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Genetics: The Science of Heredity
■
Section Summary
Mendel’s Work
Key Concepts
■
What were the results of Mendel’s experiments, or crosses?
■
What controls the inheritance of traits in organisms?
Heredity is the passing of physical characteristics from parents to offspring.
Gregor Mendel was curious about the different forms of characteristics, or
traits, of pea plants. Mendel’s work was the foundation of genetics, the
scientific study of heredity.
A new organism begins to form when egg and sperm join in the process
called fertilization. Before fertilization can happen in pea plants, pollen
must reach the pistil of a pea flower through pollination. Pea plants are
usually self-pollinating, meaning pollen from a flower lands on the pistil of
the same flower. Mendel developed a method by which he cross-pollinated,
or “crossed,” pea plants.
Mendel crossed two pea plants that differed in height. He crossed
purebred tall plants with purebred short plants. These parent plants, the P
generation, were purebred because they always produced offspring with
the same trait as the parent. In all of Mendel’s crosses, only one form of the
trait appeared in the F1 generation. However, in the F2 generation, the
“lost” form of the trait always reappeared in about one fourth of the
plants. From his results, Mendel reasoned that individual factors, one from
each parent, control the inheritance of traits. Today, scientists call the factors
that control traits genes. The different forms of a gene are called alleles.
An organism’s traits are controlled by the alleles it inherits from its
parents. Some alleles are dominant, while other alleles are recessive. A
dominant allele is one whose trait always shows up in the organism when
the allele is present. A recessive allele is hidden whenever the dominant
allele is present. A trait controlled by a recessive allele will only show up if
the organism does not have the dominant allele.
In Mendel’s cross, the purebred tall plant has two alleles for tall stems.
The purebred short plant has two alleles for short stems. The F 1 plants are all
hybrids: they have two different alleles for the trait—one allele for tall stems
and one for short stems. Geneticists use a capital letter to represent a
dominant allele and a lowercase version of the same letter for the recessive
allele.
Mendel’s discovery was not recognized during his lifetime. In 1900, three
different scientists rediscovered Mendel’s work. Because of his work,
Mendel is often called the Father of Genetics.
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Guided Reading and Study
Mendel’s Work
This section describes how Gregor Mendel identified the method by which
characteristics are passed from parents to their offspring.
Use Target Reading Skills
As you read, complete the outline about Mendel’s work. Use the red headings for the
main idea and the blue headings for the supporting ideas.
I. Mendel’s experiments
A. crossing pea plants
B.
C.
D.
II.
A.
B.
C.
D.
Introduction
1.
Gregor Mendel experimented with hundreds of pea plants to
understand the process of ________________________.
Match the term with its definition.
Term
____ 2. heredity
Definition
a. The scientific study of heredity
____ 3. genetics
b. Physical characteristics
____ 4. traits
c. The passing of traits from parents
to offspring
Mendel’s Experiments
In a flower, the female sex cells, or eggs, are produced by the
________________________. Pollen, which contains the male sex cells, is
produced by the ________________________.
6.
What are purebred organisms?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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Genetics: The Science
of Heredity
5.
Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
Mendel’s Work
■
Guided Reading and Study
(continued)
7. Complete the flowchart below, which summarizes Mendel's first
experiment with pea plants.
Mendel's Experiment
Purebred tall plants are crossed with purebred
F1 offspring are all
plants.
.
F1 offspring are allowed to self-pollinate.
F2 offspring are
and
.
8. Circle the letter of other traits in garden peas that Mendel studied.
a. seed size, seed shape, seed color
b. seed color, pod color, flower shape
c. flower size, pod shape, seed coat color
d. pod color, seed shape, flower position
9. Two forms of the trait of seed shape in pea plants are
________________________ and ________________________.
Dominant and Recessive Alleles
10. Circle the letter of each sentence that is true about alleles.
a. Recessive alleles are never present when dominant alleles are present.
b. Alleles are different forms of a gene.
c. A trait controlled by a dominant allele always shows up in the organism when
the allele is present.
d. Recessive alleles hide dominant alleles.
11. Is the following sentence true or false? Only pea plants that have two recessive
alleles for short stems will be short. ________________________
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Guided Reading and Study
Match the pea plant with its combination of alleles.
Pea Plant
Combination of Alleles
____ 12. purebred short
a. Two alleles for tall stems
____ 13. purebred tall
b. One allele for tall stems and one allele
for short stems
____ 14. hybrid tall
c. Two alleles for short stems
15. A dominant allele is represented by a(n) ________________________
letter.
16. A recessive allele is represented by a(n) ________________________
letter.
17. How might a geneticist write the alleles to show that a tall pea plant has
one allele for tall stems and one allele for short stems?
________________________
18. Is the following sentence true or false? Some scientists during Mendel's
time thought Mendel should be called the Father of Genetics.
________________________
Genetics: The Science
of Heredity
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Review and Reinforce
Mendel’s Work
Understanding Main Ideas
Study the diagram. Then answer the following questions on a separate sheet of paper.
P Generation
F1 Generation
×
1.
2.
3.
4.
5.
F2 Generation
×
What trait in pea plants is being studied in the cross above?
What are the two alleles of this trait?
Which allele is the dominant allele? Explain how you know.
Which allele is the recessive allele? Explain.
What alleles do the F1 offspring have? Explain which allele was inherited
from which parent.
Building Vocabulary
Match each term with its definition by writing the letter of the correct definition on
the line beside the term.
a. the passing of traits from parents
to offspring
____
6. genetics
____
7. alleles
____
8. traits
____
9. recessive allele
____ 10. genes
b. an organism with two different alleles
for a trait
c. factors that control traits
d. physical characteristics of organisms
____ 11. hybrid
____ 12. heredity
e. an allele whose trait always shows up in
the organism
____ 13. dominant allele
f. the different forms of a gene
g. the scientific study of heredity
h. an allele whose trait is masked in the
presence of a dominant allele
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Genetics: The Science of Heredity
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Enrich
The Test Cross
When an organism has a trait controlled by a dominant allele, it can either
be a hybrid or a purebred. To find out which, geneticists can use a test cross.
In a test cross, the organism with the trait controlled by a dominant allele is
crossed with an organism with a trait controlled by a recessive allele. If all
offspring have the trait controlled by the dominant allele, then the parent is
probably a purebred. If any offspring has the recessive trait, then the
dominant parent is a hybrid. Study the test cross below, then answer the
questions.
Test Cross
Test Cross
H = short hair
h = long hair
H = Short hair
h = Long hair
×
Generation
PPGeneration
hh
H ?
F1 Generation
H ?
H ?
hh
H ?
Answer the following questions on a separate sheet of paper.
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Genetics: The Science
of Heredity
1. Is the long-haired cat in the P generation a hybrid or a purebred? Explain
your answer.
2. Is the short-haired cat in the P generation a hybrid or a purebred? Explain
your answer.
3. If the short-haired cat in the P generation were purebred, what would you
expect the offspring to look like?
4. In horses, the allele for a black coat (B) is dominant over the allele for a
brown coat (b). A cross between a black horse and a brown horse
produces a brown foal. Is the black horse a hybrid or a purebred? Explain.
5. In guinea pigs, the allele for a smooth coat (S) is dominant over the allele
for a rough coat (s). Explain how you could find out whether a guinea pig
with a smooth coat is a hybrid or a purebred.
Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Skills Lab
Take a Class Survey
Problem
Are traits controlled by dominant alleles more common than traits
controlled by recessive alleles?
Skill Focus
developing hypotheses, interpreting data
Materials
mirror (optional)
Procedure
PART 1 Dominant and Recessive Alleles
1. Write a hypothesis reflecting your ideas about the problem question.
________________________________________________________________________
________________________________________________________________________
2. For each of the traits listed in the data table on the next page, work with
a partner to determine which trait you have. Circle that trait in your
data table.
3. Count the number of students in your class who have each trait. Record
that number in your data table. Also record the total number of students.
PART 2 Are Your Traits Unique?
4. Look at the circle of traits in your text. All the traits in your data table
appear in the circle. Place the eraser end of your pencil on the trait in
the small central circle that applies to you—either free ear lobes or
attached ear lobes.
5. Look at the two traits touching the space your eraser is on. Move your
eraser onto the next description that applies to you. Continue using your
eraser to trace your traits until you reach a number on the outside rim of
the circle. Share that number with your classmates.
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Genetics: The Science of Heredity
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Skills Lab
Data Table
Total Number or Students ________________________
Trait 1
Number
Trait 2
A
Free ear lobes
Attached ear lobes
B
Hair on fingers
No hair on fingers
C
Widow’s peak
No widow’s peak
D
Curly hair
Straight hair
E
Cleft chin
Smooth chin
F
Smile dimples
No smile dimples
Number
Analyze and Conclude
Write your answers in the spaces provided.
1. Observing The traits listed under Trait 1 in the data table are controlled
by dominant alleles. The traits listed under Trait 2 are controlled by
recessive alleles. Which traits controlled by dominant alleles were shown
by a majority of students? Which traits controlled by recessive alleles
were shown by a majority of students?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
3. Developing Hypotheses Do your data support the hypothesis you
proposed in Step 1? Write an answer with examples.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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Genetics: The Science
of Heredity
2. Interpreting Data How many students ended up on the same number on
the circle of traits? How many students were the only ones to have their
number? What do the results suggest about each person’s combination of
traits?
________________________________________________________________________
Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
Take a Class Survey
■
Skills Lab
(continued)
Design an Experiment
Do people who are related to each other show more genetic similarity than
unrelated people? Write a hypothesis. Then design an experiment to test
your hypothesis. Obtain your teacher’s permission before carrying out your
investigation.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Section Summary
Probability and Heredity
Key Concepts
■
What is probability and how does it help explain
the results of genetic crosses?
■
What is meant by genotype and phenotype?
■
What is codominance?
Probability is a number that describes how likely it is that an event will
occur. The principles of probability predict what is likely to occur, not
necessarily what will occur. For example, in a coin toss, the coin will land
either heads up or tails up. Each of these two events is equally likely to
happen. In other words, there is a 1 in 2 chance that a tossed coin will land
heads up, and a 1 in 2 chance that it will land tails up. A 1 in 2 chance can be
expressed as a fraction, 1/2, or as a percent, 50 percent. The result of one coin
toss does not affect the result of the next toss. Each event is independent of
another.
When Gregor Mendel analyzed the results of his crosses in peas, he
carefully counted all the offspring. Over time, he realized that he could
apply the principles of probability to his crosses. Mendel was the first
scientist to recognize that the principles of probability can be used to predict
the results of genetic crosses.
A tool that applies the laws of probability to genetics is a Punnett square.
A Punnett square is a chart that shows all the possible combinations of
alleles that can result from a genetic cross. Geneticists use Punnett squares to
show all the possible outcomes of a genetic cross and to determine the
probability of a particular outcome. In a Punnett square, all the possible
alleles from one parent are written across the top. All the possible alleles
from the other parent are written down the left side. The combined alleles in
the boxes of the Punnett square represent all the possible combinations in the
offspring. In a genetic cross, the allele that each parent will pass on to its
offspring is based on probability.
Two useful terms that geneticists use to describe organisms are genotype
and phenotype. An organism’s phenotype is its physical appearance, or
visible traits. An organism’s genotype is its genetic makeup, or allele
combinations. When an organism has two identical alleles for a trait, the
organism is said to be homozygous for that trait. An organism that has two
different alleles for a trait is said to be heterozygous for that trait.
For all of the traits in peas that Mendel studied, one allele was dominant
while the other was recessive. This is not always the case. In an inheritance
pattern called codominance, the alleles are neither dominant nor recessive.
As a result, both alleles are expressed in the offspring. Codominant alleles
are written as capital letters with superscripts to show that neither is
recessive.
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Guided Reading and Study
Probability and Heredity
This section explains what probability is and how the laws of probability can be used
in the study of genetics.
Use Target Reading Skills
After you read the section, reread the paragraphs that contain definitions of Key
Terms. Use all the information you have learned to write a definition of each Key
Term in your own words.
Principles of Probability
1.
A number that describes how likely it is that an event will occur is called
________________________.
2.
Circle the letter of each answer that equals the probability that a tossed
coin will land heads up.
a. 1 in 2
b. 1/2
c. 50 percent
d. 20 percent
3.
Is the following sentence true or false? When you toss a coin 20 times,
you will always get 10 heads and 10 tails. ________________________
4.
If you toss a coin five times and it lands heads up each time, can you
expect the coin to land heads up on the sixth toss? Explain.
________________________________________________________________________
________________________________________________________________________
Genetics: The Science
of Heredity
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Genetics: The Science of Heredity
■
Guided Reading and Study
Probability and Heredity (continued)
Probability and Genetics
5.
When Mendel crossed two hybrid plants for stem height (Tt), what
results did he always get?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
6.
Mendel realized that the principles of probability could be used to
________________________ the results of genetic crosses.
7.
A chart that shows all the possible combinations of alleles that can result
from a genetic cross is called a(n) ________________________.
8.
Write in the alleles of the parents and the possible allele combinations of
the offspring in the Punnett square below. (Note that both parents are
tall. Three of the offspring are tall and one is short.)
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Genetics: The Science of Heredity
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Guided Reading and Study
Phenotypes and Genotypes
Match the term with its definition.
Term
____ 9. phenotype
____ 10. genotype
____ 11. homozygous
____ 12. heterozygous
Definition
a. Describes an organism with two
identical alleles for a trait
b. An organism’s physical appearance,
or visible traits
c. An organism’s genetic makeup, or
allele combinations
d. Describes an organism that has two
different alleles for a trait
13. Mendel used the term ________________________ to describe
heterozygous pea plants.
Codominance
14. Is the following sentence true or false? In codominance, the alleles are
neither dominant nor recessive. ________________________
15. In cattle, red hair and white hair are codominant. Cattle with both white
hair and red hair are ________________________.
Genetics: The Science
of Heredity
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Review and Reinforce
Probability and Heredity
Understanding Main Ideas
Complete the two Punnett squares below, and then answer the questions on a
separate sheet of paper.
1. Punnett Square A:
B
2. Punnett Square B:
b
B
Bb
bb
b
Bb
bb
3. In the cross between two black guinea pigs shown in Punnett Square A,
what is the probability that an offspring will be black? White?
4. Is it possible that the cross between two black guinea pigs in Punnett
Square A would not produce a white guinea pig? Explain.
5. What color are the guinea pig parents in the cross shown in Punnett Square B?
6. Which guinea pig parent(s) in Punnett Square B is homozygous? Which
is heterozygous? Explain how you know.
7. Calculate the probability that an offspring will be black in the cross in
Punnett Square B. What is the probability that an offspring will be white?
Building Vocabulary
Match each term with its definition by writing the letter of the correct definition on
the line beside the term.
____
8. heterozygous
____
9. Punnett square
____ 10. genotype
____ 11. codominance
____ 12. probability
____ 13. homozygous
____ 14. phenotype
a. a chart that shows all the possible combinations of
alleles that can result from a genetic cross
b. a number that describes how likely it is that an event
will occur
c. an organism that has two identical alleles for a trait
d. an organism’s physical appearance
e. an organism’s genetic makeup, or allele combinations
f. an organism that has two different alleles for a trait
g. inheritance pattern in which the alleles are neither
dominant nor recessive
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Genetics: The Science of Heredity
■
Enrich
Genetic Crosses With Two Traits
In his work with garden peas,
Mendel also set up crosses in which
he studied the inheritance of two
P Generation
traits at one time. For example, he
crossed tall plants having green
pods (TTGG) with short plants
having yellow pods (ttgg). The F1
offspring showed both traits
controlled by dominant alleles, tall
and green. Mendel allowed the F 1
F1 Generation
offspring to self-pollinate. The F2
offspring had four different
phenotypes: tall plants with green
pods, tall plants with yellow pods,
short plants with green pods, and
short plants with yellow pods.
These results led Mendel to
formulate the Law of Independent
Assortment, which states that
alleles of one gene separate or
assort independently of alleles of
F2 Generation
another gene. In other words, the
distribution of alleles of one gene
does not affect the distribution of
alleles for another gene.
Study the Punnett square of a
genetic cross between two pea
plants with two different traits.
Then answer the questions that follow.
×
TTGG
ttgg
TtGg
TG
Tg
tG
tg
TTGG
TTGg
TtGG
TtGg
TTGg
TTgg
TtGg
Ttgg
TtGG
TtGg
ttGG
ttGg
TtGg
Ttgg
ttGg
ttgg
TG
Tg
tG
tg
Answer the following questions on a separate sheet of paper.
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Genetics: The Science
of Heredity
1. What are all the possible combinations of alleles that each F 1 parent can
pass on to the offspring?
2. What are the possible genotypes of the F 2 offspring? What are the
possible phenotypes of the F 2 offspring?
3. What is the probability that an F2 offspring will be tall with green pods?
What is the probability that an F2 offspring will be short with yellow
pods?
Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Skills Lab
Make the Right Call!
Problem
How can you predict the possible results of genetic crosses?
Skills Focus
making models, interpreting data
Materials
2 small paper bags
3 blue marbles
marking pen
3 white marbles
Procedure
1. Label one bag “Bag 1, Female Parent.” Label the other bag “Bag 2, Male Parent.”
Then read over Part 1, Part 2, and Part 3 of this lab. Write a prediction on another
sheet of paper about the kinds of offspring you expect from each cross.
PART 1 Crossing Two Homozygous Parents
2. Place two blue marbles in Bag 1. This pair of marbles represents the female parent’s
alleles. Use the letter B to represent the dominant allele for blue color.
3. Place two white marbles in Bag 2. Use the letter b to represent the recessive allele for
white color.
4. For Trial 1, remove one marble from Bag 1 without looking in the bag. Record the
result in your data table. Return the marble to the bag. Again, without looking in the
bag, remove one marble from Bag 2. Record the result in your data table. Return the
marble to the bag.
5. In the column labeled Offspring’s Alleles, write BB if you removed two blue
marbles, bb if you removed two white marbles, or Bb if you removed one blue
marble and one white marble.
6. Repeat Steps 4 and 5 nine more times.
PART 2 Crossing Homozygous and Heterozygous Parents
7. Place two blue marbles in Bag 1. Place one white marble and one blue marble
in Bag 2.
8. Repeat Steps 4 and 5 ten times, and record your data in the data table for Part 2.
PART 3 Crossing Two Heterozygous Parents
9. Place one blue marble and one white marble in Bag 1. Place one blue marble and one
white marble in Bag 2.
10. Repeat Steps 4 and 5 ten times, and record your data in the data table for Part 3.
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Skills Lab
Data Table: Part 1
Trial
Allele From Bag 1
(Female Parent)
Allele From Bag 2
(Male Parent)
Offspring’s Alleles
Allele From Bag 2
(Male Parent)
Offspring’s Alleles
1
2
3
4
5
6
7
8
9
10
Data Table: Part 2
Trial
Allele From Bag 1
(Female Parent)
1
2
3
4
5
6
7
8
9
10
Data Table: Part 3
Trial
Allele From Bag 1
(Female Parent)
Allele From Bag 2
(Male Parent)
Offspring’s Alleles
Genetics: The Science
of Heredity
1
2
3
4
5
6
7
8
9
10
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
Make the Right Call!
■
Skills Lab
(continued)
Analyze and Conclude
Write your answers on a separate sheet of paper.
1. Making Models Make a Punnett square for each of the crosses you
modeled in Part 1, Part 2, and Part 3.
2. Interpreting Data According to your results in Part 1, how many
different kinds of offspring are possible when the homozygous parents
(BB and bb) are crossed? Do the results you obtained using the marble
model agree with the results shown by a Punnett square?
3. Predicting According to your results in Part 2, what percentage of
offspring are likely to be homozygous when a homozygous parent (BB)
and a heterozygous parent (Bb) are crossed? What percentage of offspring
are likely to be heterozygous (Bb)? Does the model agree with the results
shown by a Punnett square?
4. Communicating According to your results in Part 3, what different
kinds of offspring are possible when two heterozygous parents (Bb × Bb)
are crossed? What percentages of each type of offspring are likely to be
produced? Does the model agree with the results of a Punnett square?
5. Inferring For Part 3, if you did 100 trials instead of 10 trials, would your
results be closer to the results shown in a Punnett square? Explain.
6. Communicating In a paragraph, explain how the marble model
compares with a Punnett square. How are the two methods alike? How
are they different?
More to Explore
In peas, the allele for yellow seeds (Y) is dominant over the allele for green
seeds (y). What possible crosses do you think could produce a heterozygous
plant with yellow seeds (Yy)? Use the marble model and Punnett squares to
test your predictions.
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Section Summary
The Cell and Inheritance
Key Concepts
■
What role do chromosomes play in inheritance?
■
What events occur during meiosis?
■
What is the relationship between chromosomes and genes?
In the early 1900s, scientists were working to identify the cell structures that
carried Mendel’s hereditary factors, or genes. In 1903, Walter Sutton
observed that sex cells in grasshoppers had half the number of chromosomes
as the body cells. He also noticed that each grasshopper offspring had
exactly the same number of chromosomes in its body cells as each of the
parents. He reasoned that the chromosomes in body cells actually occurred
in pairs, with one chromosome in each pair coming from the male and the
other coming from the female.
From his observations, Sutton concluded that genes are located on
chromosomes. He proposed the chromosome theory of inheritance.
According to the chromosome theory of inheritance, genes are carried
from parents to their offspring on chromosomes.
Organisms produce sex cells during meiosis. Meiosis is the process by
which the number of chromosomes is reduced by half to form sex cells—
sperm and eggs. During meiosis, the chromosome pairs separate and are
distributed to two different cells. The resulting sex cells have only half as
many chromosomes as the other cells in the organism. When they combine,
each sex cell contributes half the number of chromosomes to produce
offspring with the correct number of chromosomes.
Punnett squares show the results of meiosis. When chromosome pairs
separate, so do the alleles carried on the chromosomes. One allele from each
pair goes to each sex cell.
Chromosomes are made up of many genes joined together like beads
on a string. Each chromosome contains a large number of genes, each gene
controlling a particular trait. Each chromosome pair has the same genes. The
genes are lined up in the same order on both chromosomes. However, the
alleles for some of the genes might differ from each other, making the
organism heterozygous for some traits. If the alleles are the same, the
organism is homozygous for those traits.
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Guided Reading and Study
The Cell and Inheritance
This section describes how one set of chromosomes from each parent is passed on to
the offspring.
Use Target Reading Skills
As you read, identify the evidence that supports the hypothesis that genes are found
on chromosomes. Write the evidence in the graphic organizer below.
Evidence
Hypothesis
Grasshoppers: 24 chromosomes
in body cells, 12 in sex cells
Chromosomes are important
in inheritance.
Chromosomes and Inheritance
1.
Circle the letter of each sentence that is true about what Sutton observed
about chromosome number.
a. Grasshopper sex cells have half the number of chromosomes as body cells.
b. Grasshopper body cells have half the number of chromosomes as sex cells.
c. Grasshopper body cells and sex cells have the same number of chromosomes.
d. When grasshopper sex cells join, the fertilized egg has the same number of
chromosomes as the body cells of the parents.
2.
What is the chromosome theory of inheritance?
________________________________________________________________________
________________________________________________________________________
Meiosis
3.
What is meiosis?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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Genetics: The Science
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________________________________________________________________________
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Genetics: The Science of Heredity
■
Guided Reading and Study
The Cell and Inheritance (continued)
4.
Complete the cycle diagram, which describes the events that occur
during meiosis.
Parent cell has four chromosomes
pairs.
arranged in
Sex cells combine to produce offspring.
Each offspring has
chromosomes, one pair from each parent.
Chromosome pairs
and are
distributed to sex cells. Each sex cell has
chromosomes.
5.
A Punnett square is a shorthand way to show the events that occur
during ________________________.
6.
Is the following sentence true or false? During meiosis, the two alleles
for each gene stay together. ________________________
7.
If the male parent cell is heterozygous for a trait, Tt, what alleles could
the sperm cells possibly have?
________________________________________________________________________
________________________________________________________________________
A Lineup of Genes
8.
How many pairs of chromosomes do human body cells contain?
________________________
9.
How are the genes lined up in a pair of chromosomes?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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Genetics: The Science of Heredity
■
Review and Reinforce
The Cell and Inheritance
Understanding Main Ideas
Complete the table below by filling in the spaces with the correct stage of meiosis—
Beginning, Meiosis I, Meiosis II, End.
Event
Stage in Meiosis
The double-stranded chromosomes move to the
center of the cell. The centromeres separate.
1. ________________________
Two cells form, each with half the number of
chromosomes. Each chromosome still has two
chromatids.
2. ________________________
Four sex cells form with half the number of
chromosomes as the parental cells.
3. ________________________
The chromosomes are copied.
4. ________________________
Answer the following questions in the spaces provided.
5. What is the chromosome theory of inheritance?
________________________________________________________________________
________________________________________________________________________
6. Why is it important that sex cells have half the number of chromosomes
as body cells?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Building Vocabulary
Fill in the blank to complete the statement.
7. The process by which the number of chromosomes is reduced by half to
form sex cells is called ________________________.
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Genetics: The Science
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________________________________________________________________________
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Genetics: The Science of Heredity
■
Enrich
A Model of Meiosis
Follow the procedure below to make a model of meiosis.
Materials
different colors of pipe cleaners or yarn
beads
macaroni
string
glue
scissors
marker
construction paper or poster board
Procedure
1. Study the diagram of the stages of meiosis in your textbook.
2. Decide how you can use the materials listed above, or other materials of
your choice, to make a model of meiosis. Your model should include the
beginning of meiosis, meiosis I, meiosis II, and the end of meiosis.
3. Create your model. Begin with at least six copied pairs of chromosomes.
Label the stages of meiosis and all the important structures. On a separate
sheet of paper, write a description in your own words of what happens in
each stage of meiosis.
Analyze and Conclude
Answer the following questions on a separate sheet of paper.
What is meiosis?
What must happen before meiosis can begin?
What happens to chromosomes during meiosis I?
What happens to chromosomes during meiosis II?
Compare the sex cells produced by meiosis to the parent cell. Why is the
difference between the sex cells and parent cell important?
6. Why are chromosomes important to heredity?
1.
2.
3.
4.
5.
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Genetics: The Science of Heredity
■
Section Summary
The DNA Connection
Key Concepts
■
What forms the genetic code?
■
How does a cell produce proteins?
■
How can mutations affect an organism?
Today, scientists know that genes control the production of proteins in the
cells of an organism. Proteins determine the size, shape, and other traits of
organisms. Recall that chromosomes are composed mostly of DNA. A DNA
molecule is made up of four nitrogen bases—adenine (A), thymine (T),
guanine (G), and cytosine (C). The order of the nitrogen bases along a gene
forms a genetic code that specifies what type of protein will be produced.
In the genetic code, a group of three DNA bases codes for one specific amino
acid.
During protein synthesis, the cell uses information from a gene on a
chromosome to produce a specific protein. Protein synthesis occurs on the
ribosomes in the cytoplasm of the cell. DNA, however, is located in the cell
nucleus. Before protein synthesis occurs, a genetic “messenger,” called
ribonucleic acid or RNA, is made based on a code in the DNA. RNA is
similar to DNA, except RNA has only one strand and it has uracil instead of
thymine.
In the first step of protein synthesis, the DNA molecule “unzips” and
directs the production of messenger RNA. There are several types of RNA
involved in protein synthesis. Messenger RNA copies the coded message
from the DNA in the nucleus, and carries it to the ribosomes in the
cytoplasm. Transfer RNA carries amino acids and adds them to the growing
protein.
Sometimes changes called mutations occur in a gene or chromosome.
Mutations can cause a cell to produce an incorrect protein during protein
synthesis. As a result, the organism’s trait, or phenotype, may be different
from what it normally would have been. If a mutation occurs in a body cell,
the mutation affects only the cell that carries it. However, if a mutation
occurs in a sex cell, the mutation can be passed on to an offspring and affect
the offspring’s phenotype. Some mutations are the result of small changes in
an organism’s hereditary material. Others occur when chromosomes don’t
separate correctly during meiosis.
Some of the changes brought about by mutations are harmful to an
organism. A few mutations, however, are helpful, and still others are neither
harmful nor helpful. A mutation is harmful if it reduces an organism’s
chance for survival and reproduction. Whether or not a mutation is harmful
depends partly on the organism’s environment. For example, a white lemur
may not survive in the wild, but the mutation has no effect on its ability to
survive in a zoo.
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Genetics: The Science of Heredity
■
Guided Reading and Study
The DNA Connection
This section tells how the DNA molecule is related to genes, chromosomes, and the
inheritance of traits.
Use Target Reading Skills
As you read, complete the flowchart below to show protein synthesis. Put the steps
of the process in separate boxes in the flowchart in the order in which they occur.
Protein Synthesis
DNA provides code to form messenger RNA.
Messenger RNA attaches to ribosome.
The Genetic Code
Circle the letter of each sentence that is true about genes, chromosomes,
and proteins.
a. Genes control the production of proteins in an organism’s
cells.
b. Proteins help determine the size, shape, and other traits of an
organism.
c. Chromosomes are made up mostly of proteins.
d. A single gene on a chromosome contains only one pair of
nitrogen bases.
2.
What are the four nitrogen bases that make up a DNA molecule?
________________________________________________________________________
________________________________________________________________________
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1.
Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
The DNA Connection
3.
■
Guided Reading and Study
(continued)
What is the genetic code?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
4.
One group of three nitrogen bases codes for one ________________________.
How Cells Make Proteins
5.
During protein synthesis, the cell uses information from a
________________________ on a chromosome to produce a specific
________________________.
6.
Proteins are made on ________________________ in the cytoplasm of the cell.
7.
Complete this Venn diagram to show some of the similarities and
differences between DNA and RNA. Tell where each nucleic acid is
located and what bases it contains.
RNA
DNA
Stays inside
the nucleus
Adenine
Guanine
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Genetics: The Science of Heredity
■
Guided Reading and Study
8. List the two kinds of RNA and describe their jobs.
a. _____________________________________________________________________
________________________________________________________________________
________________________________________________________________________
b. _____________________________________________________________________
________________________________________________________________________
________________________________________________________________________
9. Circle the letter of the first step in protein synthesis.
a. Transfer RNA carries amino acids to the ribosome.
b. The ribosome releases the completed protein chain.
c. Messenger RNA enters the cytoplasm and attaches to a ribosome.
d. DNA “unzips” to direct the production of a strand of messenger RNA.
10. Circle the letter of the last step in protein synthesis.
a. Transfer RNA carries amino acids to the ribosome.
b. The protein chain grows longer as each transfer RNA molecule
adds an amino acid.
c. Messenger RNA enters the cytoplasm and attaches to a ribosome.
d. DNA “unzips” to direct the production of a strand of messenger RNA.
Mutations
11. What is a mutation?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
12. How can mutations affect protein synthesis in cells?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Genetics: The Science
of Heredity
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Genetics: The Science of Heredity
■
Guided Reading and Study
The DNA Connection (continued)
13. Circle the letter of each sentence that is true about mutations.
a. Cells with mutations will always make normal proteins.
b. Some mutations occur when one nitrogen base is substituted for another.
c. Some mutations occur when chromosomes don’t separate correctly during
meiosis.
d. Mutations that occur in a body cell can be passed on to an offspring.
14. Mutations can be a source of genetic ________________________.
15. Is the following sentence true or false? All mutations are harmful.
________________________
16. Mutations that are ________________________ improve an organism’s
chances for survival and reproduction.
17. Whether a mutation is harmful or helpful depends partly on an
organism’s ________________________.
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Genetics: The Science of Heredity
■
Review and Reinforce
The DNA Connection
Understanding Main Ideas
Complete the table below by stating whether each mutation is helpful, harmful, or neutral
to the organism.
Mutation
Effect
White lemur (in a zoo)
1. ________________________
White lemur (in the wild)
2. ________________________
Antibiotic resistance in bacteria
3. ________________________
Answer the following questions on the lines provided.
4. Describe what occurs during protein synthesis.
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
5. What is the genetic code?
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
Building Vocabulary
Fill in the blank to complete each statement.
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Genetics: The Science
of Heredity
6. A ________________________ is any change in a gene or chromosome.
7. A type of RNA that carries amino acids and adds them to the growing
protein is called ________________________ .
8. ________________________ is RNA that copies the coded message from
the DNA in the nucleus and carries the message into the cytoplasm.
Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Enrich
The Genetic Code
The genetic code is made up of groups of three nitrogen bases in the
messenger RNA. Each three-base group, called a codon, codes for one amino
acid. The table below shows the genetic code. To find the amino acid that is
coded for by the codon UGG in messenger RNA, look in the row of the first
base in the codon—U. Then move to the box that is specified by the second
base in the codon—G. Finally, look down the list of amino acids in the box
until you find the one in row “G,” the third base in the codon. You should
find that UGG is the codon for tryptophan.
Transfer RNA matches up with the messenger RNA at the ribosome to
deliver the correct amino acid to the growing protein chain. Transfer RNA
has a three-base code called an anticodon that matches up with the codon in
the messenger RNA.
Answer the following questions on a separate sheet of paper.
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Third Base in Codon
First Base in Codon
The Genetic Code (messenger RNA)
1. If the DNA
sequence of
Lysine
Arginine
Isoleucine
Threonine
A
a gene was
Lysine
Arginine
Methionine
Threonine
G
TACTTACCGAGC
Serine
Isoleucine
Threonine
A Asparagine
U
Asparagine
Serine
Isoleucine
Threonine
TAGACT, then
C
what is the
sequence of the
Glutamic acid Glycine
Valine
Alanine
A
messenger RNA?
Glutamic acid Glycine
Valine
Alanine
G
Valine
Alanine
G Aspartic acid Glycine
2. Use the genetic
U
Aspartic acid Glycine
Valine
Alanine
code to identify
C
the sequence of
"Stop" codon "Stop" codon Leucine
Serine
amino acids
A
"Stop" codon Tryptophan
Leucine
Serine
encoded by the
G
Cysteine
Phenylanaline Serine
U Tyrosine
U
messenger RNA
Tyrosine
Cysteine
Phenylanaline Serine
C
that you identified
in Question 1.
Glutamine
Arginine
Leucine
Proline
A
3. What are the
Glutamine
Arginine
Leucine
Proline
G
sequences of the
Arginine
Leucine
Proline
C Histidine
U
Histidine
Arginine
Leucine
Proline
anticodons for the
C
transfer RNA
G
A
U
C
molecules that
carry each of the
Second Base in Codon
amino acids in the
protein sequence that you identified in Question 2?
4. How would the protein change if a mutation caused a base to be added, making the
mutated DNA sequence TACGTTACCGAGCTAGACT? How is the protein affected
by this mutation? (Hint: How does the extra letter change the series of bases?)
5. How would the protein change if a mutation caused one base to replace another,
making the mutated DNA sequence TACTTACCTAGCTAGACT? How does this
mutation affect protein function?
Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Key Terms
Key Terms
Use the clues below to identify Key Terms from the chapter. Write the terms below,
putting one letter in each blank. When you finish, the word enclosed in the diagonal
lines will reveal what Mendel studied.
Clues
6. Number that describes the likelihood
1. The process by which the number of
that a certain event will occur
chromosomes is reduced by half in
sex cells
7. An allele whose trait always shows
up in the organism when the allele
2. A chart that shows all possible allele
is present
combinations resulting from a genetic
cross
8. Physical characteristic of an organism
3. An organism’s physical appearance
9. A factor that controls a trait
4. RNA that copies the coded message
10. The scientific study of heredity
in DNA
11. Offspring of many generations that
5. Describes an organism that has two
have the same trait
different alleles for a trait
1. ___ ___ ___ ___ ___ ___ ___
2. ___ ___ ___ ___ ___ ___ ___
3.
4.
5.
___ ___ ___ ___ ___ ___
___ ___ ___ ___ ___ ___ ___ ___ ___
___ ___ ___ ___ ___ ___ ___ ___ ___
___ ___ ___
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
6.
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
7.
___ ___ ___ ___ ___ ___ ___ ___
___ ___ ___ ___ ___ ___
___ ___ ___ ___ ___
9.
___ ___ ___ ___
10.
11.
___ ___ ___ ___ ___ ___ ___ ___
___ ___ ___ ___ ___ ___ ___ ___
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Genetics: The Science
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8.
Name ____________________________ Date ____________________ Class ____________
Connecting Concepts
■
Genetics: The Science of Heredity
Connecting Concepts
Develop a concept map that uses the Key Concepts and Key Terms from this
chapter. Keep in mind the big ideas of this chapter. The concept map shown
is one way to organize how the information in this chapter is related. You
may use an extra sheet of paper.
is the scientific
study of
which is the
passing of
traits
from parent
to offspring
was revolutionized
by
Mendel
who experimented
with
by setting up crosses
between plants that are
hybrid
which have which have
the same two different
alleles
for a trait
Genetics
is related to the
math principle of
which can be used to
predict inheritance of
traits using a
Punnett square
which shows all the
possible combinations of
that can result from
a genetic cross
is determined by the
coding in the
DNA
which is
copied by
messenger
RNA
which is
read by
which links
to make
proteins
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Genetics: The Science of Heredity
Laboratory Investigation
TEACHER NOTES
Chromosomes and Inheritance
Key Concept
Teaching Tips
The traits of offspring are determined by the
alleles of the parents.
■
Skills Focus
observing, making models
■
Time
40 minutes
Materials (per group)
marker
craft stick
Alternate Materials: If craft sticks are unavailable, strips of cardboard or acetate could be
used. Using clear, colorless acetate strips for
recessive alleles and colored strips for dominant
alleles can be helpful.
■
(Step 2) Remind students that scientists
generally represent a dominant allele
with a capital letter, often the initial of the
word that best describes the trait, and the
recessive allele with the corresponding
lowercase letter.
(More to Explore) Remind students that
Punnett square results show only probabilities. The traits of offspring from actual matings may not match the predictions.
(More to Explore) Based on the principles of
probability, two of four, or 50%, of the young
unimonsters would have curly hair (Cc), and
two of four, or 50%, of the young unimonsters would have straight hair (cc).
Advance Preparation
Get enough craft sticks so that you have four for
each group of students (eight if students do the
More to Explore).
Genetics: The Science
of Heredity
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Laboratory Investigation
Chromosomes and Inheritance
Pre-Lab Discussion
How are traits inherited? You can investigate this question by considering an
imaginary animal called the unimonster. Suppose this animal has only one
pair of chromosomes. Chromosomes carry genes, which control different
genetic traits, such as hair color, height, and other physical characteristics.
Different forms of a gene are called alleles. The presence of different alleles
on the chromosomes of unimonsters determines whether they have one
horn or two horns. During reproduction, parent unimonsters pass on alleles
to their offspring.
In this investigation, you will determine the different allele combinations for
the offspring of two unimonsters and figure out the number of horns the
young unimonsters will have.
1. What are dominant and recessive alleles?
________________________________________________________________________
________________________________________________________________________
2. Define genotype and phenotype.
________________________________________________________________________
________________________________________________________________________
3. What does it mean to say that an organism is homozygous for a trait?
Heterozygous for a trait?
________________________________________________________________________
________________________________________________________________________
4. How do the numbers of chromosomes in cells compare with the number
of chromosomes in sex cells? During reproduction, what fraction of
chromosomes does each parent contribute to its offspring?
________________________________________________________________________
________________________________________________________________________
Problem
How can you determine the traits of a unimonster ’s offspring?
Materials (per group)
marker
craft sticks
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Laboratory Investigation
Procedure
1. Figure 1 shows a mother and a father
unimonster, each with different genetic
traits. The allele for two horns is
dominant over the allele for one horn.
Look at the drawing and answer
question 1 in Observations.
2. The mother unimonster is heterozygous.
This means that she has one allele for two
horns and one allele for one horn. Each Mother Unimonster
Father Unimonster
of her sex cells will have either a
Figure 1
chromosome with the two-horn allele or a
chromosome with the one-horn allele. Follow Figure 2
and steps 3 and 4 to make a model of the mother unimonster ’s sex
chromosomes.
3. One of the mother unimonster ’s chromosomes will
carry the two-horn allele. Write "M1" (for mother) at
one end of a craft stick. At the other end of the stick,
write H for the dominant two-horn allele.
4. The mother unimonster ’s other chromosome will
carry the one-horn allele. Write "M2" at the end of a
second stick. At the other end, write h for the recessive
one-horn allele.
Figure 2
5. The father unimonster is homozygous (hh). Follow Figure
2 to make models of the father ’s chromosomes: F1 and F2.
6. During reproduction, the sex cells produced by the mother and father
unimonsters combine to form a fertilized egg. The fertilized egg will
grow into a young unimonster. Whether the young unimonster has one
or two horns depends on the alleles on the chromosome contributed by
each parent during reproduction. In Observations, use your chromosome
models to answer questions 2–5. Remember that the allele for two horns
is dominant. Anytime the dominant allele (H) is present, the unimonster
will have two horns.
Genetics: The Science
of Heredity
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Laboratory Investigation
Chromosomes and Inheritance
(continued)
Observations
1. Which unimonster parent has the dominant allele for number of horns?
How do you know?
________________________________________________________________________
________________________________________________________________________
2. During reproduction, the sex cells containing the chromosomes M1 and
F1 combine to form a fertilized egg.
a. Which alleles are on each of the chromosomes?
________________________________________________________________________
b. Will the young unimonster have one horn or two horns? Draw the
appropriate number of horns on young unimonster 1 in Figure 3.
3. During reproduction, the sex cells containing the chromosomes M1 and
F2 combine to form a fertilized egg.
a. Which alleles are on each of the chromosomes?
________________________________________________________________________
b. Will the young unimonster have one horn or two horns? Draw the
appropriate number of horns on young unimonster 2 in Figure 3.
4. During reproduction, the sex cells containing the chromosomes M2 and
F1 combine to form a fertilized egg.
a. Which alleles are on each of the chromosomes?
________________________________________________________________________
b. Will the young unimonster have one horn or two horns? Draw the
appropriate number of horns on young unimonster 3 in Figure 3.
5. During reproduction, the sex cells containing the chromosomes M2 and
F2 combine to form a fertilized egg.
a. Which alleles are on each of the chromosomes?
________________________________________________________________________
b. Will the young unimonster have one horn or two horns? Draw the
appropriate number of horns on young unimonster 4 in Figure 3.
Young Unimonster 1
Young Unimonster 2
Young Unimonster 3
Young Unimonster 4
Figure 3
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Name ____________________________ Date ____________________ Class ____________
Genetics: The Science of Heredity
■
Laboratory Investigation
Analyze and Conclude
1. Which young unimonster(s) are homozygous and have one horn?
________________________________________________________________________
________________________________________________________________________
2. Which young unimonster(s) are heterozygous?
________________________________________________________________________
________________________________________________________________________
3. Are any young unimonster(s) homozygous with two horns? Explain.
________________________________________________________________________
________________________________________________________________________
Critical Thinking and Applications
1. If a mother unimonster is homozygous and has two horns, and a father
unimonster is homozygous and has one horn, what are the phenotypes
and genotypes of the possible offspring? Remember that the two-horn
allele is dominant.
________________________________________________________________________
________________________________________________________________________
2. Predict the phenotypes and genotypes of the offspring of a mother
unimonster and a father unimonster that are both heterozygous.
________________________________________________________________________
________________________________________________________________________
More to Explore
Repeat the lab for the traits of curly hair versus straight hair. Assume that the
curly-hair allele is dominant and the straight-hair allele is recessive. The
mother is homozygous and has straight hair, while the father is
heterozygous. Get four more craft sticks. Make all the combinations of
different alleles. Determine all of the possible genotypes and the resulting
phenotypes of the offspring. You may wish to use the Punnett square below
to record the genotypes.
C
c
c
c
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Genetics: The Science
of Heredity
Mother’s Alleles for
Curly Hair
Father’s Alleles
for Curly Hair