Chapter 4
Sex Determination and Sex-Linked Characteristics
COMPREHENSION QUESTIONS
True/False
1. In humans SRY is the male determining gene. (T)
2. Female mammals that are heterozygous for an X-linked gene have patches of
cells that express one allele and patches of cells that express the other. (T)
3. The cellular reason for X-inactivation is to reduce the amount of
nondisjunction during meiosis. (F)
4. The condition XXXY is always lethal in humans. (F)
5. The condition of having no X chromosomes, for example YO, is lethal in
humans. (T)
6. Temperature during embryonic development determines sex in reptiles like
turtles and alligators. (T)
7. In species in which females are the homogametic sex, gender of offspring is
determined by the male. (T)
8. A female with androgen insensitivity may have XY sex chromosomes rather
than XX. (T)
9. Species in which individuals have only male or only female reproductive
structures are called dioecious. (T)
10. The X and Y chromosomes are named after their characteristic shape. (F)
Multiple Choice
11. 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. a color-blind female
b. a color-blind male
c. a noncolor-blind female
d. a noncolor-blind male
Chapter 4
12. 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. 100%
*b. 0%
c. 33%
d. 25%
13. In a germ-line cell from a female grasshopper (XX-XO sex determination
system), when do the homologous X chromosomes segregate?
a. during mitosis
*b. during meiosis I, anaphase
c. during meiosis II, anaphase
d. They do not segregate; gametes contain a copy of X and a copy of Y.
14. In a germ-line cell from a human male that is dividing, when do the X and Y
chromosomes segregate?
a. during mitosis
*b. during meiosis I, anaphase
c. during meiosis II, anaphase
d. They do not segregate; gametes contain a copy of X and a copy of Y.
Use the following information for questions 15–18.
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
15. From what type of individual is this cell?
a. male
*b. female
c. hermaphrodite
d. monoecious
16. A cell from this individual begins to go through meiosis. As it goes through
meiosis II, it becomes two cells. Which of the following is a possible combination
Sex Determination and Sex-Linked Characteristics
of chromosomes in one of the two cells when it goes through metaphase of
meiosis II?
a. one chromosome with A allele, one with B allele, one Z, one W
b. one chromosome with A allele, one with a allele, one with B allele, one with b
allele, one Z, one W
c. a pair of sister chromatids with A allele, a pair of sister chromatids with B
allele, a pair of sister chromatids Z, a pair of sister chromatids W
*d. a pair of sister chromatids with a allele, a pair of sister chromatids with B
allele, a pair of sister chromatids W
e. C and D are both possible.
17. What is the probability of a gamete from this individual that has the following
genotype: alleles A and b, chromosome Z?
a. 1/2
b. 1/4
c. 1/6
*d. 1/8
18. 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. 1/8
b. 1/16
c. 9/16
*d. 9/32
Use the following information for questions 19–21.
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
19. From what type of individual is this cell?
*a. male
b. female
c. hermaphrodite
d. monoecious
Chapter 4
20. A cell from this individual begins to go through meiosis. When the cell
reaches meiosis II, it becomes two cells. Which of the following is a possible
combination of chromosomes in one of the two cells when it goes through
metaphase of meiosis II?
a. one chromosome with A allele, one with B allele, and two X chromosomes
b. one chromosome with A allele, one with a allele, one with B allele, one with b
allele, and two X chromosomes
c. a pair of sister chromatids with A allele, a pair of sister chromatids with B
allele
d. a pair of sister chromatids with a allele, a pair of sister chromatids with B
allele, a pair of sister chromatids X
*e. C and D are both possible.
21. What is the probability of a gamete from this individual that has the following
genotype: alleles A and b, chromosome X?
a. 1/2
b. 1/4
c. 1/6
*d. 1/8
22. Which of the following human genotypes is associated with Klinefelter
syndrome?
a. XXY
b. XXYY
c. XXXY
*d. All of the above.
e. None of the above.
23. 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 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.
Sex Determination and Sex-Linked Characteristics
24. 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. 0%
b. 100%
c. 75%
d. 50%
e. 25%
25. What is the sex chromosome constitution of a male duck-billed platypus?
a. XX
b. XY
c. XO
d. ZZ
*e. XXXXXYYYYY
26. An XXY chromosome constitution produces ________ development in
humans and _________ development in fruit flies.
a. female; female
b. male; male
c. female; male
*d. male; female
27. 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 broken fragment of an
autosome.
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.
28. The Lyon hypothesis helps us to understand which phenomenon in
mammals?
a. X-linked inheritance
b. evolution of the Y chromosome
*c. dosage compensation between males and females
d. development of male and female secondary sexual characteristics
Chapter 4
Short Answer
29. List three dosage compensation strategies for equalizing the amount of sex
chromosome gene products.
(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
30. List five different sex determination systems and a representative organism
for each.
(1) XX-XO
(2) XX-XY
(3) ZZ-ZW
(4) Genic balance
(5) Environmental
grasshoppers
humans
birds
Drosophila
mollusks; many turtles, crocodiles, and alligators
31. List three different mechanisms for generating sexes in dioecious species.
(1) Chromosomal sex determination (XX-XO, XX-XY, or ZZ-ZW)
(2) Genetic sex determination
(3) Environmental sex determination
Fill in the Blank
32. The sex determination system used by Drosophila is called the genic
balance system.
33. Male mammals inherit an X chromosome from the maternal parent, the
mother.
34. Species in which an organism has either male or female reproductive
structures, but not both, are dioecious.
35. Human females with XY chromosomes and a mutation in their androgen
receptor gene have androgen-insensitivity syndrome.
Sex Determination and Sex-Linked Characteristics
36. Human males, with XY chromosomes are heterogametic and produce two
different kinds of gametes, while females with XX chromosomes are
homogametic and produce only one kind.
APPLICATION QUESTIONS AND PROBLEMS
Use the following information for questions 37–40.
The following shows a boy’s karyotype.
From: http://gslc.genetics.utah.edu/disorders/karyotype/klinefelter.html
37. How many Barr bodies (condensed X chromosomes) would you predict in
his cells?
*a. One per cell
b. Two per cell
c. Three per cell
d. None
38. His 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?
7,7 (fingerprint would show just 7)
14, 14 (fingerprint would show 14)
Chapter 4
7, 14
7, 5
5, 14
39. Describe the inheritance of each possible combination of your answer to the
previous question, including the parent and meiotic stage in which an unusual
event occurred.
X5
Polymorphisms
seen
Inherited from
father
Inherited from
mother
Nondisjunction in
X7Y
X5
X14Y
Boy’s genotype
X7 X14Y
X7 X7Y
7,14
7
X14 X14Y
14
5,7
5,14
X5 Y
X5 Y
Y
Y
Y
X7
X14
X7 X14
X7 X 7
X14 X14
father,
meiosis I
father,
meiosis I
mother,
meiosis I
mother,
meiosis
II
mother,
meiosis II
Nondisjunction in meiosis I: Homologs fail to segregate (e.g., X5 and Y or X7
and X14).
Nondisjunction in meiosis II: Sister chromatids fail to separate (e.g., X7 and
X7, or X14 and X14, or both).
40. The boy 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 karyotype?
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.
Use the following information for question 41-42.
In humans, the presence of the SRY gene, normally on Y, determines maleness.
In Drosophila, an X: A (autosome) ratio of 0.5 determines maleness.
41. Explain the genders of human and Drosophila XXY individuals.
Sex Determination and Sex-Linked Characteristics
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.
42. Explain the genders of human and Drosophila XO individuals.
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 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.
43. 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.
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
chromosomes.
Use the following information for questions 44–45.
The following diagram shows two pairs of autosomes and a pair of sex
chromosomes from a dioecious mammal.
A
a
B
b
Chapter 4
44. Draw the chromosomes as they would appear in separate cells at the end of
meiosis I, after a crossover between locus A and the centromere. Indicate alleles
A, a, B, and b.
B
A
B
b
a
a
b
A
45. Which of Mendel’s principles is demonstrated by alleles B and b in your
drawing? (Be more specific than “first principle” or “second principle.”)
Principle of segregation. (Alleles of a gene separate in meiosis. Or,
homologous chromosomes segregate in meiosis.)
46. 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.
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.
47. 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 while 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.
Females must produce one type of gamete, with 4 X chromosomes, while
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, while fertilization with a 2 Y gamete produces a male.
Sex Determination and Sex-Linked Characteristics
Use the following information for questions 48–49.
A man and a woman are trying to have children but are unsuccessful. As part of
a series of tests, the man is karyotyped. His 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.
man's sex chromosomes
normal X and Y
48. 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.
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.
49. Can you tell if the mutation came from the man's mother or the man's father?
Explain how you can tell.
He inherited the translocation chromosome from his father because his
mother could not have carried the SRY-containing chromosome.
Use the following information for questions 50–52.
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. Each of the next
three questions has an explanation 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.
Chapter 4
50. Explanation 1: None of the sons are color blind because…the mother does
not carry the color-blindness allele.
Not possible. If there are no aneuploidies, the mother must have inherited
the X with the recessive color-blindness allele from her father.
51. Explanation 2: None of the sons are color blind because…none of them
inherited the color-blindness allele from the mother.
Possible. She is heterozygous, so each son has a 50% chance of inheriting
the X chromosome with the dominant allele for normal color vision.
52. 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.
Not possible. X inactivation is reversed when an X chromosome is passed
to offspring.
53. 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?
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.
54. 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.
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.
Sex Determination and Sex-Linked Characteristics
Use the following information for questions 55–56.
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. She has two children with a man who is not color blind. Neither
Juliet's son (Henry) nor daughter (Roxanne) is color blind. Henry and Roxanne
have normal karyotypes. Roxanne has a son who is color blind.
55. What is Juliet's genotype for the color-blindness allele? How would you
explain her partial color blindness?
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.
56. Why aren't Henry and Roxanne color blind?
Henry is not color blind because he inherited the X with the normal redgreen color vision allele from Juliet. Roxanne is not color blind because
she is heterozygous like her mother, and does not have appreciable
mosaicism for X-inactivation in her eye tissues.
57. 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, while 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?
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.
58. 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
Chapter 4
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.
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+ black-furred
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.
59. 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.
(a) What is the chromosome constitution of a person with androgen-insensitivity
syndrome?
The chromosome constitution is XY.
(b) Explain why the athlete failed the gender test. What did the technician see in
the test and how was it interpreted?
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.
(c) Characterize the gonads of a person with androgen-insensitivity as testes,
ovaries, or intersex.
Because of the presence of a normal Y chromosome, the gonads develop
as testes.
(d) What level of male hormones would you expect to find in a person with
androgen-insensitivity syndrome?
Based on the presence of normal testes, one would expect to find normal
levels of male hormones in these individuals.
Sex Determination and Sex-Linked Characteristics
(e) Explain the development of female external anatomy in individuals with
androgen-insensitivity syndrome.
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.
(f) 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?
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.
60. (a) Predict the sexual phenotype of a person who is XY but whose Y
chromosome carries a deletion of the SRY gene. Explain your prediction.
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.)
(b) How might an XY person with a deletion of the SRY gene be distinguished
from a person with androgen-insensitivity syndrome?
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.
61. Compare and contrast the patterns of inheritance expected for Y-linked and
X-linked recessive inheritance in humans.
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.
Chapter 4
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.
62. Explain how dosage balance is achieved between X-linked genes and
autosomal genes in mammals.
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.