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Modern Genetic Analysis Chapter 6. Gene Interaction
Problems

1. | Both recombination and complementation can take two different recessive mutants and from them produce a wild type. How would you explain the difference between these two processes?

2. | Erminette fowls have mostly light-colored feathers with an occasional black one, giving a flecked appearance. A cross of two erminettes produced a total of 48 progeny, consisting of 22 erminettes, 14 blacks, and 12 pure whites. What genetic basis of the erminette pattern is suggested? How would you test your hypothesis?

3. | In the multiple allelic series that determines coat color in rabbits, c+ codes for agouti, cch is chinchilla, and ch is Himalayan. Dominance is in the order c+ > cch > ch. In a cross of c+/cch × cch/ch, what proportion of progeny will be chinchilla? See answer

4.

Black, sepia, cream, and albino are all coat colors of laboratory guinea pigs. Individual animals (not necessarily from pure lines) showing these colors were intercrossed; the results are tabulated below, where we are using the abbreviation A for albino; B, black; C, cream; and S, sepia, for the phenotypes:

a. Deduce the inheritance of these coat colors, using gene symbols of your own choosing. Show all parent and progeny genotypes.
b. If the black animals in crosses 7 and 8 are crossed, what progeny proportions can you predict using your model?
See answer

5. | In a maternity ward, four babies become accidentally mixed up. The ABO types of the four babies are known to be O, A, B, and AB. The ABO types of the four sets of parents are determined. Indicate which baby belongs to each set of parents: (a) AB × O, (b) A × O, (c) A × AB, (d) O × O.

6. | Two fruit flies of wild-type phenotype were crossed, and in the progeny there were 202 females and 98 males. a. What is unusual about this result? b. Provide a genetic explanation. c. Provide a test of your hypothesis. See answer

7.

A pure-breeding strain of squash that produced disk-shaped fruits (see the figure below) was crossed with a pure-breeding strain having long fruits. The F1 had disk fruits, but the F2 showed a new phenotype, sphere, and was composed of the following proportions:

Propose an explanation for these results, and show the genotypes of the P, F1, and F2 generations.

(Illustration from P. J. Russell, Genetics, 3d ed. Harper- Collins, 1992.)

8.

Wild-type snapdragons (Antirrhinum) make a red anthocyanin pigment and have red petals. Two pure anthocyanin-less mutant lines of Antirrhinum were developed, one in California and one in Holland. They looked identical in having no red pigment at all, manifested as white (albino) flowers. However, when petals from the two lines were ground up together in buffer in the same test tube, the solution, which appeared colorless at first, gradually turned red.
a. What control experiments should an investigator conduct before proceeding with further analysis?
b. What could account for the production of the red color in the test tube?
c. According to your explanation for part b, what would be the genotypes of the two lines?
d. If the two white lines are crossed, what would you predict the phenotypes of the F1 and F2 to be? What genetic principle is illustrated by the F1 results? The F2 results?

9.

Four homozygous recessive mutant lines of Drosophila melanogaster (labeled 1 through 4) showed abnormal leg coordination, which made walking highly erratic. These lines were intercrossed; the phenotypes of the F1 animals are shown in the following grid in which "+" means the F1 showed wild-type walking and "" means abnormal walking:

a. What type of test does this analysis represent?
b. How many different genes were mutated in creating these four lines?
c. Invent wild and mutant symbols and write out full genotypes for all four lines and the F1s.
d. Do these data tell us which genes are linked? If not, how could linkage be tested?
e. Do these data tell us the total number of genes involved in leg coordination in this animal?

10.

Three independently isolated tryptophan-requiring mutants of haploid yeast are called trpB, trpD, and trpE. Cell suspensions of each are streaked on a plate of nutritional medium supplemented with just enough tryptophan to permit weak growth for a trp strain. The streaks are arranged in a triangular pattern so that they do not touch one another. Luxuriant growth is noted at both ends of the trpE streak and at one end of the trpD streak (see the following figure).

a. Do you think complementation is involved?
b. Briefly explain the patterns of luxuriant growth.
c. In what order in the tryptophan-synthesizing pathway are the enzymatic steps defective in trpB, trpD, and trpE?
d. Why was it necessary to add a small amount of tryptophan to the medium in order to demonstrate such a growth pattern?

11.

The frizzle fowl is much admired by poultry fanciers. It gets its name from the unusual way that its feathers curl up, giving the impression that it has been (in the memorable words of animal geneticist F. B. Hutt) "pulled backwards through a knothole." Unfortunately, frizzle fowls do not breed true; when two frizzles are intercrossed, they always produce 50 percent frizzles, 25 percent normal, and 25 percent with peculiar woolly feathers that soon fall out, leaving the birds naked.
a. Give a genetic explanation for these results, showing genotypes of all phenotypes, and provide a statement of how your explanation works.
b. If you wanted to mass-produce frizzle fowls for sale, which types would be best to use as a breeding pair?

See answer

12.

Marfan's syndrome is a disorder of the fibrous connective tissue characterized by many symptoms, including long, thin digits, eye defects, heart disease, and long limbs. (Flo Hyman, the American volleyball star, suffered from Marfan's syndrome. She died soon after a game from a ruptured aorta.)
a. Use the pedigree shown above to propose a mode of inheritance for Marfan's syndrome.
b. What genetic phenomenon is shown by this pedigree?
c. Speculate on a reason for such a phenomenon.

(Illustration from J. V. Neel and W. J. Schull, Human Heredity. University of Chicago Press, 1954.)

13. | A woman owned a fine purebred albino female poodle (an autosomal recessive phenotype) and wanted to mate her to produce a litter of white puppies. She took the dog to a breeder, who said he would mate the female with an albino stud male, also from a pure stock. When six puppies were born, they were all black, so the woman sued the breeder, claiming that he replaced the stud male with a black dog, giving her six unwanted puppies. You are called in as an expert witness, and the defense asks you if it is possible to produce black offspring from two pure-breeding recessive albino parents. What testimony do you give?

14.

The petals of the plant Collinsia parviflora are normally blue, giving the species its common name "blue-eyed Mary." Two pure-breeding lines were obtained from color variants found in nature; the first line had pink petals and the second line had white petals. The following crosses were made between pure lines, with the results shown:

a. Explain these results genetically. Define allele symbols you use and show the genetic constitution of parents, F1, and F2.
b. A cross between a certain blue F2 plant and a certain white F2 plant gave progeny of which 3/8 were blue, 1/8 were pink, and 1/2 were white. What must the genotypes of these two F2 plants have been?

Unpacking the Problem

a. What is the character being studied?
b. What is the wild-type phenotype?
c. What is a variant?
d. What are the variants in this problem?
e. What does "in nature" mean?
f. In what way would the variants have been found in nature? (Describe the scene.)
g. At which stages in the experiments would seeds be used?
h. Would the way of writing a cross "blue × white" (for example) mean the same as "white × blue"? Would you expect similar results? Why or why not?
i. In what way do the first two rows in the table differ from the third row?
j. Which phenotypes are dominant?
k. What is complementation?
l. Where does the blueness come from in the progeny of the pink × white cross?
m. What genetic phenomenon does the production of a blue F1 from pink and white parents represent?
n. List any ratios you can see.
o. Are there any monohybrid ratios?
p. Are there any dihybrid ratios?
q. What does observing monohybrid and dihybrid ratios tell you?
r. List four modified Mendelian ratios that you can think of.
s. Are there any modified Mendelian ratios in the problem?
t. What do modified Mendelian ratios indicate generally?
u. What do the specific modified ratio or ratios in this problem indicate?
v. Draw chromosomes representing the meioses in the parents in the cross blue × white, and meiosis in the F1.
w. Repeat for the cross blue × pink.

15.

Most flour beetles are black, but several color variants are known. Crosses of pure-breeding parents produced the following results in the F1 generation, and intercrossing the F1 from each cross gave the ratios shown for the F2 generation. The phenotypes are abbreviated Bl, black; Br, brown; Y, yellow; and W, white.

a. From these results, deduce and explain the inheritance of these colors.
b. Write out the genotypes of each of the parents, the F1, and the F2 in all crosses.
c. Arrange the genes and pigments into a biochemical pathway or pathways.

16. | An allele A that is not lethal when homozygous causes rats to have yellow coats. The allele R of a separate gene that assorts independently produces a black coat. Together, A and R produce a grayish coat, whereas a and r produce a white coat. A gray male is crossed with a yellow female, and the F1 is 3/8 yellow, 3/8 gray, 1/8 black, and 1/8 white. Determine the genotypes of the parents. See answer

17.

The genotype r/r ; p/p gives fowl a single comb, R/ ; P/ gives a walnut comb, r/r ; P/ gives a pea comb, and R/ ; p/p gives a rose comb (see the figure on the top of the facing page). Note that the genes are unlinked.

a. What comb types will appear in the F1 and in the F2 in what proportions if single-combed birds are crossed with birds of a true-breeding walnut strain?
b. What are the genotypes of the parents in a walnut × rose mating from which the progeny are 3/8 rose, 3/8 walnut, 1/8 pea, and 1/8 single?
c. What are the genotypes of the parents in a walnut × rose mating from which all the progeny are walnut?
d. How many genotypes produce a walnut phenotype? Write them out.

18.

The allele B gives mice a black coat, and b gives a brown one. The genotype e/e for another, independently assorting gene prevents expression of B and b, making the coat color beige, whereas E/ permits expression of B and b. Both genes are autosomal. In the following pedigree, black symbols indicate a black coat, pink symbols indicate brown, and white symbols indicate beige:

a. What is the name given to the type of gene interaction in this example?
b. What are the genotypes of the individuals in the pedigree? (If there are alternative possibilities, state them.)

19.

Assume that two pigments, red and blue, mix to give the normal wild-type purple color of petunia petals. Separate biochemical pathways synthesize the two pigments, as shown in the top two rows of the accompanying diagram. "White" refers to compounds that are not pigments. (Total lack of pigment results in a white petal.) Red pigment forms from a yellow intermediate that normally is at a concentration too low to color petals. A third pathway whose compounds do not contribute pigment to petals normally does not affect the blue and red pathways, but if one of its intermediates (white3) should build up in concentration, it can be converted to the yellow intermediate of the red pathway. In the diagram, A to E represent enzymes; their corresponding genes, all of which are unlinked, may be symbolized by the same letters.

Assume that wild-type alleles are dominant and code for enzyme function, and that recessive alleles represent lack of enzyme function. Deduce which combinations of true-breeding parental genotypes could be crossed to produce F2 progenies in the following ratios:
a. 9 purple:3 green:4 blue
b. 9 purple:3 red:3 blue:1 white
c. 13 purple:3 blue
d. 9 purple:3 red:3 green:1 yellow
e. Which of the mutations is acting as a suppressor?
f. Which of the mutations is showing recessive epistasis?

(Note: blue mixed with yellow makes green; assume that no mutations are lethal.)

See answer

20.

The following pedigree shows the inheritance of deaf-mutism:

a. Provide an explanation for the inheritance of this rare condition in the two families in generations I and II, showing genotypes of as many individuals as possible using symbols of your own choosing.
b. Provide an explanation for the production of only normal ind-1p1.6ividuals in generation III, making sure your explanation is compatible with the answer to part a.

21.

The following pedigree is for blue sclera (bluish thin outer wall to the eye) and brittle bones:

a. Are these two disorders caused by the same gene or two different genes? State your reasons clearly.
b. Is the allele or alleles autosomal or sex-linked?
c. Does the pedigree show any evidence of incomplete penetrance or expressivity? If so, make the best calculations that you can of these measures.

22.

In minks, wild types have an almost black coat. Breeders have developed many pure lines of color variants for the mink coat industry. Two such pure lines are platinum (blue-gray) and aleutian (steel-gray). These lines were used in crosses, with the following results:

a. Devise a genetic explanation of these three crosses. Show genotypes for parents, F1, and F2 in the three crosses, and make sure you show the alleles of each gene you hypothesize in every individual.
b. Predict the F1 and F2 phenotypic ratios from crossing sapphire with platinum and aleutian pure lines.

23.

A geneticist is interested in the genes that interact to synthesize leucine (an amino acid) in the haploid filamentous fungus Aspergillus. He treats haploid cells with ultraviolet light to increase the mutation rate and obtains five haploid leucinerequiring mutants (a to e), all of which need leucine to be added to the medium in order for them to grow. (Without leucine, none of them will grow.)

Experiment I. He first makes heterokaryons between them to check on their functional relationships. He obtains the following results, where "+" indicates that the heterokaryon grew, and "" that the heterokaryon did not grow, on a medium lacking leucine:

Experiment II. The geneticist then intercrosses the mutants in all possible combinations. From each cross, he tests 500 ascospore progeny by inoculating them onto a medium lacking leucine. The results are in the following table. The numbers represent the number of progeny out of 500 that could synthesize their own leucine (and grow on a medium containing no leucine):

a. In both I and II leucine-independent strains are created. Explain the different origins of these in the two experiments.
b. Explain the pattern of + and growth in the heterokaryons in the different pairings in experiment I.
c. Explain the different frequencies of leucineindependent strains in experiment II. (Note that the two leucine-independent progeny from the b × d cross were found not to be due to mutation back to wild type.)
d. Draw a diagram that summarizes the results of both experiments.

24.

Wild-type Neurospora (a haploid fungus) can synthesize its own adenine from inorganic components in the growth medium. In a mutational analysis of the synthetic pathway for making adenine, 10 adenine-requiring mutations of Neurospora were obtained, and heterokaryons were made in all pairwise combinations. The results are shown in the following table in which a plus sign means the heterokaryon grew and a negative sign means the heterokaryon did not grow:

a. How many genes were involved in these mutations?

Mutants 1,3, and 4 were tested for growth on the compounds CAIR, AIR, and SAICAR, all chemically related to adenine. The results were as follows, where "+" means growth and "," none:

b. Explain these results.

Mutants 1, 3, and 4 were intercrossed, and 1000 ascospores were plated from each cross onto medium lacking adenine, to select for wild types. The results were as follows:

c. Explain these results and integrate them with previous results to provide a summary statement about this genetic system.

25. | In a plant the gene e+ codes for a certain enzyme. The gene r+ codes for a regulatory protein that is needed to bind to the regulatory region of gene e+ in order for transcription to occur. A recessive null mutation is obtained in each of the genes, r and e, respectively. A dihybrid e+/e ; r+/r is allowed to self. In the progeny what will be the proportion of individuals with enzyme? (Assume lack of enzyme is not lethal and that the genes are on different chromosomes.) See answer

26.

It is known that a certain Drosophila protein (call it P) is necessary for normal flight. This protein is known to be composed of only one polypeptide chain. Two pure lines of mutants (lines 1 and 2) were obtained that could not fly. When crossed to wild type, they both gave the same result:

When the two pure lines were intercrossed, the results were

Protein from all members of the F2 was run on electrophoretic gels and stained using an antibody that is specific for protein P. It was found that the F2 individuals could be grouped into six different types as follows:

(The thin bands represent very weak antibody staining.)
a. Provide a general explanation for the molecular results revealed by the gel analysis. In particular, state the difference in the molecular nature of the two original mutations.
b. According to your explanation, what are the genotypes of the six F2 types found?
c. What is the expected frequency of type 4?
d. Which of the six F2 gel types represent the 9/16 that are phenotypically wild-type?
e. Draw the expected gel pattern of the two parental mutants and the F1.

See answer

Pattern Recognition Problems

In Problems 27 through 32, diagrams show phenotypes and the results of breeding analyses. Deduce the genotypes of the individuals shown in each diagram as far as possible. All organisms are diploid.

27. |

28. |

29. | See answer

30. |

31. |

32. | Using the diagram, deduce genotype and dominance and predict the genotypes in the F2s.

33.

The wing covers of grain beetles exhibit one of three phenotypes, showing a diamond, a spot, or a stripe. The following crosses were made between various strains that were not necessarily pure-breeding:

a. Invent allele symbols, explain clearly the action of all the alleles you invent, then write the full genotypes of the parents and progeny in all three crosses. (If your explanation involves more than one gene, make sure all the genes are represented in the genotypes in every cross.)
b. What phenotypic ratio would result from crosses between the spotted progeny in cross 3?
See answer

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