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Modern Genetic Analysis Chapter 7. Gene Mutations
Problems

1. | A certain species of plant produces flowers with petals that are normally blue. Plants with the mutation w produce white petals. In a plant of genotype w/w, one w allele reverts in the development of a petal. What detectable outcome would this reversion produce in the resulting petal? Would this mutation be inherited?

2. | How would you select revertants of the yeast allele pro-1? This allele confers an inability to synthesize the amino acid proline, which can be synthesized by wild-type yeast and which is necessary for growth. See answer

3. | Suppose that you want to determine whether caffeine induces mutations in higher organisms. Describe how you might do so (include control tests).

4. | In a large maternity hospital in Copenhagen, there were 94,075 births. Ten of the infants were achondroplastic dwarfs. (Achondroplasia is an autosomal dominant, showing full penetrance.) Only two of the dwarfs had a dwarf parent. What is the mutation frequency to achondroplasia in gametes? Do you have to worry about reversion rates in this calculation? Explain. See answer

5. | One of the jobs of the Hiroshima-Nagasaki Atomic Bomb Casualty Commission was to assess the genetic consequences of the blast. One of the first things that they studied was the sex ratio in the offspring of the survivors. Why do you suppose they did this?

6.

Cells of a haploid wild-type Neurospora strain were mutagenized with EMS. Large numbers of these cells were plated and grown into colonies at 25°C on complete medium (containing all possible nutrients). These strains were tested on minimal medium and complete medium at both 25°C and 37°C. There were several mutant phenotypes, as shown in the accompanying sketch. The large circles represent luxuriant growth, the spidery symbols represent weak growth, and a blank means no growth at all. How would you categorize the types of mutants represented by isolates 1 through 5?

7.

Differentiate between the following pairs:
a. Transitions and transversions
b. Silent and neutral mutations
c. Missense and nonsense mutations
d. Frameshift and nonsense mutations

8. | Why are frameshift mutations more likely than missense mutations to result in proteins that lack normal function? See answer

9. | Describe the Streisinger model for frameshift formation. Show how this model can explain mutational hot spots in the lacI gene of E. coli.

10. | Diagram two different mechanisms for deletion formation. How do DNA sequencing experiments suggest these possibilities?

11. | Describe two spontaneous lesions that can lead to mutations. See answer

12. | Compare the mechanism of action of 5-BU with EMS in causing mutations. Explain the specificity of mutagenesis for each agent in light of the proposed mechanism.

13. | Compare the two different systems required for the repair of AP sites and in the removal of photodimers.

14. | Describe the repair systems that operate after replication.

15. | Normal ("tight") auxotrophic mutants will not grow at all in the absence of the appropriate supplement to the medium. However, in mutant hunts for auxotrophic mutants, it is common to find some mutants (called "leaky") that grow very slowly in the absence of the appropriate supplement but normally in the presence of the supplement. Propose an explanation for the molecular action of the leaky mutants in a biochemical pathway. See answer

16.

A botanist interested in the chemical reactions whereby plants capture light energy from the sun decided to dissect this process genetically. She decided that leaf fluorescence would be a useful mutant phenotype to select because it would show something wrong with the process whereby electrons are transferred from chlorophyll. Therefore four fluorescent (fl) mutants were obtained in the plant Arabidopsis after mutagenesis. All were found to be recessive in the simple Mendelian manner. Homozygous stocks of the mutants were intercrossed, and the F1s were testcrossed each to a strain that was homozygous recessive for all the genes involved in that cross. The results are shown in the table below.

a. How many genes are represented by these mutants?
b. What can you deduce about the chromosomal location of the genes?
c. Use your own gene symbols to explain the F1 and testcross results.

17.

a.Why is it impossible to induce nonsense mutations (represented at the mRNA level by the triplets UAG, UAA, and UGA) by treating wild-type strains with mutagens that cause only A·T G·C transitions in DNA?
b. Hydroxylamine (HA) causes only G·C A·T transitions in DNA. Will HA produce nonsense mutations in wild-type strains?
c. Will HA treatment revert nonsense mutations?

See answer

18.

Several auxotrophic point mutants in Neurospora are treated with various agents to see if reversion will take place. The following results were obtained (a plus sign indicates reversion; HA causes only G·C A·T transitions).

a. For each of the five mutants, describe the nature of the original mutation event (not the reversion) at the molecular level. Be as specific as possible.
b. For each of the five mutants, name a possible mutagen that could have caused the original mutation event. (Spontaneous mutation is not an acceptable answer.)
c. In the reversion experiment for mutant 5, a particularly interesting prototrophic derivative is obtained. When this type is crossed with a standard wild-type strain, the progeny consist of 90 percent prototrophs and 10 percent auxotrophs. Give a full explanation for these results, including a precise reason for the frequencies observed.

19.

You are using nitrosoguanidine to "revert" mutant nic-2 (nicotinamide-requiring) alleles in Neurospora. You treat cells, plate them on a medium without nicotinamide, and look for prototrophic colonies. You obtain the following results for two mutant alleles. Explain these results at the molecular level, and indicate how you would test your hypotheses.
a. With nic-2 allele 1, you obtain no prototrophs at all.
b. With nic-2 allele 2, you obtain three prototrophic colonies A, B, and C, and you cross each separately with a wild-type strain. From the cross prototroph A × wild type, you obtain 100 progeny, all of which are prototrophic. From the cross prototroph B × wild type, you obtain 100 progeny, of which 78 are prototrophic and 22 are nicotinamide requiring. From the cross prototroph C × wild type, you obtain 1000 progeny, of which 996 are prototrophic and 4 are nicotinamide requiring.

See answer

20.

Devise imaginative screening procedures for detecting:
a. Nerve mutants in Drosophila.
b. Mutants lacking flagella in a haploid unicellular alga.
c. Supercolossal-sized mutants in bacteria.
d. Mutants that overproduce the black compound melanine in normally white haploid fungus cultures.
e. Individual humans (in large populations) whose eyes polarize incoming light.
f. Negatively phototrophic Drosophila or unicellular algae.
g. UV-sensitive mutants in haploid yeast.

21. | A certain compound that is an analog of the base cytosine can become incorporated into DNA. It normally hydrogen-bonds just like cytosine, but it quite often isomerizes to a form that hydrogen-bonds like thymine. Do you expect this compound to be mutagenic and, if so, what types of changes might it induce at the DNA level?

22.

We will learn in Chapter 12 that the entire genome of the yeast Saccharomyces cerevisiae has been sequenced. This sequencing has led to the identification of all the open reading frames (ORFs) in the genome (gene-sized sequences with appropriate translational initiation and termination signals). Some of these ORFs are previously known genes with established functions; however, the remainder are unassigned reading frames (URFs). To deduce the possible functions of the URFs, they are being systematically, one at a time, converted into null alleles by using in vitro "knockout" techniques (see Chapter 11). To date, the results are as follows:
15. percent are lethal when knocked out;
25. percent show some mutant phenotype (altered morphology, altered nutrition, and so forth); and
60. percent show no detectable phenotype at all and resemble wild type.

Explain the possible molecular genetic basis of these three mutant categories, inventing examples where possible.

23.

A man and a woman with no record of genetic disease in their families have one child with neurofibromatosis (autosomal dominant) and another child who is unaffected. The penetrance of neurofibromatosis is close to 100%.
a. Explain the birth of the affected child.
b. How would you counsel the parents if they contemplated having another child?

See answer

24.

A haploid strain of Aspergillus nidulans carried an auxotrophic met-8 mutation conferring a requirement for methionine. Several million asexual spores were plated on minimal medium, and two prototrophic colonies grew and were isolated. These prototrophs were crossed sexually with two different strains, with the progeny shown in the body of the following table, where met+ means that methionine is not required for growth and met means that methionine is required for growth.

a. Explain the origin of both of the original prototrophic colonies.
b. Explain the results of all four crosses, using clearly defined gene symbols.

Unpacking the Problem

Before trying to solve this problem, follow the instructions or answer the questions that are relevant to the experimental system.

a. Draw a labeled diagram that shows how this experiment was done. Show test tubes, plates, and so forth.
b. Define all the genetic terms in this problem.
c. Many problems show a number next to the auxotrophic mutation's symbolhere the number 8 next to met. What does the number mean? Is it necessary to know in order to solve the problem?
d. How many crosses were actually made? What were they?
e. Represent the crosses by using genetic symbols.
f. Is the question about somatic mutation or germinal mutation?
g. Is the question about forward mutation or reversion?
h. Why was there such a small number of prototrophic colonies (two) found on the plate?
i. Why didn't the several million asexual spores grow?
j. Do you think any of the millions of spores that did not grow were mutant? Dead?
k. Do you think the wild type used in the crosses was prototrophic or auxotrophic? Explain.
l. If you had the two prototrophs from the plates and the wild-type strain in three different culture tubes, could you tell them apart just by looking at them?
m. How do you think the met-8 mutation was obtained in the first place? (Show with a simple diagram. note: Aspergillus is a filamentous fungus.)
n. Is the concept of recombination relevant to any part of this problem?
o. What progeny do you predict from crossing met- 8 with a wild type? What about met- 8 × met- 8 ? What about wild type × wild type?
p. Do you think this is a random meiotic progeny analysis or a tetrad analysis?
q. Draw a simple life cycle diagram of a haploid organism showing where meiosis takes place in the cycle.
r. Consider the 3/4 : 1/4 ratio. In haploids, crosses heterozygous for one gene generally give progeny ratios based on halves. How can this idea be extended to give ratios based on quarters?

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