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Modern Genetic Analysis Chapter 4. The Inheritance of Genes
Problems

1.

Assume you have some diploid eukaryotic cells that will divide asexually in special nutrient solution in a test tube. Your starting point is cells that have not yet entered premitotic S phase (replication phase). You put the cells into nutrient solution that contains radioactive nucleotides that can be incorporated into newly synthesized DNA by DNA polymerase. Using red for a radioactive DNA strand and black for nonradioactive, draw the DNA double helix of a chromosome
a. At the start of the experiment (before premitotic S phase)
b. After chromatid formation
c. Before premitotic S phase in the daughter cells
d. Before premitotic S phase in their daughter cells (that is, after two divisions in the solution of radioactive nucleotides)

2. | Draw a graph of DNA content against time in a diploid cell that first goes through mitosis and then through meiosis.

3. | Why is DNA synthesis continuous on one template and discontinuous on the other?

4.

Consider the following segment of DNA, which is part of a much longer molecule constituting a chromosome.

If the DNA polymerase starts replicating this segment from the left,
a. Which will be the template for the leading strand?
b. Draw the molecule when the DNA polymerase is halfway along this segment.
c. Draw the two complete daughter molecules.
d. Is your diagram in b compatible with bidirectional replication from a single origin, the usual mode of replication?

Pattern Recognition Problems

In Problems 5 through 9 diagrams show phenotypes and the results of breeding analyses. Deduce the genotypes of the individuals shown in each diagram as far as possible, using allelic symbols that take into account any evidence of dominance in the data.

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6. |

7. |

8. |

9. |

10. | If you had a fruit fly (Drosophila melanogaster) that was of phenotype A/, what test would you make to determine if it was A/A or A/a? See answer

11. | Two black guinea pigs were mated and over several years produced 29 black and 9 white offspring. Explain these results, giving the genotypes of parents and progeny. See answer

12. | Holstein cattle normally are black and white. A superb black and white bull, Charlie, was purchased by a farmer for $100,000. The progeny sired by Charlie were all normal in appearance. However, certain pairs of his progeny, when interbred, produced red and white progeny at a frequency of about 25 percent. Charlie was soon removed from the stud lists of the Holstein breeders. Explain precisely why, using symbols.

13.

The plant blue-eyed Mary grows on Vancouver Island and on the lower mainland of British Columbia. The populations are dimorphic for purple blotches on the leavessome plants have blotches, and others don't. Near Nanaimo, one plant in nature had blotched leaves. This plant, which had not yet flowered, was dug up and taken to a laboratory, where it was allowed to self. Seeds were collected and grown into progeny. One randomly selected (but typical) leaf from each of the progeny is shown in the figure below.

a. Formulate a concise genetic hypothesis to explain these results. Explain all symbols and show all genotypic classes (and the genotype of the original plant).
b. How would you test your hypothesis? Be specific.

14.

In nature, the plant Plectritis congesta is dimorphic for fruit shape; that is, individual plants bear either wingless or winged fruits, as shown in the figure in the facing column.

Plants were collected from nature before flowering and were crossed or selfed with the following results:

Interpret these results, and derive the mode of inheritance of these fruit-shape phenotypes, using symbols. What do you think the explanation is for the results indicated by the asterisk?

15.

The accompanying pedigree is for a rare, but relatively mild, hereditary disorder of the skin.

a. Is the disorder inherited as a recessive or a dominant phenotype? State reasons for your answer.
b. Give genotypes for as many individuals in the pedigree as possible. (Invent your own defined allele symbols.)
c. Consider the four unaffected children of parents III-4 and III-5. In all four-child progenies from parents of these genotypes, what proportion is expected to contain all unaffected children?

16. | The accompanying pedigree was obtained for a rare kidney disease. a. Deduce the inheritance of this condition, stating your reasons. b. If individuals 1 and 2 marry, what is the probability that their first child will have the kidney disease? See answer

17.

A cross is made between a leucine-requiring haploid yeast strain of genotype leu and a wild-type strain that can make its own leucine (leu+).
a. What will be the genotype of the transient meiocyte?
b. What progeny genotypes will be produced and what will be their proportions?
c. Draw diagrams that show possible structures of the alleles at the DNA level, and the nature of their protein products.

18. | The recessive allele s causes Drosophila to have small wings, and the s+ allele causes normal wings. This gene is known to be X-linked. (Note: it is not the same gene discussed in the chapter.) If a small-winged male is crossed with a homozygous wild-type female, what ratio of normal to small-winged flies can be expected in each sex in the F1? If F1 flies are intercrossed, what F2 progeny ratios are expected? What progeny ratios are predicted if F1 females are backcrossed to their father?

19.

Duchenne muscular dystrophy is sex-linked and usually affects only males. Victims of the disease become progressively weaker, starting early in life.
a. What is the probability that a woman whose brother has the disease will have an affected child?
b. If your mother's brother (your uncle) had Duchenne's disease, what is the probability that you have received the allele?
c. If your father's brother had the disease, what is the probability that you have received the allele?

See answer

20.

The accompanying pedigree concerns a certain rare disease that is incapacitating but not fatal.

a. Determine the mode of inheritance of this disease.
b. Write the genotype of each individual according to your proposed mode of inheritance.
c. If you were this family's doctor, how would you advise the three couples in the third generation about the likelihood of having an affected child?

21.

A mutant allele in mice causes a bent tail. Six pairs of mice were crossed. Their phenotypes and those of their progeny are given below. N is normal phenotype; B is bent phenotype. Deduce the mode of inheritance of this phenotype.

a. Is it recessive or dominant?
b. Is it autosomal or sex-linked?
c. What are the genotypes of all parents and progeny?
See answer

22.

A new mutant strain of the haploid fungus Neurospora was found. It showed erratic stop-start growth and, so, was called "stopper" (Neurospora usually grows continuously). The stopper strain was crossed to another mutant strain, which had a yellow color instead of the usual orange. The cross was done reciprocally, using each strain as either maternal or paternal parent. The results were as follows:

a. What can you conclude about the location of these genes in the cell?
b. Explain clearly the difference between the results of the reciprocal crosses.
See answer

23.

A man whose grandfather had galactosemia (a rare autosomal recessive disease) married a woman whose sister had galactosemia. The woman is now pregnant with their first child.
a. Draw the pedigree as described.
b. What is the probability that this child will have galactosemia?
c. If the first child does have galactosemia, what is the probability a second child will have it?

24.

A man whose grandfather on his mother's side had favism (a rare X-linked recessive sensitivity to broad beans) marries a woman whose uncle on her mother's side also had favism.
a. Draw the pedigree as described.
b. What is the probability that their first child will have favism?
c. If the first child does have favism, what is the probability a second child will have favism?

25.

The following pedigree is of a rare hereditary disease of the nervous system:

a. Do you think the disease is recessive or dominant? (Explain.)
b. Do you think it is autosomal or X-linked? (Explain.)
c. Under your scheme, what is the probability that the first child of the couple A and B will have the disease?

26.

In Mimulus (the monkey flower) plant stems can be either smooth or hairy, and this difference is determined by a simple allelic difference. Examine the following crosses and give the genotypes of the parental and progeny plants in each cross. (Invent your own gene symbols and define their meaning.)

See answer

27. | Represent the progeny of the cross A/a × A/a both as a grid and as a pedigree diagram. What are the strengths and weaknesses of each representational method?

28.

Consider the following pedigrees for two different rare diseases:

a. Deduce the most likely mode of inheritance of these two diseases and show your reasoning.
b. Calculate the probabilities of couple A and B and of couple C and D having an affected child.

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