1. The Chromosomal Basis of Inheritance
Chapter 15
The Chromosomal Basis of Inheritance
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

2. Morgan’s Experimental Evidence Imagine that Morgan had chosen a different organism for his genetics experiments. Which of the following species would have made a better choice than fruit flies?
a plant that could be self-pollinated
a species with many small chromosomes
a species with more genetic diversity
a species with a one-week generation time
all of the above
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

3. Morgan’s Experimental Evidence Imagine that Morgan had used a grasshopper (2N = 24 and an XX, XO sex determination system). 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 A
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4. The Chromosomal Basis of Sex Think about bees and ants, groups in which males are haploid. 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

5. The Chromosomal Basis of Sex In some Drosophila species 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 occurs in all offspring of a male with the mutation.
This mutation occurs in all male but no female offspring of a male with the mutation.
This mutation occurs in all offspring of a female with the mutation.
This mutation occurs in all male but no female offspring of a female with the mutation.
This mutation occurs in all offspring of both males and females with the mutation.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

6. The Chromosomal Basis of Sex Imagine that a deleterious recessive allele occurs on the W chromosome of a chicken (2N = 78). Where would it be most likely to appear first in a genetics experiment?
in a male because there is no possibility of the presence of a normal, dominant allele
in a male because it is haploid
in a female because there is no possibility of the presence of a normal, dominant allele
in a female because all alleles on the W chromosomes are dominant to those on the Z chromosome
none of the above
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

7. Inheritance of Sex-Linked Genes 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 orange because the functional allele O converts eumelanin into phaeomelanin.
The phenotype of o-Y males is black/brown because the non-functional 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 non-functional allele o does not convert eumelanin into phaeomelanin.
The phenotype of O-Y males is orange because the non-functional 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

8. Inheritance of Sex-Linked Genes 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 females is orange because the functional allele O converts eumelanin into phaeomelanin.
The phenotype of o-Y females is black/brown because the non-functional allele o does not convert eumelanin into phaeomelanin.
The phenotype of OO and Oo females is orange because the functional allele O converts eumelanin into phaeomelanin.
The phenotype of Oo females 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 non-functional allele o does not convert eumelanin into phaeomelanin.
The phenotype of O-Y females is orange because the non-functional 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: This is the second of two questions on the sex-linked trait of orange coat in cats. It focuses on the color of females, the action of the enzyme that converts eumelanin (brown/black pigment) to phaeomelanin (orange pigment), and the development of the Barr body. Female genotypes will be OO (phenotype orange), oo (phenotype black/brown), or Oo (mixed orange and black/brown). In OO females, the eumelanin is converted to phaeomelanin, and in oo females, the eumelanin is unchanged. In Oo females, hairs produced by cells in which the inactivated X chromosome has the O allele are black/brown because the active allele o does not affect the eumelanin. In areas of the Oo females where the cells contain inactivated X chromosomes with the o allele, the hairs are orange because the enzyme produces phaeomelanin.
To answer this question a student must 1) know that females have two copies of the gene, 2) understand that a functional allele produces an enzyme that catalyzes a chemical reaction, and 3) realize that the development of a Barr body results in patches of different colored fur. Answers a, b, and e are incorrect because they show females with Y chromosomes. Answer c is incorrect because it does not take Barr body formation into account. Answer d is correct.
Related to material on textbook pages and Figure 15.8.
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9. X Inactivation in Female Mammals Imagine two species of mammals that differ in the timing of Barr body formation during development. Both species have genes that determine coat color, O for the dominant orange fur and o for the recessive black/brown fur, on the X chromosome. In species A, the Barr body forms during week 1 of a 6-month pregnancy whereas in species B, the Barr body forms during week 3 of a 5-month pregnancy. What would you predict about the coloration of heterozygous females (Oo) in the two species?
Both species will have similar sized patches of orange and black/brown fur.
Species A will have smaller patches of orange or black/brown fur than will species B.
The females of both species will show the dominant fur color, orange.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

10. Mapping the Distance between Genes 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

11. Triploid species are usually sterile (unable to reproduce) whereas tetraploids are often fertile. Which of the following are likely good explanations of these facts?
In mitosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners.
In meiosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners.
In mitosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners.
In meiosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

12. Chromosomal rearrangements can occur after chromosomes break
Chromosomal rearrangements can occur after chromosomes break. Which of the following statements are 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 is one that is homozygous for a duplication of that same gene because loss of a function can be lethal.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

13. 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 babies increases with increasing age of the mother and that 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 Down syndrome children. The graph on the next slide shows the incidence of Down syndrome as a function of maternal age. Which of the following policies would you implement?
No testing of pregnant women should be conducted and all the health care money should be used for training of Down syndrome children.
The health care system should provide testing only for women over 30.
The health care system should provide testing only for women over 40.
The health care system should require termination of all Down syndrome fetuses.
The health care system should provide training for the 30% most seriously affected children only.
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14. Copyright © 2008 Pearson Education, Inc
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

15. The lawyer for a defendant in a paternity suit asked for DNA testing of a baby girl. Which of the following set of results would demonstrate that the purported father was not actually the genetic father of the child?
The mitochondrial DNA of the child and “father” did not match.
DNA sequencing of chromosome #5 of the child and “father” did not match.
The mitochondrial DNA of the child and “mother” did not match.
DNA sequencing of chromosome #5 of the child and “mother” did not match.
The mitochondrial DNA of the child and “father” matched but the mitochondrial DNA of the child and “mother” did not.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.