12.Examine the karyotypes shown in Figures 6.1 and 6.2a. Are the individuals from whom these karyotypes were made males or females?

*13.Which types of chromosome mutations

*14.A chromosome has the following segments, where • represents the centromere:

AB•CDEFG

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.)

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15.A chromosome initially has the following segments:

AB•CDEFG

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

16.The following diagram represents two nonhomologous chromosomes:

AB•CDEFG

RS•TUVWX

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

17.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.

*18.A wild-type chromosome has the following segments:

ABC•DEFGHI

Researchers have found individuals that are heterozygous for each of 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.

*19. 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-containing or deficient chromosomes often results in children with syndromes characterized by developmental delay, intellectual disability, the 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?

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

AB•CDEFG

AB•CDVWX

RS•TUEFG

RS•TUVWX

Draw the pairing arrangement of these chromosomes in prophase I of meiosis.

*21.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.

*22.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.

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

24. 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 completed development. Mice that were trisomic for all other chromosomes died in the course of development. For some of these trisomics, the researchers plotted 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 and provide a possible explanation for the results.

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

26.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 individuals with the following sets of chromosomes have?

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

28.Suppose that species I in Figure 6.25 had 2n = 10 and species II in the figure had 2n = 12. How many chromosomes would be present in the allotetraploid at the bottom of the figure?

29.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 could 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 6.24).

30.Assume that the autotriploid cell in Figure 6.24 has 3n = 30 chromosomes. For each of the gametes produced by this cell, give the chromosome number of the zygote that would result if the gamete fused with a normal haploid gamete.

31.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 seeds. 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.

32.What would be the chromosome number of progeny resulting from the following crosses in wheat (see Figure 6.26)? What type of polyploid (allotriploid, allotetraploid, etc.) would result from each cross?

33.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).