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Chapter 7 Study Guide
Section 7.1 Chromosomes and Phenotype
Key Concept: The chromosomes on which genes are located can affect the expression of
traits.
Vocabulary
carrier
sex-linked gene
X chromosome inactivation
Main Idea: Two copies of each autosomal gene affect phenotype.
1. What are sex chromosomes?
Chromosomes that determine an organism’s sex.
2. What are autosomes?
The other chromosomes that don’t directly affect the sex.
3. How is a carrier different from a person who has a genetic disorder?
A carrier is heterozygous, so the disorder is masked. A person who has the
disorder is homozygous recessive.
4. Complete the two Punnett squares below to compare autosomal recessive disorders
with autosomal dominant disorders. Fill in the possible genotypes for offspring, and
write in the phenotype (no disorder, carrier, or disorder) for each.
A
u
t
o
s
o
m
a
l
R
e
c
e
s
si
v
e
Autosomal
Dominant
D
D
DD
d
Dd
d
Dd
dd
A
ut
o
s
o
m
al
R
e
c
e
s
si
v
e
Autosomal
Dominant
D
D
DD
d
Dd
d
Dd
dd
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Autosomal Recessive: DD, no disorder; Dd, carrier;
Dd, carrier; dd, disorder.
Autosomal Dominant:
DD, disorder; Dd, disorder; Dd, disorder; dd, no disorder
Main Idea: Males and females can differ in sex-linked traits.
5.
6.
What are sex-linked genes?
Genes located on the sex chromosomes.
Fill in the Punnett square below to show the pattern of inheritance for sex
chromosomes.
S
e
x
C
h
r
o
m
o
s
o
m
e
I
n
h
e
ri
t
a
n
c
e
X
X
XX
X
XX
Y
XY
XY
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Sex Chromosome Inheritance: XX, female; XX, female; XY, male; XY, male
7.
8.
9.
In humans, how does a gamete from a male determine the sex of offspring?
Because there is only one copy of each gene, all traits are expressed.
For what are genes on the Y chromosome responsible?
Development of the male offspring.
How are sex-linked genes expressed differently in the phenotypes of males and
females? Females are XX so there is another copy of each gene to mask certain
things, while males have XY so nothing is masked by a dominant allele.
Vocabulary Check
10. The verb carry means “to transport.” How is the everyday meaning of carry related to
the meaning of the term carrier in genetics? Carrier do not show the trait but
transport it to the next generation.
11. What is X chromosome inactivation?
When one X chromosome randomly turns off.
Section 7.2 Complex Patterns of Inheritance
Key Concept: Phenotype is affected by many different factors.
Vocabulary
incomplete dominance
codominance
polygenic trait
Main Idea: Phenotype can depend on interactions of alleles.
1.
2.
3.
How is incomplete dominance different from a dominant and recessive relationship?
Instead of having one or the other color, you have an in between color.
How is codominance different from a dominant and recessive relationship?
Instead of being one color or a mixture of colors, both colors are expressed fully
and separately.
What is a multiple-allele trait? A trait that is determined by more than one pair of
alleles.
For questions 4–7, complete the table below. Describe how phenotypes appear in
incomplete dominance and codominance. Then describe an example of each.
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Interaction
Phenotype
Example
Incomplete
dominance
4. A Mixture
5. Pink and White can make
Pink.
Codominance
6. Patches
7. Calico cats have patches
of black and orange.
blood type (A, B, AB)
Main Idea: Many genes may interact to produce one trait.
Take notes on polygenic traits and epistasis below.
8.
Polygenic Traits: Polygenic traits are produced by two or more genes.
9.
Epistasis: A single gene that interferes with expression of other genes.
Main Idea: The environment interacts with genotype.
10. Why is genotype not the only factor that affects phenotype? Because the
environment can cause changes to ones appearance.
11. List and explain two examples of how environment and genotype can interact.
Turtle eggs in warmer climates are female. One twin may be taller than the other
because of nutrients.
Vocabulary Check
12. The prefix in- means “not.” How is the meaning of this prefix related to the meaning of
incomplete dominance? The trait is not completely dominant.
13. The prefix co- means “together.” How is the meaning of this prefix related to the
meaning of codominance? The traits are dominant together.
14. The prefix poly- means “many,” and the term genic means “related to genes.” How do
these word parts combine to give the meaning of polygenic? It means traits that are
related to many genes.
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Section 7.3 Gene Linkage and Mapping
Key Concept: Genes can be mapped to specific locations on chromosomes.
Vocabulary
linkage map
Main Idea: Gene linkage was explained through fruit flies.
1.
What is gene linkage?
When certain traits are inherited together.
2.
Why were fruit flies useful in Morgan’s research?
They were cheap and grew quickly.
What is the difference between a wild type and a mutant type?
The mutant type has white eyes, no wings and a different body color, compared to
the wild type having red eyes, wings and a brown body.
What did Morgan conclude from his research on fruit flies?
Linked genes were on the same chromosomes and that chromosomes were
inherited separately. Also, homologous chromosomes exchange genes.
Complete the chart below to take notes about the discovery of gene linkage. The first
box has been done for you.
3.
4.
5.
Mendel
Punnett, Bateson
Morgan
Genes assort
independently of one
another.
Some genes are linked,
so some are inherited
together.
Chromosomes are
inherited separately
during meiosis. But,
genes are not inherited
together every time.
Main Idea: Linkage maps estimate distances between genes.
6.
How is the distance between two genes related to the chance they are inherited
together? The closer they are, more likely they will be inherited together. The
farther apart they are, the more likely they will be separated during meiosis.
7.
What hypothesis was the basis of Sturtevant’s research? Frequency of cross-over
during meiosis was related to distance of genes.
8.
What is a linkage map? The relative location, or loci, of genes on a chromosome.
9.
How are cross-over frequencies related to linkage maps? The higher the cross-over
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frequency, the farther away the genes are on the linkage map.
10. What do linkage maps show about genes on a chromosome? The order and relative
position of those genes.
Section 7.4 Human Genetics and Pedigrees
Key Concept: A combination of methods is used to study human genetics.
Vocabulary
pedigree
karyotype
Main Idea: Human genetics follows the patterns seen in other organisms.
1.
How does genetic inheritance follow similar patterns in all sexually reproducing
organisms? A child gets half of their chromosomes from their mother and half from
their father.
chromosomes assort independently during meiosis; relationships among alleles are the same
2.
How are single-gene traits useful in studying human genetics?
You can easily find out which traits are dominant or recessive.
inheritance is straight-forward
Main Idea: Females can carry sex-linked genetic disorders.
3.
Who can be carriers of autosomal disorders? Males and Females.
4.
Why can females, but not males, be carriers of sex-linked genetic disorders?
Many disorders are caused by genes on the X chromosome.
Main Idea: A pedigree is a chart for tracing genes in a family.
5.
What is a pedigree? A chart that can help trace phenotypes and genotypes in a
family to determine who carries recessive alleles.
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6.
How are phenotypes used in pedigree analysis? The phenotypes help figure out
possible genotypes.
7.
What information on a pedigree can tell you whether a gene is on an autosome or on a
sex chromosome? The number of males and females that have that gene.
8.
Complete the two charts below to follow the logic necessary to fill out a pedigree for a
sex-linked gene. Use XD and Xd for the dominant and recessive X-linked genes,
respectively.
Tracing Sex-Linked Genes
Phenotype →
Female, recessive phenotype
Male, recessive phenotype
Female, dominant phenotype
Parental
Phenotype →
Female carrier, normal
male
must have
→
must have →
must have
→
Genotype
must have
→
XDXd
must have
→
Parental
Genotype
→
XdXd
XdY
could have
→
Offspring
Genotype
must have
→
XDXd, XDY
could have
→
XDXD, XDXd,
XDY, XdY
Female carrier,
male with recessive
phenotype
must have
→
XDXd, XdY
could have
→
XDXd, XdXd,
XDY, XdY
Female with recessive
phenotype, normal
male
must have
→
XdXd, XDY
could have
→
XDXd, XDXd,
XdY, XdY
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Female with recessive
phenotype, male with
recessive phenotype
must have
→
XdXd, XdY
could have
→
XdXd, XdXd,
XdY, XdY
Main Idea: Several methods help map human chromosomes.
9.
What are two methods that are used to directly study human chromosomes?
Pedigree and Karyotype. . Genetic Screening
10. What does a karyotype show about chromosomes?
Changes in chromosomes such as loss or gain of part of a chromosome.
Vocabulary Check
11. What is a karyotype?
It shows a picture of the chromosomes in a cell.
Study Guide Questions Copyright © McDougal Littell/Houghton Mifflin Company
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