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Modern Genetic Analysis Chapter 8. Chromosome Mutations
Problems

1. | List the diagnostic features (genetic or cytological) that are used to identify the following chromosomal alterations: a. deletions b.duplications c. inversions d.reciprocal translocations

2.

The normal sequence of nine genes on a certain Drosophila chromosome is 123·456789, where the dot represents the centromere. Some flies were isolated with chromosome aberrations that have the following structures:
a. 123·476589 b.123·46789
c. 1654·32789 d.123·4566789

Name each type of chromosome rearrangement and draw diagrams to show how each would synapse with the normal chromosome.

See answer

3.

The two loci P and Bz are normally 36 m.u. apart on the same arm of a certain plant chromosome. A paracentric inversion spans about one-quarter of this region but does not include either of the loci. What approximate recombinant frequency between P and Bz would you predict in plants that are
a. heterozygous for the paracentric inversion?
b. homozygous for the paracentric inversion?

See answer

4.

Certain mice called waltzers execute bizarre steps in contrast with the gait of normal mice. The difference between normal mice and the waltzers is genetic, with waltzing being a recessive characteristic. W. H. Gates crossed waltzers with homozygous normals and found, among several hundred normal progeny, a single waltzing mouse (). When mated with a waltzing , she produced all waltzing offspring. Mated with a homozygous normal , she produced all normal progeny. Some and of this normal progeny were intercrossed, and there were no waltzing offspring among their progeny. T. S. Painter examined the chromosomes of waltzing mice that were derived from some of Gates's crosses and that showed a breeding behavior similar to that of the original, unusual waltzing . He found that these individuals had 40 chromosomes, just as in normal mice or the usual waltzing mice. In the unusual waltzers, however, one member of a chromosome pair was abnormally short. Interpret these observations as completely as possible, both genetically and cytologically.

(Problem 4 is from A. M. Srb, R. D. Owen, and R. S. Edgar, General Genetics, 2d ed. W. H. Freeman and Company, 1965.)

5.

Six bands in a salivary gland chromosome of Drosophila are shown in the following figure, along with the extent of five deletions (Del1 to Del5):

Recessive alleles a, b, c, d, e, and f are known to be in the region, but their order is unknown. When the deletions are combined with each allele, the following results are obtained:

In this table, a minus sign means that the deletion is missing the corresponding wild-type allele (the deletion uncovers the recessive) and a plus sign means that the corresponding wild-type allele is still present. Use these data to infer which salivary band corresponds to each gene.

(Problem 5 is from D. L. Hartl, D. Friefelder, and L. A. Snyder, Basic Genetics. Jones and Bartlett, 1988.)

6. | A fruit fly was found to be heterozygous for a paracentric inversion. However, it was impossible to obtain flies that were homozygous for the inversion even after many attempts. What is the most likely explanation for this inabiliy to produce a homozygous inversion? See answer

7.

The Neurospora un3 locus is near the centromere on chromosome 1, and crossovers between un3 and the centromere are very rare. The ad3 locus is on the other side of the centromere of the same chromosome, and crossovers occur between ad3 and the centromere in about 20 percent of meioses (no multiple crossovers occur).
a. What types of linear asci do you predict and in what frequencies in a normal cross of un3 ad3 × wild type? (Specify genotypes of spores in the asci.)
b. Most of the time such crosses behave predictably, but, in one case, a standard un3 ad3 strain is crossed with a wild type isolated from a field of sugar cane in Hawaii. The results follow:

Explain these results, and state how you could test your idea.

8.

Orangutans are an endangered species in their natural environment (the islands of Borneo and Sumatra), so a captive-breeding program has been established using orangutans currently held in zoos throughout the world. One component of this program is research into orangutan cytogenetics. This research has shown that all orangutans from Borneo carry one form of chromosome 2, as shown in the following diagram, and all orangutans from Sumatra carry the other form. Before this cytogenetic difference became known, some matings were carried out between animals from different islands, and 14 hybrid progeny are now being raised in captivity.

a. What term or terms describe the differences between these chromosomes?
b. Draw the chromosomes 2, paired during the first meiotic prophase, of such a hybrid orangutan. Be sure to show all the landmarks indicated in the preceding diagram, and label all parts of your drawing.
c. In 30 percent of meioses, there will be a crossover somewhere in the region between bands p1.1 and q1.2. Draw the gamete chromosomes 2 that would resut from a meiosis in which a single crossover occurred within band q1.1.
d. What fraction of the gametes produced by a hybrid orangutan will give rise to viable progeny, if these are the only chromosomes that differ between the parents?

(Problem 8 is from Rosemary Redfield.)

See answer

9.

In corn, the genes for tassel length (alleles T and t) and rust resistance (alleles R and r) are known to be on separate chromosomes. In the course of making routine crosses, a breeder noticed that one T/t ; R/r plant gave unusual results in a testcross with the double-recessive pollen parent t/t; r/r. The results were

a. What key features of the data are different from expected results?
b. State a concise hypothesis that explains the results.
c. Show genotypes of parents and progeny.
d. Draw a diagram showing the arrangement of alleles on the chromosomes.
e. Explain the origin of the two classes of progeny having three and five members.

Unpacking the Problem

a. What to "a gene for tassel length" and "a gene for rust resistance" mean?
b. Does it matter that the precise meaning of the allelic symbols T, t, R, and r is not given? Why or why not?
c. How do the terms gene and allele, as used here, relate to the concepts of locus and gene pair? (A concept map would be one way of answering this question.)
d. What prior experimental evidence would give the corn geneticists the idea that the two genes are on separate chromosomes?
e. What do you imagine "routine crosses" are to a corn breeder?
f. What term is used to describe genotypes of the type T/t ; R/r ?
g. What is a "pollen parent"?
h. What are testcrosses and why do geneticists find them so useful?
i. What progeny types and frequencies might the breeder have been expecting from the testcross?
j. Describe how the observed progeny differ from expectations.
k. What does the approximate equality of the first two progeny classes tell you?
l. What does the approximate equality of the second two progeny classes tell you?
m. What were the gametes from the unusual plant and what were their proportions?
n. Which gametes were in the majority?
o. Which gametes were in the minority?
p. Which of the progeny types seem to be recombinant?
q. Which allele combinations appear to be linked in some way?
r. How can there be linkage of genes supposedly on separate chromosomes?
s. What do these majority and minority classes tell us about the genotypes of the parents of the unusual plant?
t. What is a corn cob?
u. What does a normal corn cob look like (sketch one and label it)?
v. What do the corn cobs from this cross look like (sketch one)?
w. What exactly is a kernel?
x. What effect could lead to the absence of half the kernels?
y. Did half the kernels die? If so, was the female or the male parent the reason for the death?

Now try to solve the problem.

10.

Yellow body in Drosophila is caused by a mutant allele e of a gene located at the tip of the X chromosome (the wild-type allele causes gray body). In a radiation experiment, a wild-type male was irradiated with X rays and then crossed with a yellow-bodied female. Most of the male progeny were yellow, as expected, but the scanning of thousands of flies revealed two gray-bodied (phenotypically wild type) males. These gray-bodied males were crossed with yellow females, with the following results:

a. Explain the origin and crossing behavior of gray male 1.
b. Explain the origin and crossing behavior of gray male 2.

11. | Two mutations in Neurospora, ad-3 and pan-2, are located on chromosomes 1 and 6, respectively. An unusual ad-3 line arises in the laboratory, giving the following results: Explain all three results with the aid of clearly labeled diagrams. (note: In Neurospora, ascospores with extra chromosomal material survive and are the normal black color, whereas ascospores lacking any chromosome region are white and inviable.) See answer

12. | In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr /pr that has standard chromosomes is crossed with a Pr /Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F1 is semisterile and phenotypically Pr (a seed color). A backcross with the parent with standard chromosomes gives 764 semisterile Pr; 145 semisterile pr; 186 normal Pr; and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

13. | A male rat that is phenotypically normal shows reproductive anomalies when compared with normal male rats, as shown in the following table. Propose a genetic explanation for these unusual results, and describe how your idea could be tested.

14. | Distinguish between Klinefelter, Down, and Turner syndromes.

15. | Show how one could make an allotetraploid between two related diploid species of plants in both of which 2n=28. See answer

16.

Deduce the phenotypic proportions in the progeny of the following crosses of autotetraploids in which the a+/a locus is very close to the centromere. (Assume that the four homologous chromosomes of any one type pair randomly two-by-two and that only one copy of the a+ allele is necessary for the wild-type phenotype.)
a.

b.
c.
d.
See answer

17.

In Drosophila, trisomics and monosomics for the tiny chromosome 4 are viable, but nullisomics and tetrasomics are not. The b locus is on this chromosome. Deduce the phenotypic proportions in the progeny of the following crosses of trisomics.
a.

b.
c.

18.

A woman with Turner syndrome is found to be colorblind (X-linked recessive phenotype). Both her mother and her father have normal vision.
a. Explain the simultaneous origin of Turner syndrome and colorblindness by the abnormal behavior of chromosomes at meiosis.
b. Can your explanation distinguish whether the abnormal chromosome behavior occurred in the father or the mother?
c. Can your explanation distinguish whether the abnormal chromosome behavior occurred at the first or second division of meiosis?
d. Now assume that a colorblind Klinefelter man has parents with normal vision, and answer parts a, b, and c.

19.

a.How would you synthesize a pentaploid (5x)?
b. How would you synthesize a triploid (3x) of genotype A/a/a?
c. You have just obtained a rare recessive mutation a* in a diploid plant, which Mendelian analysis tells you is A/a*. From this plant, how would you synthesize a tetraploid (4x) of genotype A/A/a*/a*?
d. How would you synthesize a tetraploid of genotype A/a/a/a?
e. How would you synthesize a plant that is resistant to a chemical herbicide? (Assume that mutation to this trait is very infrequent and you are lucky to get one mutant.)

20.

The New World cotton species Gossypium hirsutum has a 2n chromosome number of 52. The Old World species G. thurberi and G. herbaceum each have a 2n number of 26. Hybrids between these species show the following chromosome pairing arrangements at meiosis:

Draw diagrams to interpret these observations phylogenetically, clearly indicating the relationships between the species. How would you go about proving that your interpretation is correct?

(Problem 20 is adapted from A. M. Srb, R. D. Owen, and R. S. Edgar, General Genetics, 2d ed. W. H. Freeman and Company, 1965.)

21.

Several kinds of sexual mosaicism are well documented in humans. Suggest how each of the following examples may have arisen by nondisjunction at mitosis:
a. XX/XO (that is, there are two cell types in the body, XX and XO)
b. XX/XXYY c.XO/XXX
d. XX/XY e.XO/XX/XXX

See answer

22.

Suppose you have a line of mice that has cytologically distinct forms of chromosome 4. The tip of the chromosome can have a knob (called 4K) or a satellite (4S) or neither (4). Here are sketches of the three types:

You cross a 4K 4S female with a 4 4 male and find that most of the progeny are 4K 4 or 4S 4, as expected. However, you occasionally find some rare types as follows (all other chromosomes are normal):

Explain the rare types that you have found. Give, as precisely as possible, the stages at which they originate, and state if they originate in the male parent, the female parent, or the zygote. (Give reasons briefly.)

23.

In Drosophila, a cross (cross 1) was made between two mutant flies, one homozygous for the recessive mutation bent wing (b) and the other homozygous for the recessive mutation eyeless (e). The mutations e and b are alleles of two different genes that are known to be very closely linked on the tiny autosomal chromosome 4. All the progeny were wild-type phenotype. One of the female progeny was crossed with a male of genotype b e / b e; call this cross 2. Most of the progeny of cross 2 were of the expected types, but there was also one rare female of wild-type phenotype.
a. Explain what the common progeny are expected to be from cross 2.
b. Could the rare wild-type female have arisen by (1) crossing-over? (2) nondisjunction? Explain.
c. The rare wild-type female was testcrossed to a male of genotype b e / b e (cross 3). The progeny were

Which of the explanations in part b are compatible with this result? Explain the genotypes and phenotypes of progeny of cross 3 and their proportions.

24.

A cross is made in tomatoes between a female plant that is trisomic for chromosome 6 and a normal diploid male plant that is homozygous for the recessive allele for potato leaf (p / p).
a. A trisomic F1 plant is backcrossed to the potato-leaved male. What is the ratio of normal-leaved plants to potato-leaved plants when you assume that p is on chromosome 6?
b. What is the ratio of normal- to potato-leaved plants when you assume that p is not located on chromosome 6?

25.

A tomato geneticist attempts to assign five recessive genes to specific chromosomes by using trisomics. She crosses each homozygous mutant (2n) to each of three trisomics, involving chromosomes 1, 7, and 10. From these crosses, the geneticist selects trisomic progeny (which are less vigorous) and backcrosses them to the appropriate homozygous recessive. The diploid progeny from these crosses are examined. Her results, in which the ratios are wild type:mutant, are as follows:

Which of the genes can the geneticist assign to which chromosomes? (Explain your answer fully.)

26.

In the fungus Ascobolus (similar to Neurospora), ascospores are normally black. The mutation f, producing fawn ascospores, is in a gene just to the right of the centromere on chromosome 6, whereas mutation b, producing beige ascospores, is in a gene just to the left of the same centromere. In a cross of fawn and beige parents (+ f × b +), most octads show four fawn and four beige ascospores, but three rare exceptional octads were found, as shown below. In the sketch, black is the wild-type phenotype, a vertical line is fawn, a horizontal line is beige, and an empty circle represents an aborted (dead) ascospore.

a. Provide reasonable explanations for these three exceptional octads.
b. Diagram the meiosis that gave rise to octad 2.

© 1999 by W. H. Freeman and Company. All rights reserved.