Raven/Johnson Biology 8e
Chapter 13
1. Why is the white-eye phenotype always observed in males carrying the white-eye allele?
a. Because the trait is dominant
b. Because the trait is recessive
c. Because the allele is located on the X chromosome and males only have one X
d. Because the allele is located on the Y chromosome and only males have Y
chromosomes
The correct answer is c—
A. Answer a is incorrect. The white-eye gene must be recessive since heterozygous females
have red eyes.
The correct answer is c—
B. Answer b is incorrect. The white-eye gene is recessive; however, this does not explain
why it is always observed in males.
The correct answer is c—Because the allele is located on the X chromosome and males only have
one X
C. Answer c is correct. Because the gene is located on the X chromosome, and the Y
chromosome lacks most X chromosome genes, any male carrying the white-eye allele will
show the phenotype.
The correct answer is c—
D. Answer d is incorrect. The allele is located on the X chromosome as demonstrated by the
testcross. The Y chromosome does not carry many genes that are expressed.
2. An autosome is a chromosome that—
a. contains genetic information to determine the sex of an organism
b. determines all other traits of an organism other than sex
c. is only inherited from the mother (maternal inheritance)
d. has no matching chromosome within an organism’s genome
The correct answer is b—
A. Answer a is incorrect. The sex of an organism is determined by the sex chromosomes.
The correct answer is b—determines all other traits of an organisms other than sex
B. Answer b is correct. Autosomes are the chromosomes that carry the majority of genetic
information that determines the overall phenotype of an organism. The sex chromosomes,
in contrast, determine the sex of the organism.
The correct answer is b—
C. Answer c is incorrect. The autosomes are inherited from both parents.
The correct answer is b—
D. Answer d is incorrect. The autosomes exist as homologous pairs.
3. Sex linkage in humans occurs when—
a. an allele is located on both the X and Y chromosomes
b. all allele is located on the X chromosome
c. an allele is located on an autosome
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d. a phenotype is only observed in females
The correct answer is b—
A. Answer a is incorrect. An allele is sex-linked if it is present on only one of the sex
chromosomes. Few of the genes found on the Y chromosome are expressed.
The correct answer is b—an allele is located on the X chromosome
B. Answer b is correct. The phenotype of genes present on the X chromosome will be
observed in human males since there is not a second X available. Males cannot be
heterozygous for a recessive allele located on the X chromosome.
The correct answer is b—
C. Answer c is incorrect. Autosomal inheritance is the usual mode of inheritance and does
not include sex-linked traits.
The correct answer is b—
D. Answer d is incorrect. Female humans are XX. There are two alleles for any gene present
on the X chromosome; this will prevent the expression of a recessive phenotype in
heterozygous individuals.
4. What are Barr bodies?
a. Barr bodies are X chromosomes inactivated to prevent overexpression of the
alleles found on the X chromosome in females.
b. Barr bodies are highly condensed Y chromosomes in males.
c. Barr bodies are X chromosomes inactivated to allow for expression of the malesspecific phenotype.
d. Barr bodies are inactive autosomal chromosomes specific to females.
The correct answer is a—Barr bodies are X chromosomes inactivated to prevent overexpression
of the alleles found on the X chromosome in females.
A. Answer a is correct. Inactivation of one X chromosome in females allows for dosage
compensation.
The correct answer is a—
B. Answer b is incorrect. Although Y chromosomes are highly condensed, a Barr body is an
inactivated X chromosome.
The correct answer is a—
C. Answer c is incorrect. X inactivation only occurs in females to allow for dosage
compensation.
The correct answer is a—
D. Answer d is incorrect. Inactivation only applies to the X chromosome, not to autosomal
chromosomes.
5. How does maternal inheritance of mitochondrial genes differ from sex linkage?
a. Mitochondrial genes do not contribute to the phenotype of an individual.
b. Because mitochondria are inherited from the mother, only females are affected.
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Chapter 13
c. Since mitochondria are inherited from the mother, females and males are equally
affected.
d. Mitochondrial genes must be dominant. Sex-linked traits are typically recessive.
The correct answer is c—
A. Answer a is incorrect. Mitochondrial genes influence the phenotype of an individual since
they can alter the function of the individual’s mitochondria.
The correct answer is c—
B. Answer b is incorrect. Maternal inheritance means that both males and females receive
mitochondria from their mothers.
The correct answer is c—Since mitochondria are inherited from the mother, females and males
are equally affected.
C. Answer c is correct. The mitochondria found in the fertilized egg become the
mitochondria for the new individual, independent of gender.
The correct answer is c—
D. Answer d is incorrect. The idea of dominant and recessive does not apply in this case
since there are no “homologous” mitochondria.
6. What cellular process is responsible for genetic recombination?
a. Independent assortment
b. Separation of the homologues in meiosis 1
c. Separation of the chromatids during meiosis II
d. Crossing over
The correct answer is d—
A. Answer a is incorrect. Independent assortment refers to the distribution of homologous
chromosomes during meiosis. It does not result in genetic recombination.
The correct answer is d—
B. Answer b is incorrect. Genetic recombination refers to changes in the genes present on a
single chromosome.
The correct answer is d—
C. Answer c is incorrect. Anaphase of meiosis II does not alter the genetic makeup of the
sister chromatids.
The correct answer is d—Crossing over
D. Answer d is correct. Crossing over results in a physical change in the genes present on a
chromosome.
7. The number of map units between two genes is determined by—
a. the recombination frequency
b. the frequency of parental types
c. the total number of genes within a given piece of DNA
d. the number of linked genes within a chromosome
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The correct answer is a—the recombination frequency
A. Answer a is correct. Map units are defined as percent recombination.
The correct answer is a—
B. Answer b is incorrect. The recombination frequency is the value used to determine map
units.
The correct answer is a—
C. Answer c is incorrect. The number of genes on a chromosome is not relevant to the
determination of map units.
The correct answer is a—
D. Answer d is incorrect. The number of linked genes is not relevant to the determination of
map units.
8. How many map units separate two alleles if the recombination frequency is 0.07?
a. 700 cM
b. 70 cM
c. 7 cM
d. 0.7 cM
The correct answer is c—
A. Answer a is incorrect. One centimorgan (cM) is equal to a recombination frequency of
1%. The recombination frequency would have to be 700% for this answer to be correct.
The correct answer is c—
B. Answer b is incorrect. One centimorgan (cM) is equal to a recombination frequency of
1%. The recombination frequency would have to be 70% for this answer to be correct.
The correct answer is c—7 cM
C. Answer c is correct. One centimorgan (cM) is equal to a recombination frequency of 1%.
A recombination frequency of 0.07% equals 7 cM.
The correct answer is c—
D. Answer d is incorrect. One centimorgan (cM) is equal to a recombination frequency of
1%. The recombination frequency would have to be 0.007 for this answer to be correct.
9. Multiple crossovers lead to—
a. restoration of the paternal combination of genes
b. increased genetic diversity
c. increased numbers of recombinant progeny
d. anueploidy
The correct answer is a—restoration of the paternal combination of genes
A. Answer a is correct. Two crossovers will return the alleles to their original location.
The correct answer is a—
B. Answer b is incorrect. Crossovers represent exchanges between homologous
chromosomes; they do not alter the genes themselves.
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Chapter 13
The correct answer is a—
C. Answer c is incorrect. Multiple crossovers can restore the parental combination, therefore
even though crossovers occurred, the progeny will have the parental phenotype.
The correct answer is a—
D. Answer d is incorrect. Anueploidy occurs when there is too much or too little genetic
information. Crossovers only exchange information.
10. The disease sickle cell anemia is a caused by—
a. an altered expression of the Hb gene
b. a change of a single amino acid in the protein hemoglobin
c. a change in the Hb gene
d. both b and c
The correct answer is d—
A. Answer a is incorrect. This disease is a consequence of a change in the gene and the gene
product, not in the expression of the gene.
The correct answer is d—
B. Answer b is incorrect. Although there is a single amino acid change in the sickle cell
hemoglobin, the change in the amino acid is the result of a change in the gene.
The correct answer is d—
C. Answer c is incorrect. The change in the Hb gene alters the Hb protein. It is the altered
hemoglobin protein that represents the disease phenotype.
The correct answer is d—both b and c
D. Answer d is correct. The disease is the result of the combined effects of a change in a
gene leading to a change in a protein
11. What determines whether an individual is a genetic mosaic?
a. The presence of different alleles on the autosomal chromosomes
b. The inactivation of an allele on an autosomal chromosome
c. The inactivation of an allele on the X chromosome of a heterozygous female
d. The inactivation of an allele on the X chromosome of a homozygous male
The correct answer is c—
A. Answer a is incorrect. An individual that is heterozygous is not a genetic mosaic.
The correct answer is c—
B. Answer b is incorrect. Autosomal chromosomes do not undergo inactivation.
The correct answer is c—The inactivation of an allele on the X chromosome of a heterozygous
female
C. Answer c is correct. The cells of a female have only one active X chromosome. They are
a mosaic since the specific X chromosome that has been inactivated varies between cells.
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The correct answer is c—
D. Answer d is incorrect. The X chromosome does not become inactive in a male.
12. Down syndrome is the result of—
a. a single-base substitution on human chromosome 21
b. nondisjunction of chromosome 21 during meiosis
c. inactivation of chromosome 21
d. nondisjunction of chromosome 21 during mitosis in early development
The correct answer is b—
A. Answer a is incorrect. Down syndrome is an example of a trisomy. Trisomy results when
there is an extra copy of a chromosome occurs in a cell.
The correct answer is b—nondisjunction of chromosome 21 during meiosis
B. Answer b is correct. The failure of the homologues of chromosome 21 to separate during
anaphase of meiosis I leads to the trisomic condition.
The correct answer is b—
C. Answer c is incorrect. Down syndrome is the result of too much genetic information.
Inactivation removes genetic information from a cell.
The correct answer is b—
D. Answer d is incorrect. Nondisjunction must occur during meiosis (the production of
gametes) in order for the organism to become trisomic.
13. Which of the following examples of nondisjunction of sex chromosomes is lethal?
a. XXX
b. XXY
c. OY
d. XO
The correct answer is c—
A. Answer a is incorrect. Triple X females are viable. They have a single active X
chromosome and two Barr bodies.
The correct answer is c—
B. Answer b is incorrect. Klinefelter males are viable.
The correct answer is c—OY
C. Answer c is correct. An embryo that lacks an X chromosome cannot survive.
The correct answer is c—
D. Answer d is incorrect. These individuals are viable.
14. What is imprinting?
a. The blending of a phenotype due to the genetic contribution of both parents
b. The expression of a dominant allele
c. The development of a phenotype in response to interactions between distinct
alleles
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d. The expression of different phenotypes dependent upon the parent from which the
allele was inherited
The correct answer is d—
A. Answer a is incorrect. Imprinting occurs when one allele is inactive. The phenotype
would appear to be homozygous.
The correct answer is d—
B. Answer b is incorrect. Imprinting results in variable expression of an allele, depending on
whether that allele was inherited from the mother or father, not on whether the allele is
dominant or recessive.
The correct answer is d—
C. Answer c is incorrect. Interactions between distinct alleles leading to an altered phenotype
are called epistasis.
The correct answer is d—The expression of different phenotypes dependent upon the parent from
which the allele was inherited
D. Answer d is correct. Specific alleles can be inactivated by methylation of the DNA. For
some genes the methylation of an allele follows a pattern based on whether the source of
the alleles is maternal or paternal
15. Which of the following is NOT a method used in genetic counseling?
a. Ultrasound
b. Chorionic villi sampling
c. Amniocentesis
d. Pedigree analysis
The correct answer is a—Ultrasound
A. Answer a is correct. Ultrasound can provide a visual image of a developing fetus;
however, it does not provide any information about genotype.
The correct answer is a—
B. Answer b is incorrect. Chorionic villi sampling is a method that provides fetal cells that
can be used to examine chromosome number and other biochemical tests.
The correct answer is a—
C. Answer c is incorrect. Amniocentesis provides a sample of fetal cells that can be
examined using karyotyping.
The correct answer is a—
D. Answer d is incorrect. Pedigree analysis is a noninvasive method used by genetic
counselors to predict the possibility of a genetic defect being inherited by a child based on
family history.
Raven/Johnson Biology 8e
Chapter 13
Challenge Questions
1. Color blindness is caused by a sex-linked, recessive gene. If a woman, heterozygous
for the color blind allele, marries a man with normal color vision, what percentage of
their children will be color blind? What sex will the color blind children be?
Answer—Use a simple Punnett square to calculate the percentage of color blind children.
X (normal)
Y
X (color blind)
X (normal)
XX
XX
(normal)(color blind)
(normal)(normal)
XY
XY
(color blind)
(normal)
Theoretically, 25% of the children from this cross will be color blind. All of the color blind
children will be male and 50% of the males will be color blind.
2. What conditions would have to exist to produce a color blind female?
Answer—A female would have to inherit the color blind gene from both parents. For this to
happen the father would have to be color blind and the mother would have to be heterozygous for
the color blind allele.
X (color blind)
Y
X (color blind)
X (normal)
XX
XX
(color blind)(color blind)
(color blind)(normal)
XY
XY
(color blind)
(normal)
3. Imagine that the genes for seed color and seed shape are located on the same
chromosome. A cross was made between two, true-breeding plants. One plant
produces green wrinkled seed (rryy) and the second parent produced round yellow
seeds (RRYY). A testcross is made between the F1 generation with the following
results:
green, wrinkled
645
green, round
36
yellow, wrinkled
29
yellow, round
590
Calculate the distance between the two loci.
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Chapter 13
Answer—To answer this question it is first necessary to determine which of the progeny are
expressing the parental phenotypes and which are the recombinants (that is, the progeny with
phenotypes that are different from the parental). In this problem the original parents had the
phenotypes green, wrinkled and yellow, round. That means that the recombinants are the green,
round and yellow, wrinkled.
The next step is to add up the total number of recombinant (36 + 29 = 65) and then divide
that number by the total number of progeny (645 + 36 + 29 + 590 = 1300) – 65/1300 = 0.05.
Each 1% of recombination is a map unit. For this problem the number of map units
between the two genes is 5 cM.
4. Is it possible to have a calico cat that is male? Why or why not?
Answer—Male calico cats are very rare. The coloration that is associated with calico cats is the
product of X inactivation. X inactivation only occurs in females as a response to dosage levels of
the X-linked genes. Females that are heterozygous for coat color will have different regions or
patches of colored fur, representing the differential inactivation of the black or orange pigment
genes. The only way to get a male calico is to be heterozygous for the color gene and to be the
equivalent of a Klinefelters male (XXY).