bsci222 chapter 4

4. A female with androgen-insensitivity syndrome, a sex-linked recessive condition, has:

D) a pair of testes that produce testosterone.

21. 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:

D) female in Drosophila but male in humans.

12. An XXY chromosome constitution produces _____ development in humans and _____

D) male; female

74. 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.

a

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 mutation in the androgen receptor gene

65. 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?

(1) 7, 7 (fingerprint would show just 7) (2) 14, 14 (fingerprint would show 14) (3) 7, 14 (4) 7, 5 (5) 5, 14

85. 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

84. 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

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

(1) XX-XO; grasshoppers (2) XX-XY; humans (3) ZZ-ZW; birds (4) X:A; Drosophila (5) Environmental mollusks; many turtles, crocodiles, and alligators

71. 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 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.

(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.

41. 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?

0 %

50. If a male bird that is heterozygous for a recessive Z-linked mutation is crossed to a wildtype female, what proportion of the progeny will be mutant males?

0%

51. 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?

1/8

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.

1/8

49. 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?

1/8

52. 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.)

1/8

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

27. Because of the presence of a normal Y chromosome, the gonads develop as testes.

28. What level of male hormones would you expect to find in a person with androgeninsensitivity syndrome?

28. Based on the presence of normal testes, one would expect to find normal levels of male hormones in these individuals.

42. 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?

3/16

86. 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.

86. 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.

87. Explain how dosage balance is achieved between X-linked genes and autosomal genes in mammals.

87. 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.

46. 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?

9/32

55. 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 wildtype 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.

A

20. 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 mutation has occurred in the SRY gene making it inactive.

33. 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.

19. 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 piece of chromosomal material containing an active SRY gene is found attached to one of the X chromosomes.

7. 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.

39. 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

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

A) one per somatic cell

13. The sex determination system used by Drosophila is called:

A) the X:A sex determination system.

24. Explain the genders of human and diploid 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 normally diploid, so there are two of each autosome. If there is a single X chromosome, the X:A ratio is 1/2, or 0.5, which is a male.

23. Explain the genders of human and diploid Drosophila XXY individuals.

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 normally 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.

54. 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

B

60. A new mutation in Drosophila is 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.

B

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

B) It would likely develop into a sterile male with reduced testes.

8. In a germ-line cell from a female grasshopper (XX-XO sex determination system), when do the homologous X chromosomes segregate?

B) during meiosis I, anaphase

9. In a germ-line cell from a human male that is dividing, when do the X and Y chromosomes segregate?

B) during meiosis I, anaphase

14. With the XX-XO sex determination system, generally:

B) male offspring have one X chromosome, and it is inherited from their mother.

64. 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.

26. 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?

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.

63. 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 X chromosomes.

56. 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

C

57. 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

C

61. 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.

C

58. 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

D

53. 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.

E

59. Miniature wings in Drosophila are due to an X-linked allele (Xm) 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

E

62. 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

E

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

E) many turtles and alligators

34. 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.

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

35. 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.

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

36. 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.

He inherited the translocation chromosome from his father because his mother could not have carried the SRY-containing chromosome.

67. 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?

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

88. 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.

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.

1. What is the role of the SRY gene in humans?

It is located on the Y chromosome and initiates the developmental pathway toward the male phenotype.

68. 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?

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.

66. Describe the inheritance of each possible combination of your answer to question 61, 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. including the parent and meiotic stage in which an unusual event occurred

Nondisjunction in meiosis I: Homologs fail to segregate (e.g., X5 and Y or X7 and X14).

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?

Some of the boys are XXY

32. 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.)

40. Which statement BEST summarizes our current understanding of the origin of the Y chromosome?

The Y chromosome is thought to have been derived along with the X chromosome from a pair of autosomes.

25. 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?

The chromosome constitution is XY

31. 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

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.

30. 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?

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.

29. Explain the development of female external anatomy in individuals with androgeninsensitivity 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.

69. 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?

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 t/t and Xe/Xe. One-quarter of the F2 progeny will be t/ t, 1/4 will be XeY. Therefore, 1/16 of the progeny will be black-padded, pointy-eared males.

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

YY

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

XXXXXYYYYY

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

XXY

18. In which of the following phenotypic females do testes develop?

XY with the X-linked recessive condition of androgen-insensitivity syndrome

70. 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. 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.

76. 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

b

72. 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.

c

75. 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

c

78. 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

c

81. A male has Klinefelter syndrome with the chromosome constitution of XXXY. How many Barr bodies should be found in the nuclei of cells of this individual? A) 0 B) 1 C) 2 D) 3 E) 4

c

73. 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.

d

80. 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

d

82. How is dosage compensation accomplished in Drosophila? A) One of the two X chromosomes is inactivated in females. B) Drosophila has no obvious mechanism of dosage compensation. C) The Y chromosome has an increased level of gene expression. D) The activity of genes on the male X chromosome is doubled but not that of genes on the X chromosome of females. E) The activity of genes on half of the autosomes is doubled and the activity of genes on the X chromosomes of both males and females is decreaed.

d

83. Assume that an XX mouse is homozygous for a mutation in the Xist gene that causes the gene to not be expressed. What would be the expected consequences of such a situation? A) The mouse would have two Barr bodies instead of one. B) The mouse would develop as a male. C) The mouse would have a Y chromosome. D) Both X chromosomes would be active. E) Nondisjunction would occur at each meiotic event.

d

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

dioecious.

77. 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

e

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. From what type of individual is this cell?

female

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.

heterogametic; homogametic

47. 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.

male

During the evolution of the human Y chromosome, all of the following are assumed to occur

many of the genes on the early X chromosome that were responsible for critical cellular functions got moved to the evolving Y chromosome

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

monoecious

44. 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?

one chromosome with an a allele, one chromosome with a B allele, one W

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?

one chromosome with the A allele, one chromosome with the a allele