Question 25.15

How would you respond to someone who said that models are useless in studying population genetics because they represent oversimplifications of the real world?

Question 25.16

Voles (Microtus ochrogaster) were trapped in fields in southern Indiana and genotyped for a transferrin locus. The following numbers of genotypes were recorded, where T^E and T^F represent different alleles.

[Tom McHugh/Photo Researchers.]

Calculate the genotypic and allelic frequencies of the transferrin locus for this population.

Question 25.17

Jean Manning, Charles Kerfoot, and Edward Berger studied genotypic frequencies at the phosphoglucose isomerase (GPI) locus in the cladoceran Bosmina longirostris (a small crustacean known as a water flea). At one location, they collected 176 of the animals from Union Bay in Seattle, Washington, and determined their GPI genotypes by using electrophoresis (J. Manning, W. C. Kerfoot, and E. M. Berger. 1978. Evolution 32:365-374).

Determine the genotypic and allelic frequencies for this population.

Question 25.18

Orange coat color of cats is due to an X-linked allele (X^O) that is codominant with the allele for black (X⁺). Genotypes of the orange locus of cats in Minneapolis and St. Paul, Minnesota, were determined, and the following data were obtained:

Calculate the frequencies of the X^o and X⁺ alleles for this population.

Question 25.19

Use the graph shown in Figure 25.3 to determine which genotype is most frequent when the frequency of the A allele is:

• a. 0.2
• b. 0.5
• c. 0.8

Question 25.20

A total of 6129 North American Caucasians were blood typed for the MN locus, which is determined by two codominant alleles, L^M and L^N. The following data were obtained:

Carry out a chi-square test to determine whether this population is in Hardy–Weinberg equilibrium at the MN locus.

Question 25.21

Assume that the phenotypes of lady beetles shown in Figure 25.1 are encoded by the following genotypes:

• a. For the lady beetles shown in the figure, calculate the frequencies of the genotypes and frequencies of the alleles.
• b. Use a chi-square test to determine if the lady beetles shown are in Hardy Weinberg equilibrium.

Question 25.22

Most black bears (Ursus americanus) are black or brown in color. However, occasional white bears of this species appear in some populations along the coast of British Columbia. Kermit Ritland and his colleagues determined that white coat color in these bears results from a recessive mutation (G) caused by a single nucleotide replacement in which guanine substitutes for adenine at the melanocortin-1 receptor locus (mcr1), the same locus responsible for red hair in humans (K. Ritland, C. Newton, and H. D. Marshall. 2001. Current Biology 11:1468–1472). The wild-type allele at this locus (A) encodes black or brown color. Ritland and his colleagues collected samples from bears on three islands and determined their genotypes at the mcr1 locus.

[Wendy Shattil/Alamy.]

• a. What are the frequencies of the A and G alleles in these bears?
• b. Give the genotypic frequencies expected if the population is in Hardy–Weinberg equilibrium.
• c. Use a chi-square test to compare the number of observed genotypes with the number expected under Hardy–Weinberg equilibrium. Is this population in Hardy–Weinberg equilibrium? Explain your reasoning.

Question 25.23

Genotypes of leopard frogs from a population in central Kansas were determined for a locus (M) that encodes the enzyme malate dehydrogenase. The following numbers of genotypes were observed:

• a. Calculate the genotypic and allelic frequencies for this population.
• b. What would the expected numbers of genotypes be if the population were in Hardy–Weinberg equilibrium?

Question 25.24

Full color (D) in domestic cats is dominant over dilute color (d). Of 325 cats observed, 194 have full color and 131 have dilute color.

• a. If these cats are in Hardy–Weinberg equilibrium for the dilution locus, what is the frequency of the dilute allele?
• b. How many of the 194 cats with full color are likely to be heterozygous?

Question 25.25

Tay–Sachs disease is an autosomal recessive disorder. Among Ashkenazi Jews, the frequency of Tay–Sachs disease is 1 in 3600. If the Ashkenazi population is mating randomly for the Tay–Sachs gene, what proportion of the population consists of heterozygous carriers of the Tay–Sachs allele?

Question 25.26

In the plant Lotus corniculatus, cyanogenic glycoside protects the plant against insect pests and even grazing by cattle. This glycoside is due to a simple dominant allele. A population of L. corniculatus consists of 77 plants that possess cyanogenic glycoside and 56 that lack the compound. What is the frequency of the dominant allele responsible for the presence of cyanogenic glycoside in this population?

Question 25.27

Color blindness in humans is an X-linked recessive trait. Approximately 10% of the men in a particular population are color blind.

• a. If mating is random for the color-blind locus, what is the frequency of the color-blind allele in this population?
• b. What proportion of the women in this population are expected to be color blind?
• c. What proportion of the women in the population are expected to be heterozygous carriers of the color-blind allele?

Question 25.28

The human MN blood type is determined by two codominant alleles, L^M and L^N. The frequency of L^M in Eskimos on a small Arctic island is 0.80.

• a. If random mating takes place in this population, what are the expected frequencies of the M, MN, and N blood types on the island?
• b. If the inbreeding coefficient for this population is 0.05, what are the expected frequencies of the M, MN, and N blood types on the island?

Question 25.29

Demonstrate mathematically that full-sib mating (F = ) reduces the heterozygosity by with each generation.

Question 25.30

The forward mutation rate for piebald spotting in guinea pigs is 8 × 10⁻⁵; the reverse mutation rate is 2 × 10⁻⁶.

If no other evolutionary forces are assumed to be present, what is the expected frequency of the allele for piebald spotting in a population that is in mutational equilibrium?

Question 25.31

For three years, Gunther Schlager and Margaret Dickie estimated the forward and reverse mutation rates for five loci in mice that encode various aspects of coat color by examining more than 5 million mice for spontaneous mutations (G. Schlager and M. M. Dickie. 1966. Science 151:205-206). The numbers of mutations detected at the dilute locus are as follows:

Calculate the forward and reverse mutation rates at this locus. If these mutations rates are representative of rates in natural populations of mice, what would the expected equilibrium frequency of dilute mutations be?

Question 25.32

In Figure 25.10, each blue dot represents one copy of the A allele and each red dot represents each copy of the a allele. Calculate the frequencies of the A allele in population II before and after migration. Explain why the frequency of A in population II changed after migration.

Question 25.33

In German cockroaches, curved wing (cv) is recessive to normal wing (cv⁺). Bill, who is raising cockroaches in his dorm room, finds that the frequency of the gene for curved wings in his cockroach population is 0.6. In his friend Joe’s apartment, the frequency of the gene for curved wings is 0.2. One day Joe visits Bill in his dorm room, and several cockroaches jump out of Joe’s hair and join the population in Bill’s room. Bill estimates that, now, 10% of the cockroaches in his dorm room are individual roaches that jumped out of Joe’s hair. What is the new frequency of curved wings among cockroaches in Bill’s room?

Question 25.34

A population of water snakes is found on an island in Lake Erie. Some of the snakes are banded and some are unbanded; banding is caused by an autosomal allele that is recessive to an allele for no bands. The frequency of banded snakes on the island is 0.4, whereas the frequency of banded snakes on the mainland is 0.81. One summer, a large number of snakes migrate from the mainland to the island. After this migration, 20% of the island population consists of snakes that came from the mainland.

• a. If both the mainland population and the island population are assumed to be in Hardy–Weinberg equilibrium for the alleles that affect banding, what is the frequency of the allele for bands on the island and on the mainland before migration?
• b. After migration has taken place, what is the frequency of the banded allele on the island?

Question 25.35

Pikas are small mammals that live at high elevation in the talus slopes of mountains. Most populations located on mountain tops in Colorado and Montana in North America are isolated from one another: the pikas don’t occupy the low-elevation habitats that separate the mountain tops and don’t venture far from the talus slopes. Thus, there is little gene flow between populations. Furthermore, each population is small in size and was founded by a small number of pikas.

A group of population geneticists propose to study the amount of genetic variation in a series of pika populations and to compare the allelic frequencies in different populations. On the basis of the biology and distribution of pikas, predict what the population geneticists will find concerning the within- and between-population genetic variation.

Question 25.36

What proportion of the populations shown in Figure 25.13 reached fixation by generations 10, 25, and 30? How does the proportion of populations that reach fixation due to genetic drift change over time?

Question 25.37

In a large, randomly mating population, the frequency of the allele (s) for sickle-cell hemoglobin is 0.028. The results of studies have shown that people with the following genotypes at the beta-chain locus produce the average numbers of offspring given:

• a. What will the frequency of the sickle-cell allele (s) be in the next generation?
• b. What will the frequency of the sickle-cell allele be at equilibrium?

Question 25.38

Two chromosomal inversions are commonly found in populations of Drosophila pseudoobscura: Standard (ST) and Arrowhead (AR). When treated with the insecticide DDT, the genotypes for these inversions exhibit overdominance, with the following fitnesses:

What will the frequencies of ST and AR be after equilibrium has been reached?

Question 25.39

In a large, randomly mating population, the frequency of an autosomal recessive lethal allele is 0.20. What will the frequency of this allele be in the next generation if the lethality takes place before reproduction?

742

Question 25.40

The fruit fly Drosophila melanogaster normally feeds on rotting fruit, which may ferment and contain high levels of alcohol. Douglas Cavener and Michael Clegg studied allelic frequencies at the locus for alcohol dehydrogenase (Adh) in experimental populations of D. melanogaster (D. R. Cavener and M. T. Clegg. 1981. Evolution 35:1–10). The experimental populations were established from wild-caught flies and were raised in cages in the laboratory. Two control populations (C1 and C2) were raised on a standard cornmeal-molasses-agar diet. Two ethanol populations (E1 and E2) were raised on a cornmeal-molasses-agar diet to which was added 10% ethanol. The four populations were periodically sampled to determine the allelic frequencies of two alleles at the alcohol dehydrogenase locus, Adh^S and Adh^F. The frequencies of these alleles in the experimental populations are shown in the graph.

• a. On the basis of these data, what conclusion might you draw about the evolutionary forces that are affecting the Adh alleles in these populations?
• b. Cavener and Clegg measured the viability of the different Adh genotypes in the alcohol environment and obtained the following values:
  

  Genotype	Relative viability
  AdhF/AdhF	0.932
  AdhF/AdhS	1.288
  AdhS/AdhS	0.596

  Using these relative viabilities, calculate relative fitnesses for the three genotypes. If a population has an initial frequency of p = f(Adh^F) = 0.5, what will the expected frequency of Adh^F be in the next generation on the basis of these fitness values?

Question 25.41

A certain form of congenital glaucoma is caused by an autosomal recessive allele. Assume that the mutation rate is 10⁻⁵ and that persons having this condition produce, on the average, only about 80% of the offspring produced by persons who do not have glaucoma.

• a. At equilibrium between mutation and selection, what will the frequency of the gene for congenital glaucoma be?
• b. What will the frequency of the disease be in a randomly mating population that is at equilibrium?

Question 25.42

Examine Figure 25.15. Which evolutionary forces:

• a. Cause an increase in genetic variation both within and between populations?
• b. Cause a decrease in genetic variation both within and between populations?
• c. Cause an increase in genetic variation within populations but cause a decrease in genetic variation between populations?