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SKILL: DETERMINING RELEVANCE
Are Traits Linked?
1.5
3.0
(white eyes)
(notch wings)
Distance
between
genes on
this map
27.5
(tan body)
Fruit flies are easy to breed in a laboratory and have an assortment
of easily recognized genetic traits—different eye colors, body patterns, limb characteristics, and wing shapes. For these reasons, early
geneticists studied fruit flies to learn how certain traits were
inherited. Sometimes the experiments produced puzzling results.
Here is an example from the laboratory of Thomas Hunt Morgan.
Observations
• In a batch of fruit flies, most red-eyed individuals were
born with short wings.
• In the same batch, at least one fruit fly was born with
red eyes and normal-sized wings.
56.7
(forked bristles)
Hypotheses
Morgan and his coworkers made these hypotheses about the
inherited traits:
Genes have a particular
location on a chromosome.
A gene map shows the
location.
• The gene for red eyes and the gene for short wings
are linked together on a fruit fly’s chromosomes. These
linked genes are usually inherited together.
• Sometimes during meiosis, one of the linked genes will
“cross over” from one chromosome to a homologous
one. When this happens, a fruit fly will be born with
one but not both of the linked genes—red eyes without
short wings.
• Genes that are farthest from each other on a chromosome are most likely to become separated and cross
over during meiosis. Genes that are closest (linked) to
each other are least likely to.
Further Discoveries
By studying the results of many breeding experiments, Morgan and
his student, Alfred Sturtevant, could determine which genes were
closest and farthest from each other on the same chromosome.
From this information, they drew a simple map showing the
location of each of the fruit fly’s linked genes.
Determine Relevance
On Your Own Look at the map of a chromosome on this page.
Which of the traits are most likely to be inherited together? Which
might be most easily separated and cross over during meiosis?
As a Group Is it reasonable to think that information about a fruit
fly’s genes could apply to the genes of a human being? Discuss this
topic in a small group and see if the group can agree.
Chapter 4: Patterns of Heredity 123
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Chapter Review
In sexual reproduction, genes are passed from
parents to offspring in predictable patterns.
CONTENT REVIEW
CLASSZONE.COM
KEY CONCEPTS SUMMARY
1
alleles
Living things inherit traits
in patterns.
Offspring inherit alleles, which are forms of genes,
from their parents. Alleles can be dominant or
recessive. The alleles you have are your genotype;
the observable characteristics that come from your
genotype are your phenotype.
a gene
2
A
a
B
B
c
C
d
d
E
e
F
F
g
G
h
H
sexual reproduction
p. 102
gene p. 102
heredity p. 102
allele p. 103
phenotype p. 106
genotype p. 106
dominant p. 107
recessive p. 107
Patterns of heredity can be predicted.
Punnett squares show
possible outcomes of
heredity. Ratios and
percentages can be used
with Punnett squares to
express the probability
of particular outcomes.
B
b
BB
Bb
Bb
bb
VOCABULARY
VOCABULARY
B = black fur
b = brown fur
Punnett square p. 110
ratio p. 112
probability p. 112
percentage p. 112
B
b
3
Meiosis is a special form of cell division.
VOCABULARY
• At the beginning of meiosis I,
the parent cell has two copies
of each chromosome pair.
• During meiosis I, the
homologs of the chromosome
pair separate; there are
two cells, each with two
copies of one homolog
from each pair.
• During meiosis II, the two copies
of each homolog separate; each
daughter cell has one homolog.
gamete p. 118
egg p. 118
sperm p. 118
fertilization p. 118
meiosis p. 119
124 Unit 1: Cells and Heredity
Meiosis I
Meiosis II
1n
1n
1n
1n
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Reviewing Vocabulary
Make a frame for each of the vocabulary
terms listed below. Write the term in the center. Think about how each term is related to
the Big Idea of the chapter. Decide what information to frame it with. Use definitions,
examples, descriptions, parts, or pictures.
1. allele
3. ratio
2. heredity
4. probability
Describe how the vocabulary terms in the following pairs of words are related to each
other. Explain the relationship in a one- or
two-sentence answer. Underline each vocabulary word or term in your answers.
11. In guinea pigs, the allele for black fur (B) is
dominant, and the allele for brown fur (b) is
recessive. If a BB male mates with a Bb female,
what percentage of offspring are likely to have
black fur?
a. 100 percent
c. 50 percent
b. 75 percent
d. 25 percent
12. If one parent has two dominant alleles and
another parent has two recessive alleles, the
offspring will have
a. the recessive phenotype
b. the dominant phenotype
c. two dominant alleles
d. two recessive alleles
13. Cells that contain half the usual number of
chromosomes are
a. fertilized egg cells c. alleles
b. gametes
d. diploid cells
5. phenotype, genotype
6. dominant, recessive
Reviewing Key Concepts
Multiple Choice Choose the letter of the best
answer.
7. Which is an example of an acquired trait?
a. eye color
c. blood type
b. hair color
d. ability to read
8. The unit of heredity that determines a particular trait is known as
a. a chromosome
c. a gene
b. a gamete
d. a phenotype
9. A human female would have which set of sex
chromosomes?
a. XX
c. XY
b. YY
d. XxYy
10. If one copy of a dominant allele is present in a
genotype, then the trait the allele codes for is
a. expressed in the phenotype
b. not expressed in the phenotype
c. partially expressed in the phenotype
d. not expressed in an offspring’s phenotype
14. The process that produces haploid (1n) cells is
known as
a. mitosis
c. meiosis
b. reproduction
d. fertilization
15. What happens when fertilization occurs?
a. Two 2n cells combine in a new cell.
b. Two 1n cells combine into a new cell.
c. Two 2n daughter cells are produced.
d. Two 1n daughter cells are produced.
16. Which does not occur during meiosis?
a. Four haploid daughter cells are produced.
b. Two diploid daughter cells are produced.
c. Only cells that are gametes are produced.
d. Daughter cells are produced that contain
half the chromosomes of the parent cell.
Short Answer Write a short answer to each
question.
17. In what case would a recessive allele be
expressed in the phenotype of an offspring?
18. Describe the purpose of a Punnett square.
19. How does the number of chromosomes in a
person’s sex cells compare with the number of
chromosomes in the body cells?
Chapter 4: Patterns of Heredity 125
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Thinking Critically
20. INFER How was Mendel able to infer that each
offspring of two parent pea plants had a pair
of “factors” for a particular trait?
27. ANALYZE This diagram shows the process of
fertilization. Which of the cells shown are
haploid? Explain.
egg cell
(female gamete)
21. COMMUNICATE Briefly describe how heredity
works. Use the terms gene and chromosome
in your explanation.
22. APPLY Can a dwarf pea plant ever have a
dominant allele? Explain.
23. ANALYZE How is a Punnett Square used to
show both the genotype and phenotype of
both parents and offspring?
24. APPLY In rabbits, the allele for black fur is
dominant over the allele for white fur. Two
black rabbits have a litter of eight offspring.
Six of the offspring have black hair and two
have white hair. What are the genotypes of
the parents? Explain.
fertilized
egg cell
1n
fertilization
sperm cell
(male gamete)
2n
1n
28. SUMMARIZE Briefly describe what happens
during meiosis I and meiosis II. What is the
function of meiosis?
Use the Punnett square below to answer
the next two questions.
B
b
b
Bb
bb
b
Bb
bb
25. CALCULATE A parent has one dominant allele
for black fur (B) and one recessive allele for
white fur (b). The other parent has two recessive alleles for white fur. In this cross what is
the chance that an offspring will be born with
black fur? With white fur?
29. INFER Look again at the picture on pages
98–99. Now that you have finished the chapter, how would you change or add details to
your answer to the question on the photograph?
30. SYNTHESIZE Write one or more paragraphs
explaining how Mendel’s observations of pea
plants contributed to the study of modern
genetics. Use these terms in your explanation.
gene
allele
trait
genotype
phenotype
dominant
recessive
26. CALCULATE What is the percentage chance
that an offspring will have the recessive phenotype?
If you need to create graphs or other visuals for
your project, be sure you have grid paper, poster
board, markers, or other supplies.
126 Unit 1: Cells and Heredity
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Standardized Test Practice
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TEST PRACTICE
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Analyzing data
The chart below shows the phenotypes of pea-plant offspring.
Phenotypes of Pea Plants
Phenotype
Number of
Offspring
Regular (D)
12
Dwarf (d)
4
Use the chart to answer the questions below.
1. What percentage of pea plants showed the
dominant phenotype?
a. 100 percent
b. 75 percent
c. 50 percent
d. 25 percent
2. What percentage of pea plants showed the
recessive phenotype?
a. 100 percent
b. 75 percent
c. 50 percent
d. 25 percent
3. What is the genotype of the dwarf pea plants?
a. DD
b. Dd
c. dd
d. cannot tell
4. What are the possible genotypes of the regular
pea plants?
a. DD and dd
b. DD and Dd
c. Dd and dd
d. cannot tell
5. What are the genotypes of the parents?
a. Dd and dd
b. DD and Dd
c. Dd and Dd
d. dd and dd
6. Which statement is true, based on the data in the
chart?
a. If both parents were Dd, then none of the
offspring would be dwarf.
b. If both parents were DD, then none of the
offspring would be dwarf.
c. If one parent were Dd and the other were dd,
then none of the offspring would be regular.
d. If one parent were DD and the other parent
were dd, then none of the offspring would be
regular.
Extended Response
7. Traits for a widow’s peak hairline (W) and curly hair
(C) are controlled by dominant alleles. A family of
eight has three children with widow’s peaks. All six
children have curly hair. Use your knowledge of
heredity to write one or two paragraphs explaining
the possible genotypes of the parents.
8. A student proposes a hypothesis that traits that
are dominant are more common in the general
population than traits with recessive alleles.
Describe a procedure you might use to test this
hypothesis.
Chapter 4: Patterns of Heredity 127