
Cells in brain start to die in middle age; mental
abilities decline and movements become
uncontrollable, resulting in early death
Down
syndrome
Chromosomal disorder
Mental retardation and heart defects;
characteristic facial features; severity of disease
ranges from mild to severe
L3 Cultural Connection
Point out that several genetic disorders
tend to be more common among
certain cultures. For example, sickle cell
disease occurs more commonly in people
with African ancestry. Tay-Sachs is more
common among people with Ashkenazi
Jewish ancestry. Cystic fibrosis and PKU
occur more frequently in people of
Northern European ancestry.
Heredity and Disease
Just like earlobe shape, eye color, and other inherited traits, an abnormal
condition known as a genetic disorder can be passed from parent to
child. Genetic disorders are caused by the inheritance of an abnormal
gene or chromosome.
Genetic disorders
may be dominant or recessive, or
caused by errors in chromosome
inheritance.
Reading Tables Name two
disorders that are more common
in males.
FIGURE 13
480
Genetic Disorders Figure 13 provides information about some of
the more common genetic disorders. Many genetic disorders, such as
cystic fibrosis and hemophilia, are recessive traits. A child must receive
two abnormal copies of the gene—one from each parent—in order for
the disorder to develop. A few disorders, such as Huntington’s disease, are
dominant traits. Such disorders require just one abnormal copy of the
gene. Other genetic disorders, such as Down syndrome, are the result of
too few or too many chromosomes.
Chapter 18
I!
FY
Sun Exposure in Teens
Skin cancer is caused mainly by sun exposure during childhood and adolescence.
Sunburns at a young age are especially
likely to increase the risk of skin cancer.
According to a recent survey of more than
10,000 teens nationwide, most teens know
that too much sun exposure causes skin
480
Chapter 18
cancer, but many continue to deliberately
expose themselves to the sun, mainly
because of a simple preference for tanned
skin. In the same survey, only three in ten
teens said they regularly use sunscreen, and
eight in ten said they got a sunburn during
the past year.
L4 Building Health Skills
Diseases With a Genetic Link You have probably heard that
certain diseases run in families. What does that mean? Scientists know
that a person’s risk for many diseases increases when close relatives have
the disease. Some diseases for which a genetic link is suspected or has been
identified are breast cancer, colon cancer, high blood pressure, diabetes,
and some forms of Alzheimer’s disease.
How do diseases with a genetic link differ from genetic disorders like
those listed in Figure 13? Many different genes affect the development of
disease. In addition, those genes do not typically cause disease, but they
do increase a person’s risk.
Connect to
YOUR LIFE
Analyzing Influences Have some students
go online to research risk factors for
diseases listed in the text for which a
genetic link is suspected—breast cancer,
colon cancer, high blood pressure,
diabetes, some forms of Alzheimer’s
disease. Ask students to share what they
learn with the class. Use what they report
to start a class discussion of the relative
influences of heredity, environment,
and behavior on the risk of developing
diseases with a genetic link.
What diseases run in your family? How can you
decrease your risk for these diseases?
The Effect of Environment and Behavior Even if you have
genes that increase your risk for certain diseases, many other factors also
affect your risk. For most diseases, your environment and your behavior
affect your risk as much as or even more than your genes.
Environmental factors include such things as exposure to air
pollution and certain chemicals. The typical climate where
you live may be an environmental risk factor. Suppose, for
example, that skin cancer runs in your family. Living in a
warm, sunny climate would further increase your risk for
developing the disease.
Exposure to environmental risk factors is sometimes not
in your control, especially as a child. Other risk factors, however, are. Among the factors you can control are your habits
or behaviors. For example, using sunscreen can reduce your
risk of skin cancer. Regular physical activity can lower your risk
of high blood pressure, diabetes, and breast cancer. Eating more
fruits and vegetables can reduce your risk of colon cancer.
Making wise choices now will greatly decrease your risk for disease later on in life.
L3 Class Discussion
Point out that genetic testing saves lives
but may have drawbacks. Ask: What
might be some drawbacks of genetic
testing? (Sample answer: high cost; misuse of the information by employers or
insurance companies; coping with the
knowledge that you will probably
develop a particular disease later in life)
Ask students whether they think the
drawbacks outweigh the benefits.
Connect to Allow students to answer
YOUR LIFE this question in their
private journals.
L3 Visual Learning: Figure 14
Caption Answer: Fair complexion, multiple or abnormal moles, and a family history of skin cancer are genetic risk factors. Living in a warm, sunny climate is
an environmental risk factor. Unprotected
or excessive exposure to the sun, use of
tanning beds, and sunburns are behavioral risk factors.
Risk Factors You Can Control
• Unprotected or excessive
exposure to the sun
• Use of tanning beds
• Sunburns
Risk Factors You Can’t Control
• Fair complexion
• Multiple or abnormal moles
• Family history of skin cancer
• Climate in which you live
Using plenty of sunscreen
can reduce your risk of skin cancer.
Classifying Which risk factors for skin
cancer are genetic? Environmental?
Behavioral?
FIGURE 14
Reproduction and Heredity 481
L1 Special Needs
Students may not understand the distinction between genetic disorders and diseases with a genetic link. Make two spider
diagrams on the board to show students
the difference. Inside one of two large circles write Genetic Disorders, and inside the
other write Diseases With a Genetic Link.
Draw an arrow, labeled “Genes,” to the
first circle, and draw three arrows, labeled
“Genes,” “Behavior,” and “Environment,”
to the second circle. Explain the diagrams
to the class. Make sure students understand that they can change their behavior
and sometimes their environment to
reduce their chances of getting diseases
with a genetic link. As you make this point,
draw X’s through the arrows labeled
“Behavior” and “Environment.”
L1 Active Learning
To give special needs students a hands-on
experience to reinforce concepts of sun
protection, bring in several items for
them to handle, compare, and evaluate.
For example, you might bring in a baseball cap and wide-brimmed hat, or hats
made of tightly woven and loosely
woven fabrics. Also bring in sun lotions
and creams with different SPF values.
Have students handle the items and
discuss how they protect the body. You
may want to encourage students to add
their own items from home.
Reproduction and Heredity
481
Chapter 18, Section 4
Medical Advances Today, scientists are working hard
to develop new ways to identify and treat genetic disorders and
diseases with a genetic link. Areas of research include genetic
testing and gene therapy.
3. Assess
䊳
Genetic Testing Genetic testing involves the analysis of a
blood sample for the presence of abnormalities in specific
genes. Genetic testing has become more common in recent
years. The symptoms of some genetic disorders and most
diseases don’t show up early in life. By knowing someone
has the defective gene as early as possible—in some cases,
even before birth— doctors may be able to start therapies
that can prevent or reduce future symptoms.
䊳
Gene Therapy Scientists are currently researching a technique in which healthy copies of a gene are delivered to
the cells of a person who has a defective copy of the gene.
This therapy would ideally restore normal function to
people with a genetic disorder. Unfortunately, gene therapy has yet to be proven as an effective treatment, but the
research effort continues.
Evaluate
These assignments can help you assess
students’ mastery of the section content.
Section 4 Review
Answers appear below.
Teaching Resources
• Practice 18-4
• Section 18-4 Quiz
L2
Divide the class into small groups, and
have members of each group demonstrate their knowledge of section objectives to one another. Group members
should reread passages relating to any
objectives for which they have disagreements or uncertainties.
L4 Enrich
Teaching Resources
In the future, blood samples
from newborns may routinely undergo
genetic testing for both rare disorders
and common diseases.
FIGURE 15
Health and Community
Section 4 Review
• Enrich 18-4
Key Ideas and Vocabulary
Health and
Community
Supporting a Cause Possible organizations students might contact
include the Sickle Cell Disease
Association of America and the
Cystic Fibrosis Foundation. Web sites
for the organizations generally give
contact information and information
about how people can help. Students’
e-mails should be factually correct
and persuasive.
1. How is genetic information passed from one
generation to the next?
2. What are genes? How are they related to
chromosomes?
3. What are the causes of genetic disorders? Give two
examples of genetic disorders.
4. What three factors influence your risk for disease?
Which factors are under your control?
Critical Thinking
5. Predicting A man with free earlobes—who has
two dominant forms of the gene—marries a
woman with attached earlobes. What kind of
earlobes will their children have? Explain.
482
Supporting a Cause Contact a local
organization that supports research for a genetic
disorder. Find out how you can help support the
cause. Write an e-mail to your friends telling them
about the disease, its causes and treatment, and
how people can help.
6. Making Judgments Government funding for
research of some genetic disorders is very low
because the disorders are so rare. Is that
reasonable? Explain your position.
7. Evaluating The American Cancer Society
estimates that one third of cancer deaths in the
United States are due to unhealthy diet and lack of
physical activity. Write a paragraph evaluating this
statistic. Does it make you feel gloomy or hopeful?
Explain.
Chapter 18
Section 4 Review
1. through genes contained on the
chromosomes that a person
receives from his or her parents
2. Genes are sections of chromosomes
that determine characteristics. Each
chromosome contains many genes.
3. Genetic disorders are caused by the
inheritance of an abnormal gene or
482
Chapter 18
chromosome. Accept any two genetic disorders from Figure 13 on page 480.
4. Genes, environment, and behavior influence
your risk. Behavior and sometimes environment are under your control.
5. All of their children will have free earlobes
because they will inherit the dominant gene
for free earlobes from their father.
6. Students may or may not think it is reasonable. Their explanations should provide
logical arguments and show a correct
understanding of genetic disorders.
7. Sample answer: The statistic makes me
feel hopeful, because it means that I can
control the most important risk factors for
cancer by eating well and exercising.