1. 12
The Chromosomal Basis of Inheritance

2. Why did the improvement of microscopy techniques in the late 1800s set the stage for the emergence of modern genetics?
It revealed new and unanticipated features of Mendel’s pea plant varieties.
It allowed the study of meiosis and mitosis, revealing parallels between behaviors of the Mendelian concept of the gene and the movement/pairing of chromosomes.
It allowed scientists to see the nucleotide sequence of DNA.
It led to the discovery of mitochondria.
It showed genes functioning to direct the formation of enzymes.
Answer: B
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3. Why did the improvement of microscopy techniques in the late 1800s set the stage for the emergence of modern genetics?
It revealed new and unanticipated features of Mendel’s pea plant varieties.
It allowed the study of meiosis and mitosis, revealing parallels between behaviors of the Mendelian concept of the gene and the movement/pairing of chromosomes.
It allowed scientists to see the nucleotide sequence of DNA.
It led to the discovery of mitochondria.
It showed genes functioning to direct the formation of enzymes.
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4. Morgan and his colleagues worked out a set of symbols to represent fly genotypes. Which of the following are representative?
AaBb  AaBb
46 or 46w
w or w on X
2  3
Answer: C
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5. Morgan and his colleagues worked out a set of symbols to represent fly genotypes. Which of the following are representative?
AaBb  AaBb
46 or 46w
w or w on X
2  3
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6. Imagine that Morgan had chosen a different organism for his genetics experiments. What kind of species would have made a better choice than fruit flies?
Answer: This question is designed to make students think about the process of genetic experimentation. Choosing a plant that could be self-pollinated would allow the experimenter to self-pollinate the plant and expose recessive alleles more quickly. Choosing a species with more genetic diversity could allow recessive alleles to be detected more quickly. A shorter generation time would be beneficial in that genetic tests could be conducted more quickly. Choosing a species with many small chromosomes would have made light microscopy difficult.
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7. Imagine that Morgan had used a grasshopper (2n  24, and sex is determined as follows: male has X, and female has XX) to study sex linkage. Predict where the first mutant would have been discovered.
on the O chromosome of a male
on the X chromosome of a male
on the X chromosome of a female
on the Y chromosome of a male
Answer: B. This question is designed to show students that there are a variety of sex determination mechanisms and to help students understand the significance of Morgan’s work. Answer A is incorrect because there is no O chromosome—the X chromosome has no pairing partner. Answer C is incorrect because a mutant is likely recessive and is likely to be masked by a dominant on the other X chromosome in a female. Answer D is incorrect because grasshopper males do not have a Y chromosome. Answer B is correct because a mutant on the male’s single X chromosome would not be masked by a normal allele on a second X chromosome.
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8. Imagine that Morgan had used a grasshopper (2n  24, and sex is determined as follows: male has X, and female has XX) to study sex linkage. Predict where the first mutant would have been discovered.
on the O chromosome of a male
on the X chromosome of a male
on the X chromosome of a female
on the Y chromosome of a male
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9. Think about bees and ants, which have no X and Y sex chromosomes
Think about bees and ants, which have no X and Y sex chromosomes. Males are haploid, whereas fertilization results in females, as diploid cells become females. Which of the following are accurate statements about bee and ant males when they are compared to species in which males are XY and diploid for the autosomes?
Bee males have half the DNA of bee females, whereas human males have nearly the same amount of DNA that human females have.
Considered across the genome, harmful (deleterious) recessives will negatively affect bee males more than Drosophila males.
Human and Drosophila males have sons, but bee males do not.
Inheritance in bees is like inheritance of sex-linked characteristics in humans.
none of the above
Answer: The point of this question is that there are a variety of sex determination mechanisms among different species. Answers A, B, and C are correct. Answer A is correct because bee males are haploid, but the only difference in the amount of DNA in human males and females is that the Y chromosome is slightly smaller than the X chromosome. Answer B is correct because deleterious alleles from the whole genome will affect male bees, but deleterious recessives on the autosomes in Drosophila males can be masked by a dominant allele. Answer C is correct because, in humans and Drosophila, half of a male’s sperm carry a Y chromosome and cause the production of male offspring, but in bees and ants males are haploid, and any egg the sperm fertilizes develops into a female. Answer D is incorrect because male bees have no sons.
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10. Think about bees and ants, which have no X and Y sex chromosomes
Think about bees and ants, which have no X and Y sex chromosomes. Males are haploid, whereas fertilization results in females, as diploid cells become females. Which of the following are accurate statements about bee and ant males when they are compared to species in which males are XY and diploid for the autosomes?
Bee males have half the DNA of bee females, whereas human males have nearly the same amount of DNA that human females have.
Considered across the genome, harmful (deleterious) recessives will negatively affect bee males more than Drosophila males.
Human and Drosophila males have sons, but bee males do not.
Inheritance in bees is like inheritance of sex-linked characteristics in humans.
none of the above
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11. Determination of sex in Drosophila is similar to that in humans
Determination of sex in Drosophila is similar to that in humans. In some species of Drosophila, there are genes on the Y chromosome that do not occur on the X chromosome. Imagine that a mutation of one gene on the Y chromosome reduces the size by half of individuals with the mutation. Which of the following statements is accurate with regard to this situation?
This mutation is transmitted to all offspring of a male with the mutation.
This mutation is transmitted to all male but no female offspring of a male with the mutation.
This mutation is transmitted to all offspring of a female with the mutation.
This mutation is transmitted to all male but no female offspring of a female with the mutation.
This mutation is transmitted to all offspring of both males and females with the mutation.
Answer: B. The point of this question is to help students understand sex linkage by looking at the effect of genes on the Y chromosome. Answer A is incorrect because half of the sperm of the mutant male will contain an X chromosome and produce normal size daughters. Answer C is incorrect because females do not have a Y chromosome and cannot carry the mutation. Answer D is incorrect because, again, females do not have a Y chromosome and thus cannot carry the mutation. Answer E is incorrect because females cannot carry the mutation (no Y chromosome) and only the half of the eggs that are fertilized by a Y-bearing sperm will receive the mutation (i.e., only half of the male offspring). Answer B is correct because all the sons of the mutant male receive his Y chromosome.
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12. Determination of sex in Drosophila is similar to that in humans
Determination of sex in Drosophila is similar to that in humans. In some species of Drosophila, there are genes on the Y chromosome that do not occur on the X chromosome. Imagine that a mutation of one gene on the Y chromosome reduces the size by half of individuals with the mutation. Which of the following statements is accurate with regard to this situation?
This mutation is transmitted to all offspring of a male with the mutation.
This mutation is transmitted to all male but no female offspring of a male with the mutation.
This mutation is transmitted to all offspring of a female with the mutation.
This mutation is transmitted to all male but no female offspring of a female with the mutation.
This mutation is transmitted to all offspring of both males and females with the mutation.
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13. In cats, a sex-linked gene affects coat color
In cats, a sex-linked gene affects coat color. The O allele produces an enzyme that converts eumelanin, a black or brown pigment, into phaeomelanin, an orange pigment. The o allele is recessive to O and produces a defective enzyme, one that does not convert eumelanin into phaeomelanin. Which of the following statements is/are accurate?
The phenotype of o-Y males is black/brown because the nonfunctional allele o does not convert eumelanin into phaeomelanin.
The phenotype of OO and Oo males is orange because the functional allele O converts eumelanin into phaeomelanin.
The phenotype of Oo males is mixed orange and black/brown because the functional allele O converts eumelanin into phaeomelanin in some cell groups (orange) and because in other cell groups the nonfunctional allele o does not convert eumelanin into phaeomelanin.
The phenotype of O-Y males is orange because the nonfunctional allele O does not convert eumelanin into phaeomelanin, while the phenotype of o-Y males is black/brown because the functional allele o converts eumelanin into phaeomelanin.
Answer: A. This question focuses on the color of males and the action of the enzyme that converts eumelanin (brown/black pigment) to phaeomelanin (orange pigment). Male genotypes will be either O-Y or o-Y, with phenotypes of either orange or black/brown, respectively. In O-Y males, the eumelanin is converted to phaeomelanin, and in o-Y males, the eumelanin is unchanged. To answer this question, a student must know that males have only one copy of the gene and must understand that a functional allele produces an enzyme that catalyzes the chemical reaction.
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14. In cats, a sex-linked gene affects coat color
In cats, a sex-linked gene affects coat color. The O allele produces an enzyme that converts eumelanin, a black or brown pigment, into phaeomelanin, an orange pigment. The o allele is recessive to O and produces a defective enzyme, one that does not convert eumelanin into phaeomelanin. Which of the following statements is/are accurate?
The phenotype of o-Y males is black/brown because the nonfunctional allele o does not convert eumelanin into phaeomelanin.
The phenotype of OO and Oo males is orange because the functional allele O converts eumelanin into phaeomelanin.
The phenotype of Oo males is mixed orange and black/brown because the functional allele O converts eumelanin into phaeomelanin in some cell groups (orange) and because in other cell groups the nonfunctional allele o does not convert eumelanin into phaeomelanin.
The phenotype of O-Y males is orange because the nonfunctional allele O does not convert eumelanin into phaeomelanin, while the phenotype of o-Y males is black/brown because the functional allele o converts eumelanin into phaeomelanin.
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15. Imagine a species with three loci thought to be on the same chromosome
Imagine a species with three loci thought to be on the same chromosome. The recombination rate between locus A and locus B is 35%, and the recombination rate between locus B and locus C is 33%. Predict the recombination rate between A and C.
The recombination rate between locus A and locus C is either 2% or 68%.
The recombination rate between locus A and locus C is probably 2%.
The recombination rate between locus A and locus C is either 2% or 50%.
The recombination rate between locus A and locus C is either 2% or 39%.
The recombination rate between locus A and locus C cannot be predicted.
Answer: C. The recombination rate between loci A and B is 35%. Locus C can be either between A and B or on the opposite side of B from A. If locus C is in between locus A and locus B (ACB), the distance between locus A and C would be 2%. Locus C cannot be on the other side of A from B (CAB) because the recombination rate would have to be higher than 35%. Thus far, answers A, B, C, and D could be correct. If locus C is on the other side of locus B from locus A (ABC), adding the two recombination rates gives a prediction of 68%, but the maximum recombination rate between two loci is 50%, the same frequency of recombinants that are on different chromosomes. Using this information, answer A cannot be correct because a recombination rate cannot be greater than 50%. Answer B could be right, but it is not the only possible placement and so answer B is incomplete. Answer C is the best answer (2% if C is between A and B, and 50% if C is on the other side of B than A).
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16. Imagine a species with three loci thought to be on the same chromosome
Imagine a species with three loci thought to be on the same chromosome. The recombination rate between locus A and locus B is 35%, and the recombination rate between locus B and locus C is 33%. Predict the recombination rate between A and C.
The recombination rate between locus A and locus C is either 2% or 68%.
The recombination rate between locus A and locus C is probably 2%.
The recombination rate between locus A and locus C is either 2% or 50%.
The recombination rate between locus A and locus C is either 2% or 39%.
The recombination rate between locus A and locus C cannot be predicted.
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17. Chromosomal rearrangements can occur after chromosomes break
Chromosomal rearrangements can occur after chromosomes break. Which of the following statements is most accurate with respect to alterations in chromosome structure?
Chromosomal rearrangements are more likely to occur in mammals than in other vertebrates.
Translocations and inversions are not deleterious because no genes are lost in the organism.
Chromosomal rearrangements are more likely to occur during mitosis than during meiosis.
An individual that is homozygous for a deletion of a certain gene is likely to be more damaged than one that is homozygous for a duplication of that same gene because loss of a function can be lethal.
Answer: D. Chromosomal rearrangements are important in evolution. For example, duplications provide raw material on which natural selection can act (e.g., the globin genes are thought to have arisen via gene duplication). This question will make students think about the consequences of chromosomal rearrangements. Answer A is not correct and should be nixed by students because there is no information in the chapter that would lead to this conclusion. Answers B and C directly contradict material in the text and are therefore incorrect.
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18. Chromosomal rearrangements can occur after chromosomes break
Chromosomal rearrangements can occur after chromosomes break. Which of the following statements is most accurate with respect to alterations in chromosome structure?
Chromosomal rearrangements are more likely to occur in mammals than in other vertebrates.
Translocations and inversions are not deleterious because no genes are lost in the organism.
Chromosomal rearrangements are more likely to occur during mitosis than during meiosis.
An individual that is homozygous for a deletion of a certain gene is likely to be more damaged than one that is homozygous for a duplication of that same gene because loss of a function can be lethal.
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19. Imagine that you could create medical policy for a country
Imagine that you could create medical policy for a country. In this country it is known that the frequency of Down syndrome increases with increasing age of the mother and that the frequency of schizophrenia and autism increases with the age of the father. In both schizophrenia and autism, the severity of characteristics varies enormously and unpredictably among affected individuals. Furthermore, financial resources are severely limited, both for testing of pregnant women and for supplemental training of children with Down syndrome. What kind of policy regarding fetal testing would you implement?
Answer: This question should stimulate a spirited discussion. It will be important to emphasize that financial resources are limited. We often act as if such funds are, or should be, unlimited. Some things to consider: Medical testing is likely to be much cheaper than training; fewer than 4% of fetuses in the highest risk group are affected; and Down syndrome incidence increases somewhat sharply at certain maternal ages. The website contains a lot of information.
In addition, there is much to add to the discussion regarding aging males and their sperm. There is a new emphasis on aging males and the frequency of autism and schizophrenia in particular. See and
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20. Recall that in Drosophila, white eyes are due to an X-linked recessive allele (Xw). Describe a genetic cross that could result in white–eyed female Drosophila.
no possible cross
white-eyed females with red-eyed males
heterozygous red-eyed females with white-eyed males
heterozygous red-eyed females with red-eyed males
Answer: C. This question tests knowledge of X-linked recessive genetics. The question is not rote application but rather requires analysis.
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21. Recall that in Drosophila, white eyes are due to an X-linked recessive allele (Xw). Describe a genetic cross that could result in white–eyed female Drosophila.
no possible cross
white-eyed females with red-eyed males
heterozygous red-eyed females with white-eyed males
heterozygous red-eyed females with red-eyed males
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22. Which statement best describes the relationship between recombination frequency and the physical distance of genes on chromosomes?
There is no relationship. All genes have random recombination frequencies.
There is no relationship. All genes have the same, fixed recombination frequencies.
The farther apart two genes are, the higher the recombination frequency
The closer together two genes are, the higher the recombination frequency
Answer: C. This question tests knowledge of the fundamental concept of genetic mapping.
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23. Which statement best describes the relationship between recombination frequency and the physical distance of genes on chromosomes?
There is no relationship. All genes have random recombination frequencies.
There is no relationship. All genes have the same, fixed recombination frequencies.
The farther apart two genes are, the higher the recombination frequency
The closer together two genes are, the higher the recombination frequency
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24. What is the expected recombination frequency for a testcross between the black and cinnabar loci?9%
48.5%
50%
57.5%
Answer: A. Going from map to predicted results.
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25. What is the expected recombination frequency for a testcross between the black and cinnabar loci?9%
48.5%
50%
57.5%
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26. What is the expected recombination frequency for a testcross between the black and brown loci?
48.5%
50%
56%
100%
Answer: B. Going from map to predicted results, understanding that max RF = 50%.
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27. What is the expected recombination frequency for a testcross between the black and brown loci?
48.5%
50%
56%
100%
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28. In tomatoes, a heterozygous plant with yellow flowers and red fruit is crossed with a recessive plant having white flowers and yellow fruit. The following distribution of offspring is observed: yellow flowers, red fruit 42.5% white flowers, yellow fruit 42.5% yellow flowers, yellow fruit 7.5% white flowers, red fruit 7.5% What conclusion can be made regarding the loci for flower color and fruit color?
The loci may be on the same chromosome more than 50 map units apart, or they may be on separate chromosomes.
The loci are on separate chromosomes.
The loci are on the same chromosome, at an unknown distance from each other.
The loci are on the same chromosome 15 map units apart.
Answer: D. Using data to construct map.
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29. In tomatoes, a heterozygous plant with yellow flowers and red fruit is crossed with a recessive plant having white flowers and yellow fruit. The following distribution of offspring is observed: yellow flowers, red fruit 42.5% white flowers, yellow fruit 42.5% yellow flowers, yellow fruit 7.5% white flowers, red fruit 7.5% What conclusion can be made regarding the loci for flower color and fruit color?
The loci may be on the same chromosome more than 50 map units apart, or they may be on separate chromosomes.
The loci are on separate chromosomes.
The loci are on the same chromosome, at an unknown distance from each other.
The loci are on the same chromosome 15 map units apart.
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30. In tomatoes, a heterozygous plant with purple stems and normal leaves is crossed with a recessive plant having green stems and broad leaves. The following distribution of offspring is observed: purple stems, normal leaves 25% green stems, broad leaves 25% purple stems, broad leaves 25% green stems, normal leaves 25% What conclusion can be made regarding the loci for stem color and leaf shape?
The loci may be on the same chromosome more than 50 map units apart, or they may be on separate chromosomes.
The loci are on separate chromosomes.
The loci are on the same chromosome, at an unknown distance from each other.
The loci are on the same chromosome 25 map units apart.
Answer: A. Using data to construct map, with ambiguity of large map distances highlighted.
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31. In tomatoes, a heterozygous plant with purple stems and normal leaves is crossed with a recessive plant having green stems and broad leaves. The following distribution of offspring is observed: purple stems, normal leaves 25% green stems, broad leaves 25% purple stems, broad leaves 25% green stems, normal leaves 25% What conclusion can be made regarding the loci for stem color and leaf shape?
The loci may be on the same chromosome more than 50 map units apart, or they may be on separate chromosomes.
The loci are on separate chromosomes.
The loci are on the same chromosome, at an unknown distance from each other.
The loci are on the same chromosome 25 map units apart.
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32. Which of the following diagrams best depicts the karyotype of a trisomy?
Answer: B. This question tests knowledge of karyotypes and ploidy.
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33. Which of the following diagrams best depicts the karyotype of a trisomy?
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34. Nondisjunction can happen in either meiosis I or meiosis II
Nondisjunction can happen in either meiosis I or meiosis II. Consider an n + 1 gamete that is formed from nondisjunction and compare the origin of the extra (+1) chromosome between the two types of nondisjunction. Select the best comparative statement.
There is no inherent difference between the two.
The +1 chromosome resulting from meiosis I nondisjunction was a homolog of its partner, while that from meiosis II was a sister chromatid of its partner.
The +1 chromosome resulting from meiosis I nondisjunction was a sister chromatid of its partner, while that from meiosis II was a homolog of its partner.
The +1 chromosome resulting from meiosis I nondisjunction results in syndromes, while that from meiosis II does not.
Answer: B. This questions tests knowledge of the high-level but important distinction between two kinds of nondisjunction.
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35. Nondisjunction can happen in either meiosis I or meiosis II
Nondisjunction can happen in either meiosis I or meiosis II. Consider an n + 1 gamete that is formed from nondisjunction and compare the origin of the extra (+1) chromosome between the two types of nondisjunction. Select the best comparative statement.
There is no inherent difference between the two.
The +1 chromosome resulting from meiosis I nondisjunction was a homolog of its partner, while that from meiosis II was a sister chromatid of its partner.
The +1 chromosome resulting from meiosis I nondisjunction was a sister chromatid of its partner, while that from meiosis II was a homolog of its partner.
The +1 chromosome resulting from meiosis I nondisjunction results in syndromes, while that from meiosis II does not.
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