Question 8.18

Examine the karyotypes shown in Figure 8.1 and Figure 8.2. Are the individuals from whom these karyotype were made males or females?

Question 8.19

Which types of chromosome mutations

• a. increase the amount of genetic material in a particular chromosome?
• b. increase the amount of genetic material in all chromosomes?
• c. decrease the amount of genetic material in a particular chromosome?
• d. change the position of DNA sequences in a single chromosome without changing the amount of genetic material?
• e. move DNA from one chromosome to a nonhomologous chromosome?

Question 8.20

A chromosome has the following segments, where · represents the centromere:

What types of chromosome mutations are required to change this chromosome into each of the following chromosomes? (In some cases, more than one chromosome mutation may be required.)

• a. A B A B · C D E F G
• b. A B · C D E A B F G
• c. A B · C F E D G
• d. A · C D E F G
• e. A B · C D E
• f. A B · E D C F G
• g. C · B A D E F G
• h. A B · C F E D F E D G
• i. A B · C D E F C D F E G

Question 8.21

A chromosome initially has the following segments:

Draw the chromosome, identifying its segments, that would result from each of the following mutations.

• a. Tandem duplication of DEF
• b. Displaced duplication of DEF
• c. Deletion of FG
• d. Paracentric inversion that includes DEFG
• e. Pericentric inversion of BCDE

Question 8.22

The following diagram represents two nonhomologous chromosomes:

What type of chromosome mutation would produce each of the following chromosomes?

• a. A B · C D
  R S · T U V W X E F G
• b. A U V B · C D E F G
  R S · T W X
• c. A B · T U V F G
  R S · C D E W X
• d. A B · C W G
  R S · T U V D E F X

Question 8.23

The Notch mutation is a deletion on the X chromosome of Drosophila melanogaster. Female flies heterozygous for Notch have an indentation on the margins of their wings; Notch is lethal in the homozygous and hemizygous conditions. The Notch deletion covers the region of the X chromosome that contains the locus for white eyes, an X-linked recessive trait. Give the phenotypes and proportions of progeny produced in the following crosses.

• a. A red-eyed, Notch female is mated with a white-eyed male.
• b. A white-eyed, Notch female is mated with a red-eyed male.
• c. A white-eyed, Notch female is mated with a white-eyed male.

Question 8.24

The green-nose fly normally has six chromosomes: two metacentric and four acrocentric. A geneticist examines the chromosomes of an odd-looking green-nose fly and discovers that it has only five chromosomes; three of them are metacentric and two are acrocentric. Explain how this change in chromosome number might have taken place.

Question 8.25

A wild-type chromosome has the following segments:

An individual is heterozygous for the following chromosome mutations. For each mutation, sketch how the wild-type and mutated chromosomes would pair in prophase I of meiosis, showing all chromosome strands.

• a. A B C · D E F D E F G H I
• b. A B C · D H I
• c. A B C · D G F E H I
• d. A B E D · C F G H I

Question 8.26

For the chromosomes shown in Figure 8.12, draw the chromatids that would result from a two-strand double crossover: one crossover between C and D and the other crossover between D and E.

Question 8.27

As discussed in this chapter, crossing over within a pericentric inversion produces chromosomes that have extra copies of some genes and no copies of other genes. The fertilization of gametes containing such duplication or deficient chromosomes often results in children with syndromes characterized by developmental delay, intellectual disability, abnormal development of organ systems, and early death. Maarit Jaarola and colleagues examined individual sperm cells of a male who was heterozygous for a pericentric inversion on chromosome 8 and determined that crossing over took place within the pericentric inversion in 26% of the meiotic divisions (M. Jaarola, R. H. Martin, and T. Ashley. 1998. American Journal of Human Genetics 63:218–224).

Assume that you are a genetic counselor and that a couple seeks counseling from you. Both the man and the woman are phenotypically normal, but the woman is heterozygous for a pericentric inversion on chromosome 8. The man is karyotypically normal. What is the probability that this couple will produce a child with a debilitating syndrome as the result of crossing over within the pericentric inversion?

Question 8.28

An individual heterozygous for a reciprocal translocation possesses the following chromosomes:

• a. Draw the pairing arrangement of these chromosomes in prophase I of meiosis.
• b. Diagram the alternate, adjacent-1, and adjacent-2 segregation patterns in anaphase I of meiosis.
• c. Give the products that result from alternate, adjacent-1, and adjacent-2 segregation.

Question 8.29

Red–green color blindness is a human X-linked recessive disorder. A young man with a 47,XXY karyotype (Klinefelter syndrome) is color blind. His 46,XY brother also is color blind. Both parents have normal color vision. Where did the nondisjunction that gave rise to the young man with Klinefelter syndrome take place? Assume that no crossing over took place in prophase I of meiosis.

Question 8.30

Junctional epidermolysis bullosa (JEB) is a severe skin disorder that results in blisters over the entire body. The disorder is caused by autosomal recessive mutations at any one of three loci that help to encode laminin 5, a major component in the dermal–epidermal basement membrane. Leena Pulkkinen and colleagues described a male newborn who was born with JEB and died at 2 months of age (L. Pulkkinen et al. 1997. American Journal of Human Genetics 61:611–619); the child had healthy unrelated parents. Chromosome analysis revealed that the infant had 46 normal-appearing chromosomes. Analysis of DNA showed that his mother was heterozygous for a JEB-causing allele at the LAMB3 locus, which is on chromosome 1. The father had two normal alleles at this locus. DNA fingerprinting demonstrated that the male assumed to be the father had, in fact, conceived the child.

• a. Assuming that no new mutations occurred in this family, explain the presence of an autosomal recessive disease in the child when the mother is heterozygous and the father is homozygous normal.
• b. How might you go about proving your explanation? Assume that a number of genetic markers are available for each chromosome.

Question 8.31

Some people with Turner syndrome are 45,X/46,XY mosaics. Explain how this mosaicism could arise.

Question 8.32

Bill and Betty have had two children with Down syndrome. Bill’s brother has Down syndrome and his sister has two children with Down syndrome. On the basis of these observations, indicate which of the following statements are most likely correct and which are most likely incorrect. Explain your reasoning.

• a. Bill has 47 chromosomes.
• b. Betty has 47 chromosomes.
• c. Bill and Betty’s children each have 47 chromosomes.
• d. Bill’s sister has 45 chromosomes.
• e. Bill has 46 chromosomes.
• f. Betty has 45 chromosomes.
• g. Bill’s brother has 45 chromosomes.

Question 8.33

In mammals, sex-chromosome aneuploids are more common than autosomal aneuploids but, in fish, sex-chromosome aneuploids and autosomal aneuploids are found with equal frequency. Offer a possible explanation for these differences in mammals and fish. (Hint: Think about why sex chromosome aneuploids are more common than autosomal aneuploids in mammals.)

Question 8.34

A young couple is planning to have children. Knowing that there have been a substantial number of stillbirths, miscarriages, and fertility problems on the husband’s side of the family, they see a genetic counselor. A chromosome analysis reveals that, whereas the woman has a normal karyotype, the man possesses only 45 chromosomes and is a carrier of a Robertsonian translocation between chromosomes 22 and 13.

• a. List all the different types of gametes that might be produced by the man.
• b. What types of zygotes will develop when each of gametes produced by the man fuses with a normal gamete produced by the woman?
• c. If trisomies and monosomies entailing chromosomes 13 and 22 are lethal, approximately what proportion of the surviving offspring are expected to be carriers of the translocation?

Question 8.35

Using breeding techniques, Andrei Dyban and V. S. Baranov (Cytogenetics of Mammalian Embryonic Development. Oxford: Oxford University Press, Clarendon Press; New York: Oxford University Press, 1987) created mice that were trisomic for each of the different mouse chromosomes. They found that only mice with trisomy 19 developed. Mice trisomic for all other chromosomes died in the course of development. For some of these trisomics, they compared the length of development (number of days after conception before the embryo died) as a function of the size of the mouse chromosome that was present in three copies (see the adjoining graph). Summarize their findings as presented in this graph and provide a possible explanation for the results.

[E. Torres, B. R. Williams, and A. Amon. 2008. Genetics 179:737–746, Fig. 2B.]

Question 8.36

Species I has 2n = 16 chromosomes. How many chromosomes will be found per cell in each of the following mutants in this species?

• a. Monosomic
• b. Autotriploid
• c. Autotetraploid
• d. Trisomic
• e. Double monosomic
• f. Nullisomic
• g. Autopentaploid
• h. Tetrasomic

Question 8.37

Species I is diploid (2n = 8) with chromosomes AABBCCDD; related species II is diploid (2n = 8) with chromosomes MMNNOOPP. What types of chromosome mutations do individual organisms with the following sets of chromosomes have?

• a. AAABBCCDD
• b. MMNNOOOOPP
• c. AABBCDD
• d. AAABBBCCCDDD
• e. AAABBCCDDD
• f. AABBDD
• g. AABBCCDDMMNNOOPP
• h. AABBCCDDMNOP

Question 8.38

Species I has 2n = 8 chromosomes and species II has 2n = 14 chromosomes. What would the expected chromosome numbers be in individual organisms with the following chromosome mutations? Give all possible answers.

• a. Allotriploidy including species I and II
• b. Autotetraploidy in species II
• c. Trisomy in species I
• d. Monosomy in species II
• e. Tetrasomy in species I
• f. Allotetraploidy including species I and II

Question 8.39

Suppose that Species I in Figure 8.28 has 2n = 10 and Species II in the figure has 2n = 12. How many chromosomes would be present in the allotetraploid at the bottom of the figure?

Question 8.40

Consider a diploid cell that has 2n = 4 chromosomes—one pair of metacentric chromosomes and one pair of acrocentric chromosomes. Suppose that this cell undergoes nondisjunction giving rise to an autotriploid cell (3n). The triploid cell then undergoes meiosis. Draw the different types of gametes that may result from meiosis in the triploid cell, showing the chromosomes present in each type. To distinguish between the different metacentric and acrocentric chromosomes, use a different color to draw each metacentric chromosome; similarly, use a different color to draw each acrocentric chromosome. (Hint: See Figure 8.27).

Question 8.41

Assume that the autotriploid cell in Figure 8.27 has 3n = 30 chromosomes. For each of the gametes produced by this cell, give the chromosome number of the resulting zygote if the gamete fused with a normal haploid gamete.

Question 8.42

Nicotiana glutinosa (2n = 24) and N. tabacum (2n = 48) are two closely related plants that can be intercrossed, but the F₁ hybrid plants that result are usually sterile. In 1925, Roy Clausen and Thomas Goodspeed crossed N. glutinosa and N. tabacum and obtained one fertile F₁ plant (R. E. Clausen and T. H. Goodspeed. 1925 Genetics 10:278–284). They were able to self-pollinate the flowers of this plant to produce an F₂ generation. Surprisingly, the F₂ plants were fully fertile and produced viable seed. When Clausen and Goodspeed examined the chromosomes of the F₂ plants, they observed 36 pairs of chromosomes in metaphase I and 36 individual chromosomes in metaphase II. Explain the origin of the F₂ plants obtained by Clausen and Goodspeed and the numbers of chromosomes observed.

Question 8.43

What would be the chromosome number of progeny resulting from the following crosses in wheat (see Figure 8.29)? What type of polyploid (allotriploid, allotetraploid, etc.) would result from each cross?

• a. Einkorn wheat and emmer wheat
• b. Bread wheat and emmer wheat
• c. Einkorn wheat and bread wheat

Question 8.44

Karl and Hally Sax crossed Aegilops cylindrica (2n = 28), a wild grass found in the Mediterranean region, with Triticum vulgare (2n = 42), a type of wheat (K. Sax and H. J. Sax. 1924. Genetics 9:454–464). The resulting F₁ plants from this cross had 35 chromosomes. Examination of metaphase I in the F₁ plants revealed the presence of 7 pairs of chromosomes (bivalents) and 21 unpaired chromosomes (univalents).

• a. If the unpaired chromosomes segregate randomly, what possible chromosome numbers will appear in the gametes of the F₁ plants?
• b. What does the appearance of the bivalents in the F₁ hybrids suggest about the origin of Triticum vulgare wheat?