Test Bank for
Chapter 4: Sex Determination and Sex-Linked Characteristics
Multiple Choice Questions
1. What is the role of the SRY gene in humans?
a. It initiates the X inactivation process in females.
b. It is located on the X chromosome and causes the X to pair with the Y chromosome
during male meiosis.
c. It is located on the Y chromosome and initiates the developmental pathway toward the
male phenotype.
d. It is located on an autosomal chromosome and represses expression of autosomal genes
in order to balance their expression level with genes on the X chromosome.
e. None of the above answers is correct.
Answer: c
Section 4.1
Comprehension
2. What is the expected outcome for a human embryo with the XXXY chromosome
constitution?
a.
b.
c.
d.
e.
It would likely develop into a female who will not respond to the hormone testosterone.
It would likely develop into a sterile male with reduced testes.
It will always abort early in development before birth.
It would likely develop into a tall female who may be slightly cognitively impaired.
It would likely develop into a fertile man with a completely normal male phenotype.
Answer: b
Section 4.1
Comprehension
3. Which of the following chromosome constitutions would never lead to a viable human baby
being born?
a.
b.
c.
d.
e.
XXX
XYY
XO (O = the absence of a second chromosome)
YY
XXY
Answer: d
Section 4.1
Comprehension
4. A female with androgen-insensitivity syndrome, a sex-linked recessive condition, has
a. two X chromosomes, both carrying mutant alleles in the gene that makes the androgen
receptor.
b. a pair of ovaries that overproduce estrogen.
c. a XXX chromosome constitution that causes her not to produce testosterone.
d. a pair of testes that produce testosterone.
e. an inactive SRY gene.
Answer: d
Section 4.1
Comprehension
5. Species in which individuals have only male or only female reproductive structures are called
a.
b.
c.
d.
e.
hermaphrodites.
diploids.
dioecious.
homogametic.
monoecious.
Answer: c
Section 4.1
Comprehension
6. In which of the following organisms is gender/sex determined by the temperature during
embryonic development?
a.
b.
c.
d.
e.
Humans
Mice
Fruit flies
Many snakes and birds
Many turtles and alligators
Answer: e
Section 4.1
Comprehension
7. In species of birds, males are the homogametic sex and females the heterogametic sex.
Which if the following is true in this system of sex determination?
a.
b.
c.
d.
e.
The gender of the offspring is determined by the female parent.
Male offspring have a ZW chromosome constitution.
The gender of the offspring is determined by the male parent.
Female offspring have a ZZ chromosome constitution.
Female and male offspring have the same chromosome constitution.
Answer: a
Section 4.1
Comprehension
8. In a germ-line cell from a female grasshopper (XX-XO sex determination system), when do
the homologous X chromosomes segregate?
a.
b.
c.
d.
During mitosis
During meiosis I, anaphase
During meiosis II, anaphase
They do not segregate; gametes contain a copy of X and a copy of Y.
Answer: b
Section 4.1
Comprehension
9. In a germ-line cell from a human male that is dividing, when do the X and Y chromosomes
segregate?
a.
b.
c.
d.
During mitosis
During meiosis I, anaphase
During meiosis II, anaphase
They do not segregate; gametes contain a copy of X and a copy of Y.
Answer: b
Section 4.1
Comprehension
10. Which of the following human genotypes is associated with Klinefelter syndrome?
a.
b.
c.
d.
e.
XXY
XXXY
XXXXY
All of the above
None of the above
Answer: d
Section 4.1
Comprehension
11. What is the sex chromosome constitution of a male duck-billed platypus?
a. XX
b. XY
c. XO
d. ZZ
e. XXXXXYYYYY
Answer: e
Section 4.1
Comprehension
12. An XXY chromosome constitution produces ________ development in humans and
_________ development in fruit flies.
a.
b.
c.
d.
e.
female; female
male; male
female; male
male; female
male, intersex
Answer: d
Section 4.1
Comprehension
13. The sex determination system used by Drosophila is called
a.
b.
c.
d.
e.
the X:A sex determination system.
the ZZ-ZW sex determination system.
the XX-XO sex determination system.
the XX-XY sex determination system.
Both b and c are correct.
Answer: a
Section 4.1
Comprehension
14. With the XX-XO sex determination system, generally
a.
b.
c.
d.
e.
female offspring have one X chromosome, and it is inherited from their father.
male offspring have one X chromosome, and it is inherited from their mother.
male offspring have one X chromosome, and it is inherited from their father.
female offspring have one X chromosome, and it is inherited from their mother.
None of the above statements is true.
Answer: b
Section 4.1
Comprehension
15. Species in which an individual organism has both male and female reproductive structures
are called
a.
b.
c.
d.
e.
monoecious.
haploid.
diploid.
dioecious.
Both c and d are correct.
Answer: a
Section 4.1
Comprehension
16. Human females with XY chromosomes and a phenotype that includes the absence of a
uterus and ovaries and the presence of testes are likely to have which of the following
mutations?
a.
b.
c.
d.
e.
A mutation in the SRY gene
A mutation in the androgen receptor gene
A deletion that removes much of the Y chromosome
They likely do not carry a mutation but may have been premature babies.
None of the above answers is correct.
Answer: b
Section 4.1
Comprehension
17. Human males, with XY chromosomes are___________ and produce two different kinds of
gametes, whereas females with XX chromosomes are ___________ and produce only one
kind.
a.
b.
c.
d.
e.
homogametic; heterogametic
dioecious; monoecious
heterogametic; homogametic
monoecious; dioecious
monoecious; heterogametic
Answer: c
Section 4.1
Comprehension
18. In which of the following phenotypic females do testes develop?
a.
b.
c.
d.
e.
XY with an deletion that removes the SRY gene
XO
XY with the X-linked recessive condition of androgen insensitivity syndrome
XX
Both a and b are correct.
Answer: c
Section 4.1
Application
19. During the evolution of the human Y chromosome, all of the following are assumed to occur
except
a. the original chromosome was an autosome that eventually evolved into the Y
chromosome.
b. one of the early events in the evolution of the Y chromosome was the acquisition or
evolution of a gene somewhat similar to the current human SRY gene.
c. many of the genes on the original ancestral chromosome suffered mutations and became
inactive during the evolution of the Y chromosome.
d. many of the genes on the early X chromosome that were responsible for critical cellular
functions got moved to the evolving Y chromosome.
e. several palindromic regions evolved or were acquired and are now present on the Y
chromosome.
Answer: d
Section 4.2
Comprehension
20. Red-green color blindness is X-linked recessive. A woman with normal color vision has a
father who is color blind. The woman has a child with a man with normal color
vision.Which phenotype is NOT expected?
a.
b.
c.
d.
A color-blind female
A color-blind male
A noncolor-blind female
A noncolor-blind male
Answer: a
Section 4.2
Comprehension
21. Which statement best summarizes our current understanding of the origin of the Y
chromosome?
a. The Y chromosome is thought to have arisen spontaneously in an ancestor of mammals
millions of years ago.
b. The Y chromosome is thought to have arisen as a fusion of two autosomes.
c. The Y chromosome is thought to have arisen as a broken fragment of the X chromosome.
d. The Y chromosome is thought to have been derived along with the X chromosome from a
pair of autosomes.
e. Both a and c are correct.
Answer: d
Section 4.2
Comprehension
22. If a female Drosophila that is heterozygous for a recessive X-linked mutation is crossed to a
wild-type male, what proportion of female progeny will have the mutant phenotype?
a.
b.
c.
d.
100%
0%
33%
25%
Answer: b
Section 4.2
Application
23. A woman is phenotypically normal, but her father had the sex-linked recessive condition of
red-green colorblindness. If she has children with a man with normal vision, what is the
probability that their first child will have normal vision and their second child will be
colorblind?
a.
b.
c.
d.
e.
1/16
3/8
3/16
3/6
8/27
Answer: c
Section 4.2
Application
24. A eukaryotic diploid cell from an organism with the ZZ-ZW sex determination system has
two pairs of autosomes and a pair of sex chromosomes, Z and W, shown below.
A
-a
B --
-b
Sex chromosomes
From what type of individual is this cell?
a.
b.
c.
d.
Male
Female
Hermaphrodite
Monoecious
Answer: b
Section 4.2
Application
25. A eukaryotic diploid cell from an organism with the ZZ-ZW sex determination system has
two pairs of autosomes and a pair of sex chromosomes, Z and W, shown below.
A
-a
B --
-b
Sex chromosomes
A diploid cell from this individual begins to go through meiosis. After the completion of
meiosis I, it becomes two cells. One of these two cells now undergoes meiosis II. Which of
the following is a possible normal combination of chromosomes in one of the subsequent two
cells after the completion of meiosis II?
a. One chromosome with the A allele, one with the B allele, one Z, one W
b. One chromosome with the A allele, one with the a allele, one with the B allele, one with
the b allele, one Z, one W
c. A pair of chromosomes with A alleles, a pair of chromosomes with B alleles, a pair of Z
chromosomes
d. One chromosome with an a allele, one chromosome with a B allele, one W
e. Both c and c are possible.
Answer: d
Section 4.2
Application
26. A eukaryotic diploid cell from an organism with the ZZ-ZW sex determination system has
two pairs of autosomes and a pair of sex chromosomes, Z and W, shown below.
A
-a
B --
-b
Sex chromosomes
What is the probability of a gamete from this individual that has the following genotype:
alleles A and b, chromosome Z?
a.
b.
c.
d.
e.
1/2
1/4
1/6
1/8
1/16
Answer: d
Section 4.2
Application
27. A eukaryotic diploid cell from an organism with the ZZ-ZW sex determination system has
two pairs of autosomes and a pair of sex chromosomes, Z and W, shown below.
A
-a
B --
-b
Sex chromosomes
Assume A and B are dominant alleles. If this individual were crossed to an individual of
genotype Aa Bb, what is the probability of a female offspring with the two dominant traits
given by alleles A and B?
a.
b.
c.
d.
e.
1/8
1/16
9/16
9/32
3/32
Answer: d
Section 4.2
Application
28. A eukaryotic diploid cell from an organism with the XX-XO sex determination system has
two pairs of autosomes and one X chromosome, shown below.
A
-a
B --
-b
From what type of individual is this cell?
a.
b.
c.
d.
e.
Male
Female
Hermaphrodite
Monoecious
Intersex
Answer: a
Section 4.2
Application
29. A eukaryotic diploid cell from an organism with the XX-XO sex determination system has
two pairs of autosomes and one X chromosome, shown below.
A
-a
B --
-b
A diploid cell from this individual begins to go through meiosis. After the completion of
meiosis I, it becomes two cells. One of these two cells now undergoes meiosis II. Which of
the following is a possible normal combination of chromosomes in one of the subsequent two
cells after the completion of meiosis II?
a. One chromosome with the A allele, one with the B allele, and two X chromosomes
b. One chromosome with the A allele, one with the a allele, one with B allele, one with b
allele, and two X chromosomes
c. One chromosome with the A allele, one chromosome with the B allele
d. One chromosome with the a allele, one chromosome with the B allele, one X
chromosome
e. Both c and d are possible.
Answer: e
Section 4.2
Application
30. A eukaryotic diploid cell from an organism with the XX-XO sex determination system has
two pairs of autosomes and one X chromosome, shown below.
A
-a
B --
-b
What is the probability of a gamete from this individual that has the following genotype:
alleles A and b, chromosome X?
a.
b.
c.
d.
e.
1/2
1/4
1/6
1/8
1/16
Answer: d
Section 4.2
Application
31. If a male bird that is heterozygous for a recessive Z-linked mutation is crossed to a wild type
female, what proportion of the progeny will be mutant males?
a.
b.
c.
d.
e.
0%
100%
75%
50%
25%
Answer: a
Section 4.2
Application
32. A woman has normal vision although her maternal grandfather (her mother’s father) had redgreen colorblindness, a sex-linked recessive trait. Her maternal grandmother and the
woman’s own father are assumed to not possess a copy of the mutant allele. The woman
marries a man with normal vision although his father was colorblind. What is the probability
that the first child of this couple will be colorblind?
a.
b.
c.
d.
e.
1/2
1/4
1/8
1/16
1/12
Answer: c
Section 4.2
Application
33. Joan is phenotypically normal, but had a child with the autosomal recessive disease cystic
fibrosis (CF) from a previous marriage. Joan’s father has hemophila A, a sex-linked recessive
condition where the blood fails to clot properly. Her father has survived due to recent
treatment advances. Joan now intends to marry Bill, who is also phenotypically normal but
who has a sister, Jill, with CF. Bill’s parents are phenotypically normal, and there is no
history of hemophilia A in his family. Assume that Joan and Bill do marry and have a child.
What is the probability that this child will have CF, but will not have hemophilia A ? (Hint:
This problem requires that you utilize concepts from Chapter 3 as well as Chapter 4.)
a.
b.
c.
d.
e.
1/8
1/12
1/24
3/32
5/32
Answer: a
Section 4.2
Challenge
34. In humans there is a genetic disorder that results from a dominant mutation present in a gene
located in the pseudoautosomal region of the Y chromosome and on the X
chromosome.Which of the following statements is correct?
a. All affected men marrying normal women will have no affected daughters.
b. All affected women marrying normal men will have affected daughters and no affected
sons.
c. All affected men marrying normal women will have affected daughers, but all the sons
will be normal.
d. All affected women marrying nomal men will have only normal sons and daughters.
e. None of the above statements is correct.
Answer: e
Section 4.2
Challenge
35. Familial vitamin-D-resistant rickets is an X-linked dominant condition in humans. If a man is
afflicted with this condition and his wife is normal, it is expected that among their children,
all the daughters would be affected and all the sons would be normal. In families where the
husband is affected and the wife is normal, this is almost always that outcome among their
children when such families have been studied. Very rarely an unexpected result occurs in
such families where a boy is born with the disorder. If the chromosomes of such unusual
boys are examined, what might be expected to be found?
a.
b.
c.
d.
e.
Some of the boys are XYY.
Some of the boys are XY but have lost the SRY gene from their Y chromosome.
Some of the boys are YY.
Some of the boys are XXY.
Some of the boys are XXX.
Answer: d
Sections 4.1 and 4.2
Challenge
36. A Barr body is a(n):
a. gene on the X chromosome that is responsible for female development.
b. patch of cells that has a phenotype different from surrounding cells because of variable X
inactivation.
c. inactivated X chromosome, visible in the nucleus of a cell that is normally from a female
mammal.
d. extra X chromosome in a cell that is the result of nondisjunction.
e. extra Y chromosome in a cell that is the result of nondisjunction.
Answer: c
Section 4.3
Comprehension
37. Which of the following statements about X inactivation in mammalian females is false?
a. Females that are heterozygous for an X-linked gene have patches of cells that express one
allele and patches of cells that express the other.
b. Some genes on the inactive X continue to be expressed after the chromosome is
inactivated.
c. X inactivation is random as to which X is inactivated and takes place early in embryonic
development
d. Inactivation is thought to be initiated by expression of the Xist gene on the X that will
remain active.
e. Once an X chromosome first becomes inactivated in a cell, that same X will remain
inactivated in somatic cells that are descendants of this cell.
Answer: d
Section 4.3
Comprehension
38. What is the apparent purpose for X inactivation in humans and other mammals?
a. It allows for the levels of expression of genes on the X chromosome to be similar in
males and females.
b. It allows for the levels of expression of genes on the autosomes to be similar to the levels
of genes on the X chromosome.
c. It suppresses the expression of genes on the Y chromosome in males.
d. It reduces the amount of nondisjunction during meiosis in females.
e. It enhances the level of pairing between the two X chromosomes during meiosis in
females.
Answer: a
Section 4.3
Comprehension
39. The Lyon hypothesis helps us to understand which phenomenon in mammals?
a.
b.
c.
d.
e.
X-linked inheritance
Evolution of the Y chromosome
Dosage compensation between males and females
Development of male and female secondary sexual characteristics
Sex determination
Answer: c
Section 4.3
Comprehension
40. Women with Turner syndrome (XO) and normal women (XX) are clearly different
phenotypically. In addition, the vast majority of XO conceptions abort before birth. However,
both XO and XX women have one active X chromosome since the X in XO women remains
active and one might expect that they would therefore have similar phenotypes. What is the
most reasonable explanation for their different phenotypes?
a. XO women do not have a copy of the SRY gene.
b. Some genes remain active on the inactive X chromosome and XX women will have two
copies of these genes expressed and XO women only one copy.
c. In XO women, the single X chromosome has no partner to pair with during mitosis so
that each cell division is delayed by pairing problems with the single X not finding a
pairing partner.
d. XO women are missing a copy of the XIST gene so that they are forced to develop
partway along the male pathway during embryogenesis.
e. Both c and d are reasonable explanations.
Answer: b
Section 4.3
Application
41. In which of the following individuals would you expect to find two Barr bodies in their
somatic cells?
a.
b.
c.
d.
e.
XX
XO
XXY
XXYY
XXX
Answer: e
Section 4.3
Application
42. Normal males (XY) and Klinefelter males (XXY) both possess only one active X
chromosome. Nonetheless there are clearly phenotypic differences between the two. What is
the most reasonable explanation as to why such differences exist?
a. The Y chromosome has higher gene expression levels when two X chromosomes are
present compared to one X.
b. The Y chromosome has lower gene expression levels when two X chromosomes are
present compare to one X.
c. Some genes remain active on inactive X chromosomes so XXY males would produce
have higher expression levels for these genes compared to XY males.
d. XXY males exhibit a higher rate of problems during mitotic divisions than XY males.
e. XXY males don’t have a copy of the SRY gene.
Answer: c
Section 4.3
Application
43. How many Barr bodies (condensed X chromosomes) would you predict in
cells?
a.
b.
c.
d.
e.
this boy’s
One per somatic cell
Two per somatic cell
Three per somatic cell
None
Either two or three depending on the tissue type
Answer: a
Section 4.3
Application
Short Answer Questions
44. List five different sex determination systems and a representative organism
for each.
Answer:
(1) XX-XO; grasshoppers
(2) XX-XY; humans
(3) ZZ-ZW; birds
(4) X:A; Drosophila
(5) Environmental mollusks; many turtles, crocodiles, and alligators
Section 4.1
Comprehension
45. Explain the genders of human and diploid Drosophila XXY individuals.
Answer: An XXY human is male. The SRY on the Y chromosome determines maleness, in
spite of the two X chromosomes. An XXY Drosophila is female. Drosophila is diploid, so
there are two of each autosome. If there are two X chromosomes, the X:A ratio is 1.0, which
is a female.
Section 4.1
Application
46. Explain the genders of human and diploid Drosophila XO individuals.
Answer: An XO human is female. In the absence of SRY from a Y chromosome, the person
will develop as a female, in spite of having only one X chromosome. An XO Drosophila is
male. Drosophila is normally diploid, so there are two of each autosome. If there is a single X
chromosome, the X:A ratio is ½, or 0.5, which is a male.
Section 4.1
Application
47. Suppose that an apparently female athlete fails a gender test and is not allowed to compete in
her event. The gender test is based on examination of cheek cells for the presence of one or
more Barr bodies. Later, it is discovered that the athlete has androgen-insensitivity
syndrome. What is the chromosome constitution of a person with androgen-insensitivity
syndrome?
Answer: The chromosome constitution is XY.
Section 4.1
Application
48. Suppose that an apparently female athlete fails a gender test and is not allowed to compete in
her event. The gender test is based on examination of cheek cells for the presence of one or
more Barr bodies. Later, it is discovered that the athlete has androgen-insensitivity
syndrome.Explain why the athlete failed the gender test. What did the technician see in the
test and how was it interpreted?
Answer: Because the athlete is XY, the technician would have seen no Barr bodies in her
cells. The absence of Barr bodies is normally characteristic of males, so the test was
interpreted to indicate that the athlete is genetically male.
Section 4.1
Application
49. Characterize the gonads of a person with androgen-insensitivity as testes, ovaries, or intersex.
Answer: Because of the presence of a normal Y chromosome, the gonads develop as testes.
Section 4.1
Application
50. What level of male hormones would you expect to find in a person with androgen
insensitivity syndrome?
Answer: Based on the presence of normal testes, one would expect to find normal levels of
male hormones in these individuals.
Section 4.1
Application
51. Explain the development of female external anatomy in individuals with androgen
insensitivity syndrome.
Answer: These individuals lack the androgen receptor on the cells of their bodies. Therefore,
the cells cannot respond to the presence of male hormones, and the external anatomy
develops according to the “default” pathway, which is female.
Section 4.1
Application
52. Suppose that an apparently female athlete fails a gender test and is not allowed to compete in
her event. The gender test is based on examination of cheek cells for the presence of one or
more Barr bodies. Later, it is discovered that the athlete has androgen-insensitivity
syndrome.Evaluate the decision of the officials to exclude the athlete from athletic
competition as a female in light of your knowledge of androgen-insensitivity syndrome. Do
you think the athlete should be allowed to compete as a female?
Answer: The decision to ban the athlete very likely was unfair. Other than the presence of
testes and the absence of internal female reproductive structures, she is anatomically female.
Although she has male hormones in her system, her muscle and other cells do not respond to
the presence of the male hormones. Very likely, she does not gain a competitive advantage
over other females by the presence of the Y chromosome, testes, and male hormones.
Section 4.1
Application
53. While doing summer field work on a remote Indonesian island, you discover a new genus of
lizard closely related to komodo dragons. You attempt to discover what sex determination
system it uses by performing a series of controlled crosses on the island, using an isolated
pair of lizards. Initially, all your crosses yield only males (in significant numbers). As fall
begins and you prepare to leave the island, you find that your last cross yielded only females
(in significant numbers). Suggest a mode of sex determination that explains this data.
Answer: The crosses yielded all males or all females from the same parents. Male and
female progeny were correlated with climatic conditions (summer versus fall).
Environmental sex determination that is dependent on temperature is a likely explanation.
Section 4.1
Application
54. Predict the sexual phenotype of a person who is XY but whose Y chromosome carries a
deletion of the SRY gene. Explain your prediction.
Answer: The SRY gene is the primary determinant of maleness in humans. If it is deleted,
the gonads will not be induced to differentiate as testes, and the individual would likely
follow the female developmental pathway. (Actually, this condition exists as Swyer
syndrome. It turns out that the gonads do not develop into functional ovaries, indicating that
genes other than the SRY also have roles in sexual differentiation. However, these
individuals develop as apparently normal females until the lack of female hormones from the
ovaries causes puberty not to be induced without hormone therapy.)
Section 4.1
Application
55. How might an XY person with a deletion of the SRY gene be distinguished from a person
with androgen-insensitivity syndrome?
Answer: A person with androgen-insensitivity syndrome would have testes and a level of
male hormones characteristic of males. An XY person with a deletion of SRY would not
have testes or high levels of male hormones.
Section 4.1
Application
56. Some organisms have multiple X and Y chromosomes and even different numbers of X and
Y chromosomes. You have discovered such a species. Females have 8 X chromosomes,
whereas males have 4 X and 2 Y. Describe the X and Y constitution of the gametes
produced by this species—both male and female—that allows these chromosome numbers to
be stably maintained.
Answer: Females must produce one type of gamete, with 4 X chromosomes, whereas males
produce two types of gametes, with 4 X chromosomes or with 2 Y chromosomes.
Fertilization using a 4 X male gamete produces an 8 X female, whereas fertilization with a 2
Y gamete produces a male.
Section 4.1
Challenge
57. A man and a woman are trying to have children but are unsuccessful. The man’s autosomes
appear normal, but his sex chromosomes, shown in the following diagram, are not. The
diagram also shows a normal male’s sex chromosomes for reference. In two to three
sentences, explain the man’s situation, including the type of chromosome mutation he
carries, the specific regions of specific chromosomes involved, and why he is male.
man's sex chromosomes
normal X and Y
Answer: He has an X with a translocation, meaning part of one chromosome has been
moved to another. The translocated part is from Y and carries SRY, which determines
maleness. He is missing the rest of Y, including the genes required for male fertility.
Section 4.1
Challenge
58. A man and a woman are trying to have children but are unsuccessful. The man’s autosomes
appear normal, but his sex chromosomes, shown in the following diagram, are not. The
diagram also shows a normal male’s sex chromosomes for reference. Can you tell if the
mutation came from the man’s mother or the man’s father? Explain how you can tell.
man's sex chromosomes
normal X and Y
Answer: He inherited the translocation chromosome from his father because his mother
could not have carried the SRY-containing chromosome.
Section 4.1
Challenge
59. Calvin Bridges crossed white-eyed females to red-eyed males and found rare red-eyed males
and white-eyed females in the progeny. Explain how he used the Drosophila sex
determination system and nondisjunction to demonstrate that the gene for red/white eye color
is on the X chromosome.
Answer: Bridges looked at the chromosomes of the rare flies under the microscope and
showed that the rare white-eyed females had two X chromosomes and one Y, and the rare
red-eyed males had only one sex chromosome. The rare white-eyed females were XwXwY;
they were white-eyed because they contained only recessive w alleles, and female because
their X: A ratio = 2:2 = 1.0. The rare red-eyed males were X+O; they were red-eyed because
of the dominant + allele, and male because their X: A ratio was 1:2 = 0.5. This demonstrated
that genes that confer phenotypes were located on X chromosomes.
Section 4.2
Application
60. You are trying to develop a new species of newt as an experimental model system. You
know that in other species of newt, green (G) is dominant to brown (g) skin color and is
determined by a sex-linked gene. You cross brown males to green females and see that in the
F1 all the males are green and all the females are brown. Which is the heterogametic sex in
your species of newt?
Answer: Because the F1 females have the recessive brown phenotype, they must be
hemizygous (i.e., they inherited a brown allele from their father and no allele from their
mother). Therefore, the females of this species are the heterogametic sex. We use the ZZ-ZW
nomenclature for species with heterogametic females. Therefore, your F1 females and males
are ZgW and ZGZg, respectively.
Section 4.2
Application
61. This boy’s parents’ X chromosomes can be distinguished by polymorphism (DNA
fingerprint) differences. One of his mother’s X chromosomes has the polymorphism allele 7
and the other has the allele 14; his father’s X has the allele 5. Assuming each parent
contributed at least one sex chromosome, what polymorphism combinations are possible in
the boy’s X chromosomes?
Answer:
(1) 7,7 (fingerprint would show just 7)
(2) 14, 14 (fingerprint would show 14)
(3) 7, 14
(4) 7, 5
(5) 5, 14
Section 4.2
Application
62. Describe the inheritance of each possible combination of your answer to question 61,
including the parent and meiotic stage in which an unusual event occurred.
Answer:
Polymorphisms seen
Inherited from father
Inherited from mother
Nondisjunction in
Boy’s genotype
X5 X7Y
5,7
X5 Y
X7
father,
meiosis I
X5 X14Y
5,14
X5 Y
X14
father,
meiosis I
X7 X14Y
7,14
Y
X7 X14
mother,
meiosis I
X7 X7Y
7
Y
X7 X7
mother,
meiosis II
Nondisjunction in meiosis I: Homologs fail to segregate (e.g., X5 and Y or X7
X14 X14Y
14
Y
X14 X14
mother,
meiosis II
and X14).
Nondisjunction in meiosis II: Sister chromatids fail to separate (e.g., X7 and
and X14, or both).
Section 4.2
Application
X7, or X14
63. The boy in questions 61 and 62 has an X-linked recessive condition that is not seen in either
parent. With this additional information, what can you conclude about the allelic composition
of his parents and how he got this condition?
Answer: His mother must be a carrier of the allele for the condition because she passed it to
the son without showing it herself. The father could not have been a carrier because he did
not show the condition. If the son has the recessive condition, the recessive allele must be on
both his X chromosomes. Therefore, the son’s two X chromosomes are the same and were
inherited from his mother. The nondisjunction occurred in the mother’s germline, in meiosis
II, when sister chromatids failed to separate.
Section 4.2
Application
64. Red-green color blindness is an X-linked recessive condition. Juliet has a bit of difficulty
passing the red-green color distinction test when she tries to get her driver’s license. Her
husband is not colorblind, and neither is her son, Henry, nor her daughter, Roxanne. Roxanne
has a son who is color blind. What is Juliet’s genotype for the color-blindness allele? How
would you explain her partial color blindness?
Answer: Juliet is heterozygous. So every diploid cell in her eyes contains both X
chromosomes, one with the defective red-green color vision allele, and one with the normal
color vision allele. She is not completely color blind because some of her cells are
inactivating the X chromosome with the recessive, defective red-green color vision allele and
expressing the normal color vision allele, allowing her to see color. She has some difficulty
with color vision because some of her cells are inactivating the X with the normal red-green
color vision allele and expressing the red-green color blind phenotype. Her eyes are actually
a mosaic of cells with two different phenotypes.
Section 4.2
Challenge
65. You cross a female rat with pink toe pads (T) and pointy ears (Xe) to a male rat with black
toe pads (t) and round ears (XE). The t and e alleles are both recessive, and the ear-shaped
gene is X-linked, whereas the toe pad color gene is autosomal. The F1 progeny all have pink
toe pads. What is the genotype of parental generation? What is the genotype of the F1
progeny? If the F1 are crossed to produce F2 progeny, what proportion of the F2 will be
black-padded, pointy-eared males?
Answer: The parental generation was T/T; Xe/Xe and t/t; XE/Y. The F1 were T/t;
XEXe and T/t;XeY. Black-padded, pointy-eared males are tt and Xe/Xe. One-quarter of the
F2 progeny will be t/t, ¼ will be XeY. Therefore, 1/16 of the progeny will be black-padded,
pointy-eared males.
Section 4.2
Challenge
66. List three dosage compensation strategies for equalizing the amount of sex chromosome gene
products.
Answer:
(1) Inactivation of one sex chromosome in the homogametic sex
(2) Halving the activity of genes on both sex chromosomes in the homogametic sex
(3) Increasing the activity of genes on the sex chromosome in the heterogametic sex
Section 4.3
Comprehension
67. List three different mechanisms for generating sexes in dioecious species.
Answer:
(1) Chromosomal sex determination (XX-XO, XX-XY, or ZZ-ZW)
(2) Genetic sex determination
(3) Environmental sex determination
Section 4.3
Comprehension
68. In a few sentences, describe what effect removal of the Xist gene would have. Include a
description of what Xist encodes and why a mutation would have this effect.
Answer: Xist encodes an RNA that coats an X chromosome to inactivate it. Null mutation
would eliminate Xist RNA, and no X chromosome would ever be inactivated. Lack of Xist
RNA would lead to improper dosage compensation in a female, but have no effect on dosage
compensation in a male.
Section 4.3
Application
69. Explain how dosage balance is achieved between X-linked genes and autosomal genes in
mammals.
Answer: In both males and females only one X chromosome is fully active, potentially
creating an imbalance in gene expression between genes on the X chromosome and
autosomes. The imbalance is prevented by upregulation of genes on the X chromosome.
Section 4.3
Application
70. Red-green color blindness is X-linked recessive. A woman with normal color vision has a
father who is red-green color blind. The woman has four sons, none of whom are color blind.
In this family there are no instances of chromosome loss or gain such as occurs due to
nondisjunction in meiosis. Below are three explanations for why none of the sons are color
blind. For each, state if color blindness is possible or not possible, then give the reason for
your choice.
(a) Explanation 1: None of the sons are color blind because the mother does not carry the
color-blindness allele.
(b) Explanation 2: None of the sons are color blind because none of them inherited the colorblindness allele from the mother.
(c) Explanation 3: None of the sons are color blind because the mother inactivated the X
chromosome with the recessive color-blindness allele, and that is the one each son inherited.
Answer:
(a) Not possible. If there are no aneuploidies, the mother must have inherited the X with the
recessive color-blindness allele from her father.
(b) Likely. She is heterozygous, so each son has a 50% chance of inheriting the X
chromosome with the dominant allele for normal color vision.
(c) Not possible. X inactivation is reversed when an X chromosome is passed to offspring.
Sections 4.2 and 4.3
Application
71. Marsupials, like cats, achieve dosage compensation by X inactivation. You are working in a
lab that has discovered a mutation on the X chromosome in marsupials in the same gene that
causes the tortoiseshell fur color phenotype in cats. You cross an X+Y black- furred male
with an XOXO orange-furred female. You expect that the X+XO female progeny will have
tortoiseshell fur (like cats). Surprisingly, you find that all the females (n = 25) have solid
orange fur. Offer a hypothesis to explain these results and describe a genetic test to support
your hypothesis.
Answer: It appears that the X+ chromosome from the male was inactivated in every female
offspring. So perhaps in marsupials, unlike in cats, X inactivation is not random. Instead, in
marsupials only the paternal X chromosome is inactivated. If this is true (and, in fact, it is),
then it may be tested genetically. Crossing an orange-furred XOY male to a X+X+ blackfurred female should produce only black-furred female progeny even though their genotype
is X+XO, the same as the orange-furred female progeny from the first cross.
Section 4.3
Challenge
72. Compare and contrast the patterns of inheritance expected for Y-linked and X-linked
recessive inheritance in humans.
Answer: Y-linked inheritance is easy to recognize because the trait is passed from a father to
all of his sons and none of his daughters and continues to pass from fathers to sons in every
generation. The trait does not affect females. An X-linked recessive trait is more common in
males but can affect females as well. A son inherits the trait from his mother, who is typically
a phenotypically normal carrier. Often, the trait will also be present in the mother’s father or
brothers or other male relatives. An affected female normally occurs from an affected father
and a carrier mother.
Section 4.2
Application