Deck 4: Sex Determination and Sex-Linked Characteristics

Question 1

What is the expected outcome for a human embryo with the XXXY chromosome constitution?

A) It would likely develop into a female who will not respond to the hormone testosterone.
B) It would likely develop into a sterile male with reduced testes.
C) It will always abort early in development before birth.
D) It would likely develop into a tall female who may be slightly cognitively impaired.
E) It would likely develop into a fertile man with a completely normal male phenotype.

Answer 1

B

Question 2

In which of the following organisms is gender/sex determined by the temperature during embryonic development?

A) humans
B) mice
C) fruit flies
D) many snakes and birds
E) many turtles and alligators

Answer 2

E

Question 3

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
E) male, intersex

Answer 3

D

Question 4

In humans, occasionally a baby is found that has the XY chromosomal karyotype but is phenotypically female. Which of the following statements might be a CORRECT explanation for at least some of these unusual cases?

A) A mutation has occurred in the SRY gene making it inactive.
B) An extra pierce of autosomal chromosome 15 is probably present in the genome but is too small to be detected.
C) A small piece of the X chromosome is missing but is too small to be detected.
D) The Xist gene on the X chromosome is mutated and inactive.
E) The ratio of number of X chromosomes to number of sets of chromosomes is incorrect.

Question 5

In humans, occasionally a baby is found that has the XX chromosomal karyotype but is phenotypically male. Which of the following statements might be a CORRECT explanation for at least some of these unusual cases?

A) A mutation has occurred in the SRY gene making it inactive.
B) An extra pierce of autosomal chromosome 15 is probably present in the genome but is too small to be detected.
C) A small piece of autosomal chromosome 15 is missing but is too small to be detected.
D) A piece of chromosomal material containing an active SRY gene is found attached to one of the X chromosomes.
E) The ratio of number of X chromosomes to number of sets of chromosomes is incorrect.

Question 6

Which of the following chromosome constitutions would never lead to a viable human baby being born?

A) XXX
B) XYY
C) XO (O = the absence of a second chromosome)
D) YY
E) XXY

Question 7

Species in which individuals have only male or only female reproductive structures are called:

A) hermaphrodites.
B) diploids.
C) dioecious.
D) homogametic.
E) monoecious.

Question 8

With the XX-XO sex determination system, generally:

A) female offspring have one X chromosome, and it is inherited from their father.
B) male offspring have one X chromosome, and it is inherited from their mother.
C) male offspring have one X chromosome, and it is inherited from their father.
D) female offspring have one X chromosome, and it is inherited from their mother.
E) male offspring have two X chromosomes, one inherited from each parent.

Question 9

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) an XXX chromosome constitution that causes her not to produce testosterone.
D) a pair of testes that produce testosterone.
E) an inactive SRY gene.

Question 10

The sex determination system used by Drosophila is called:

A) the X:A sex determination system.
B) the ZZ-ZW sex determination system.
C) the XX-XO sex determination system.
D) the XX-XY sex determination system.
E) genic sex detemination.

Question 11

What is the sex chromosome constitution of a male duck-billed platypus?

A) XX
B) XY
C) XO
D) ZZ
E) XXXXXYYYYY

Question 12

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) a mutation in the SRY gene
B) a mutation in the androgen receptor gene
C) a deletion that removes much of the Y chromosome
D) They likely do not carry a mutation but may have been premature babies.
E) a defect in Xist gene involved in X inactivation

Question 13

Species in which an individual organism has both male and female reproductive structures are called:

A) monoecious.
B) haploid.
C) diploid.
D) dioecious.
E) heterogametic.

Question 14

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.
E) just before meiosis begins

Question 15

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 answers is correct.

Question 16

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) homogametic; heterogametic
B) dioecious; monoecious
C) heterogametic; homogametic
D) monoecious; dioecious
E) monoecious; heterogametic

Question 17

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.
E) just before meiosis begins

Question 18

In which of the following phenotypic females do testes develop?

A) XY with an deletion that removes the SRY gene
B) XO
C) XY with the X-linked recessive condition of androgen-insensitivity syndrome
D) XX
E) XXX

Question 19

In species of birds, males are the homogametic sex and females the heterogametic sex. Which of the following statements is TRUE in this system of sex determination?

A) The gender of the offspring is determined by the female parent.
B) Male offspring have a ZW chromosome constitution.
C) The gender of the offspring is determined by the male parent.
D) Female offspring have a ZZ chromosome constitution.
E) Female and male offspring have the same chromosome constitution.

Question 20

Which of the following human genotypes is associated with Klinefelter syndrome?

A) XXY
B) XYY
C) XXX
D) XO
E) XX

Question 21

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.

Question 22

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?

Question 23

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
E) a color-blind male or a color-blind female

Question 24

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 the 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) Some of the boys are XYY.
B) Some of the boys are XY but have lost the SRY gene from their Y chromosome.
C) Some of the boys are YY.
D) Some of the boys are XXY.
E) Some of the boys are XXX.

Question 25

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.

Question 26

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.

Question 27

While doing summer fieldwork 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 these data.

Question 28

Explain the development of female external anatomy in individuals with androgen-insensitivity syndrome.

Question 29

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?

Question 30

Explain the genders of human and diploid DrosophilaXXY individuals.

Question 31

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.

Question 32

What level of male hormones would you expect to find in a person with androgen-insensitivity syndrome?

Question 33

List five different sex determination systems and a representative organism for each.

Question 34

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?

Question 35

Predict the sexual phenotype of a person who is XY but whose Y chromosome carries a deletion of the SRY gene. Explain your prediction.

Question 36

Explain the genders of human and diploid Drosophila XO individuals.

Question 37

Characterize the gonads of a person with androgen-insensitivity syndrome as testes, ovaries, or intersex.

Question 38

How might an XY person with a deletion of the SRY gene be distinguished from a person with androgen-insensitivity syndrome?

Question 39

In Drosophila, flies that are XXY but have a normal diploid set of autosomes have been found. Such XXY chromosome constitutions have also been found in humans. The sex of such individuals is expected to be:

A) male in both Drosophila and humans.
B) female in both Drosophila and humans.
C) male in Drosophila but female in humans.
D) female in Drosophila but male in humans.
E) intersexes in Drosophila and males in humans.

Question 40

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) The Y chromosome arose from the fusion of the X chromosome with one of the autosomes.

Question 41

Joan is phenotypically normal but had a child with the autosomal recessive disease cystic fibrosis (CF) from a previous marriage. Joan's father has hemophilia 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) 1/8
B) 1/12
C) 1/24
D) 3/32
E) 5/32

Question 42

In Drosophila yellow body color is caused by a sex-linked recessive allele and brown eye color is an autosomal recessive trait. A phenotypically normal female fly, heterozygous for both genes, is crossed to a male with normal body color and brown eyes. What proportion of their offspring should have yellow body color and brown eyes?

A) 1/16
B) 1/8
C) 1/4
D) 3/16
E) 3/8

Question 43

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 here. Assume A and B are dominant alleles. If this individual were crossed to an individual of genotype Aa Bb, what is the probability of an offspring being a female with an A__B__ genotype?

A) 1/8
B) 1/16
C) 9/16
D) 9/32
E) 3/32

Question 44

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 normal men will have only normal sons and daughters.
E) None of the statements is correct.

Question 45

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 here. From what type of individual is this cell?

A) male
B) female
C) hermaphrodite
D) monoecious
E) intersex

Question 46

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%
E) 50%

Question 47

James is a 42-year-old man with hemophilia, a sex-linked recessive condition. His daughter, Susan, who has normal blood clotting, is married to Fred who also has hemophilia. Susan and Fred are expecting their first child and an ultrasound shows that the fetus is male. What is the approximate probability that their new son will have hemophilia?

A) 1
B) 3/4
C) 2/3
D) 1/2
E) 1/4

Question 48

A eukaryotic diploid cell from an organism with the XX-XO sex determination system has two pairs of autosomes and one X chromosome, shown here. 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 chromosome with the B allele, and two X chromosomes
B) one chromosome with the A allele, one chromosome 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 a allele
D) one chromosome with the b allele, one chromosome with the B allele, one X chromosome
E) one chromosome with the a allele, one chromosome with the B allele, and two X chromosomes

Question 49

Miniature wings in Drosophila are due to an X-linked allele (X^m) that is recessive to the wild-type allele for normal long wings (X⁺). Sepia eyes are produced by an autosomal allele (se), which is recessive to the wild-type allele for red eyes (se⁺). A female that is homozygous for normal wings and has sepia eyes is crossed with a male that has miniature wings and is homozygous for red eyes. The F₁ offspring are intercrossed to produce the F₂ generation. What proportion of the F₂ females is expected to have miniature wings and sepia eyes?

A) 1/4
B) 3/16
C) 1/2
D) 1/8
E) 0

Question 50

A eukaryotic diploid cell from an organism with the XX-XO sex determination system has two pairs of autosomes and one X chromosome, shown here. From what type of individual is this cell?

A) male
B) female
C) hermaphrodite
D) monoecious
E) intersex

Question 51

A woman is phenotypically normal but her father had the sex-linked recessive condition of red-green color blindness. If she marries a man with normal vision, what is the probability that their two children will both have normal vision?

A) 4/9
B) 1/16
C) 9/16
D) 3/8
E) 3/4

Question 52

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 here. 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) one chromosome with the B allele, one with the b allele, one Z

Question 53

A new mutation inDrosophilais found called peach, which causes a peach-like body color. A peach male is crossed to a homozygous wild-type female and the offspring consists of peach females and normal males. If a heterozygous peach female is crossed to a normal male, the offspring consists of ¹/₄ peach females, ¹/₄ normal females, ¹/₄ peach males, and ¹/₄ normal males. What is the MOST likely mode of inheritance for peach?

A) It is X linked and recessive.
B) It is X linked and dominant.
C) It is autosomal and recessive.
D) It is autosomal and dominant.
E) It is Y linked.

Question 54

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 here. 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
E) 1/16

Question 55

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%

Question 56

A phenotypically normal man marries a phenotypically normal woman whose father was color blind. They have a color-blind daughter with Turner syndrome (XO). What is the BEST explanation for this result?

A) nondisjunction at meiosis II in the mother
B) nondisjunction at meiosis I in the mother
C) nondisjunction at meiosis I or meiosis II in the father
D) mitotic nondisjunction in the first division of the zygote
E) two sperm fertilized the same egg simultaneously

Question 57

You discover a new mutation in Drosophila that causes an abnormal wing shape. When a male fly with this mutation is crossed to a homozygous normal female, the F₁ are all wild-type phenotypically. The F₂ flies consist of ¹/₂ phenotypically normal females, ¹/₄ phenotypically normal males, and ¹/₄ males with abnormal wings. What is the MOST likely explanation for the new mutant allele?

A) It is X linked and recessive.
B) It is X linked and dominant.
C) It is autosomal and recessive.
D) It is autosomal and dominant.
E) It is Y linked.

Question 58

A eukaryotic diploid cell from an organism with the XX-XO sex determination system has two pairs of autosomes and one X chromosome, shown here. 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
E) 1/16

Question 59

A woman is phenotypically normal, but her father had the sex-linked recessive condition of red-green color blindness. 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 color blind?

A) 1/16
B) 3/8
C) 3/16
D) 3/6
E) 8/27

Question 60

A woman has normal vision although her maternal grandfather (her mother's father) had red-green color blindness, 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 color blind. What is the probability that the first child of this couple will be color blind?

A) 1/2
B) 1/4
C) 1/8
D) 1/16
E) 1/12

Question 61

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.

Question 62

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 compared to one X.
C) Some genes remain active on inactive X chromosomes so XXY males would 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.

Question 63

How many Barr bodies (condensed X chromosomes) would you predict in this boy's cells?

A) one per somatic cell
B) two per somatic cell
C) three per somatic cell
D) none
E) either two or three depending on the tissue type

Question 64

Women known to be heterozygous or carriers for the sex-linked recessive condition of hemophilia A were studied to determine the time required for their blood to clot. It was found that the time required for their blood to clot varied from individual to individual. The values obtained ranged from normal clotting at one extreme to clinical hemophilia at the other extreme. What is the MOST probable correct explanation for these findings?

A) Some women had only one X chromosome and it is inactive.
B) Some women had three copies of the X chromosome, which allowed them to make extra amounts of gene products for their X-linked genes.
C) The women with normal clotting times probably had a mother with hemophilia while those with abnormal clotting times probably had fathers with hemophilia.
D) Random X inactivation probably results in individuals with different proportions of cells in their bodies expressing the normal allele at the hemophilia locus.
E) In women with abnormal clotting times, there was probably an interaction between an allele of a gene on the X chromosome and an allele of an autosomal gene that reduced the expression of the X-linked gene.

Question 65

Compare and contrast the patterns of inheritance expected for Y-linked and X-linked recessive inheritance in humans.

Question 66

Which of the following statements about the sex-linked recessive trait of red-green color blindness in humans is FALSE?

A) A phenotypically normal daughter can have a color-blind father.
B) A phenotypically normal daughter can have a color-blind mother.
C) A color-blind daughter can have a normal father.
D) A color-blind daughter can have a phenotypically normal mother.
E) A color-blind son could have a color-blind father.

Question 67

A male fruit fly carries the allele for yellow body color on his X chromosome. How would this male's genotype for this body color gene be described?

A) holandric
B) heterozygous
C) homozygous
D) homogametic
E) hemizygous

Question 68

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.

Question 69

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.

Question 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 colorblind. In this family there are no instances of chromosome loss or gain such as occurs due to nondisjunction in meiosis. Following 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 color-blindness 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.

Question 71

You cross a female rat with pink toe pads (T) and pointy ears (X^e) to a male rat with black toe pads (t) and round ears (X^E). 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?

Question 72

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?

Question 73

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.

Question 74

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
E) sex determination

Question 75

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.

Question 76

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?

Question 77

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?

Question 78

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 color blind, 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?

Question 79

In which of the following individuals would you expect to find two Barr bodies in their somatic cells?

A) XX
B) XO
C) XXY
D) XXYY
E) XXX

Question 80

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) XO women have problems during development because mitosis is abnormal.