Chapter 25: Population Genetics

    1. Which of the following statements describes the members of a population? a. They must have the same allele for at least one genetic locus. b. They must be able to interbreed with one another. c. They all have the same genotype. d. They can include all of the plants and animals within a geographic area.

    2. The Hardy-Weinberg model predicts that allele frequencies will not change when all of the following assumptions are fulfilled except a. Random mating within the population b. Infinitely large population size c. No linkage d. No selection or mutation

    3. In a population in Hardy-Weinberg equilibrium, the frequency of allele A is 0.9 and the frequency of allele a is 0.1. What proportion of individuals exhibit the dominant phenotype? a. 0.01 b. 0.09 c. 0.18 d. 0.99

    4. All members of an isolated village were genotyped for a particular RFLP. Of the 1000 individuals, 200 were homozygous for the presence of the restriction site, 700 were heterozygous, and 100 were homozygous for its absence. What is the frequency of the "restriction site present" allele, p? a. 0.2 b. 0.45 c. 0.55 d. 0.9

    5. A population is in Hardy-Weinberg equilibrium for a gene locus that has four alleles at the following allelic frequencies: a1 (0.1), a2 (0.2), a3 (0.4), and a4 (0.3). What is the expected frequency of the a2a3 genotype? a. 0.08 b. 0.16 c. 0.2 d. 0.4

    6. Fitness is a measure of a. An organism's ability to survive b. The reproductive output of organisms with a particular genotype c. The reproductive output of organisms relative to the alternative genotypes d. An organism's physiological vigor

    7. Evolution by natural selection for a particular trait occurs when each of the following occurs, except a. More offspring are born than can survive. b. Some variants are more successful at surviving and reproducing than others. c. The trait is heritable. d. All of the above are necessary for evolution by natural selection to occur.

    8. Which of the following statements about directional selection is true? a. The mean value of a trait increases. b. Improvement can continue as long as selection is applied. c. Heterozygous individuals have the highest fitness. d. The variance for a trait decreases.

    9. For a locus where there are two alleles, B and b, and for which bb is homozygous lethal and BB and Bb have identical fitness, what is the expected allele frequency of b after 10 generations if the frequency of b is 0.1 at generation zero? a. 0 b. 0.05 c. 0.1 d. 0.5

    10. Which of the following statements about mutations in population genetics is true? a. Mutation frequencies can be calculated from the frequency of the homozygous recessive genotypic class alone. b. Except in situations of Hardy-Weinberg equilibrium, mutation plays a significant role in changing allelic frequencies. c. The mutation rate is equal to the allelic frequency of the recessive allele divided by the initial frequency of the recessive allele. d. Mutation is a major force in generating genetic variability, but by itself plays a relatively insignificant role in changing allelic frequencies.

    11. Which of the following statements about inbreeding in a natural population is true? a. It can increase the frequency of homozygotes. b. It increases the genetic variation in a population. c. It is likely to occur as population size increases. d. It occurs in all populations that are not in Hardy-Weinberg equilibrium.

    12. Hybrid vigor occurs when a. Members of two different inbred strains are crossed b. Individuals from closely related species interbreed c. Individuals mate nonrandomly d. F1 hybrids exhibit significant mortality

    13. Allele frequencies within a population can be altered by all of these processes except a. Migration b. Mutation c. Nonrandom mating d. Natural selection

    14. Coat color in mice is determined by two alleles. If the frequency of individuals exhibiting the recessive coat color, brown (b), is 0.25, what is the frequency of the b allele? a. 0.05 b. 0.25 c. 0.5 d. Strictly speaking, it cannot be determined from the information given.

    15. Darwin arrived at his theory of natural selection despite having no awareness of a. Whether differential reproductive success occurred among organisms b. A mechanism for the heritability of traits c. The mechanisms of sexual breeding of organisms d. The prevalence of variation in nature

    16. True or false? In the absence of selection, the frequency of recessive alleles tends to decrease over time within a population.

    17. Which of the following statements about the children of first-cousin matings is true? a. They exhibit greater levels of heterozygosity. b. They have increased mortality rates. c. They contain more unique alleles than other children. d. They have lower risk of spontaneous abortion.

    18. Which of the following statements does not provide an explanation for hybrid vigor? a. Disease-causing, homozygous recessive phenotypes from either parent are masked in hybrids. b. Offspring from a hybrid cross usually possess the best traits of two desirable parents. c. Inherently, hybrids have no deleterious mutations. Hybrids often have more than one form of many of the genes present, giving them an edge against environmental pressures.

    19. True or false? If the frequency of the M allele in the human MN blood group system is 0.65 in a population at equilibrium, then the frequency of the N allele must be 0.04.

    20. If a recessive disease is found in 50 out of 100,000 individuals, what is the frequency of the heterozygote carriers for this disease? a. 0.0005 b. 0.022 c. 0.043 d. 0.956

    21. In a population of birds in Africa, it was observed that birds with small or large beaks could efficiently crack and eat small or large seeds, respectively. Birds with intermediate beaks had trouble with both types of seeds. What type of selection would be expected to occur in this population if small and large seeds were the only types of food available to these birds? a. Disruptive b. Directional c. Stabilizing d. There would be no selection in this population.

    Hints and Answers

    1. HINT: A population, rather than the individual, is the unit of interest in population genetics. a. Incorrect. This is not necessarily the case, although it may be true in some populations. 25.1 Allele Frequencies in Population Gene Pools Vary in Space and Time (p. 618) b. Correct. Members of a population must be able to interbreed, actually or potentially. 25.1 Allele Frequencies in Population Gene Pools Vary in Space and Time (p. 618) c. Incorrect. This is not necessarily the case, although it is true for inbred lab-animal populations. 25.1 Allele Frequencies in Population Gene Pools Vary in Space and Time (p. 618) d. Incorrect. Individuals from different species are unable to interbreed, which is one of the criteria for the members of a population. 25.1 Allele Frequencies in Population Gene Pools Vary in Space and Time (p. 618)

    2. HINT: The Hardy-Weinberg law was developed by two mathematicians to calculate what happens to alleles and genotypes in an "ideal" population. a. Incorrect. With nonrandom mating, certain genotypes might be avoided, decreasing the frequency of the alleles they carry. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618) b. Incorrect. Small populations are subject to sampling errors and random effects. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618) c. Correct. Linkage doesn't affect allele frequencies. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618) d. Incorrect. Selection and mutation remove and add, respectively, alleles from the population. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618)

    3. HINT: Both homozygous dominant and heterozygous individuals exhibit the dominant trait. a. Incorrect. This is the frequency of individuals showing the recessive phenotype. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618) b Correct. The frequency of homozygous dominant AA individuals is 0.81 and the frequency of heterozygous Aa individuals is 0.18. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618) c. Incorrect. This is the frequency of the heterozygotes. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618) d. Incorrect. This is the frequency of only half of the heterozygotes. 25.2 The Hardy-Weinberg Law Describes the Relationship between Allele Frequencies and Genotype Frequencies in an Ideal Population (p. 618)

    4. HINT: Review how genotype data can be used to determine allele frequencies in a population. a. Incorrect. When calculating the allele frequency, remember that heterozygotes have only 1/2 the number of alleles as the homozygotes. 25.3 The Hardy-Weinberg Law Can Be Applied to Human Populations (p. 620) b. Incorrect. When calculating the allele frequency, remember that heterozygotes have only 1/2 the number of alleles as the homozygotes. 25.3 The Hardy-Weinberg Law Can Be Applied to Human Populations (p. 620) c. Incorrect. When calculating the allele frequency, remember that heterozygotes have only 1/2 the number of alleles as the homozygotes. 25.3 The Hardy-Weinberg Law Can Be Applied to Human Populations (p. 620) d. Correct. This is the answer obtained by adding the frequency of the allele in the homozygote (200/1000) to the frequency of the allele in the heterozygote (700/1000)(0.5). 25.3 The Hardy-Weinberg Law Can Be Applied to Human Populations (p. 620)

    5. HINT: The allele frequencies given are for both sexes. Assume that, in the final a2a3 genotype, each allele could come from either sex. a. Correct. Calculate the frequency for the female carrying a2 and the male carrying a3 and then the inverse. Therefore, a2(0.2) ´ a3(0.4) = 0.08. The inverse cross would also give 0.08. Add the frequencies of each cross together to get 0.08 + 0.08 = 0.16. 25.4 The Hardy-Weinberg Law Can Be Used for Multiple Alleles, X-Linked Traits, and Estimating Heterozygote Frequencies (p. 622) b. Incorrect. This is the frequency of the a2 allele alone in the population. 25.4 The Hardy-Weinberg Law Can Be Used for Multiple Alleles, X-Linked Traits, and Estimating Heterozygote Frequencies (p. 622) c. Incorrect. This is the answer if the genotype frequencies are considered in only one direction (as far as which parent has which allele). Assume that, in the final a2a3 genotype, each allele could come from either sex. 25.4 The Hardy-Weinberg Law Can Be Used for Multiple Alleles, X-Linked Traits, and Estimating Heterozygote Frequencies (p. 622) d. Incorrect. This is the frequency of the a3 allele alone in the population. 25.4 The Hardy-Weinberg Law Can Be Used for Multiple Alleles, X-Linked Traits, and Estimating Heterozygote Frequencies (p. 622)

    6. HINT: Evolution occurs when there are fitness differences among genotypes. a. Incorrect. Survival is relevant only to the extent that it increases relative reproductive success. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) b. Incorrect. Selection doesn't favor good genotypes; it favors genotypes that are better than others. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) c. Correct. These alleles will increase in frequency relative to other alleles. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) d. Incorrect. Fitness in population genetics does not refer to an individual's physiological vigor. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624)

    7. HINT: Charles Darwin and Alfred Russel Wallace independently identified the process of natural selection as an important force shaping natural populations. a. Incorrect. If all individuals survived, it wouldn't matter what traits they carried. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) b. Incorrect. Individuals vary within a population, and those variants that are more successful leave more offspring (carrying similar alleles). 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) c. Incorrect. Only genetic traits can be influenced by natural selection. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) d. Correct. Regardless of anything else, when these four conditions hold, natural selection is occurring. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624)

    8. HINT: Review the effects of directional selection on extreme phenotypes. a. Incorrect. The mean value of a trait may increase or decrease. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) b. Incorrect. There must be genetic variation available in which to make a selection. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) c. Incorrect. Individuals exhibiting one of the extremes of a distribution are selected. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) d. Correct. Alleles are lost since certain genotypes are not used for breeding. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624)

    9. HINT: The frequency of an allele in any future generation can be determined from its frequency at generation zero by using an equation derived from the Hardy-Weinberg equation. a. Incorrect. The b allele would (under normal circumstances) never be lost completely. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) b. Correct. The frequency of the b allele can be determined to be 0.05 by using the following equation: [qg = q0/(1 + gq0)), where qg is the frequency of allele b in generation zero, q0 is the starting frequency of allele b, and g is the number of generations that have passed. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) c. Incorrect. If the genotype bb were lethal, the frequency of the b allele would decrease over time. 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624) d. Incorrect. The frequency of the b allele can be determined by using the following equation: (qg = q0/(1 + gq0)). 25.5 Natural Selection Is a Major Force Driving Allele Frequency Change (p. 624)

    10. HINT: Mutation is the process by which DNA undergoes a structural change. The mutation may happen randomly (spontaneously) or from direct pressure. a. Incorrect. Although most mutations are recessive, some are dominant. This method would miss the dominant mutations and the heterozygotes with recessive mutations. 25.6 Mutation Creates New Alleles in a Gene Pool (p. 629) b. Incorrect. Mutations by themselves have a small effect on allele frequencies at any time. Genetic drift and natural selection working with mutations will have the greatest effect on allele frequencies. 25.6 Mutation Creates New Alleles in a Gene Pool (p. 629) c. Incorrect. The mutation rate is the number of mutation events per gene per cell generation. Mutations may result in recessive, dominant, or codominant alleles. 25.6 Mutation Creates New Alleles in a Gene Pool (p. 629) d. Correct. Mutation creates new alleles and therefore variability. Changes in allele frequencies depend on the forces of natural selection and genetic drift. 25.6 Mutation Creates New Alleles in a Gene Pool (p. 629)

    11. HINT: Inbreeding is mating among relatives a. Correct. Consequently, it can increase expression of homozygous-recessive diseases. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632) b. Incorrect. Inbreeding decreases genetic variation in a population. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632) c. Incorrect. Inbreeding is highest in small populations. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632) d. Incorrect. There are several reasons a population may not be in Hardy-Weinberg equilibrium. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632)

    12. HINT: Plant breeders frequently observe hybrid vigor. a. Correct. The offspring are highly heterozygous. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632) b. Incorrect. Interspecies hybrids do not show any vigor. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632) c. Incorrect. Nonrandom mating is not necessarily between individuals from different inbred strains. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632) d. Incorrect. F1 hybrids show decreased mortality. 25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency (p. 632)

    13. HINT: Natural selection is not the only mechanism of evolution. a. Incorrect. The addition of alleles from outside the population will change the relative frequencies. b. Incorrect. When mutation changes an allele, the overall frequencies have changed. c. Correct. Nonrandom mating does not change the allele frequencies, just the combinations of traits that co-occur. d. Incorrect. Natural selection is probably the most influential mechanism shaping the allele frequencies in a population.

    14. HINT: The frequency of individuals showing the dominant phenotype can be determined from this information. a. Incorrect. This would be true only if the population were in Hardy-Weinberg equilibrium. b. Incorrect. Which phenotypes can possess the recessive allele? c. Incorrect. Information on the proportion of heterozygotes is necessary. d. Correct. Because it is not clear whether the population is in Hardy-Weinberg equilibrium, the proportion of heterozygotes is unknown, and so the frequency of the b allele cannot be determined.

    15. HINT: Darwin formed his theory by observing animals in nature and fossils on his voyage of the HMS Beagle. a. Incorrect. Darwin observed this when he observed animals in nature. b. Correct. Mendel had solved this problem, but Darwin had not read his work. c. Incorrect. Darwin observed this in nature and among animal breeders. d. Incorrect. In his travels, Darwin was impressed with the great amount of variation among individual plants and animals.

    16. HINT: Allele frequencies change if one of the Hardy-Weinberg assumptions is violated. False. There is no inherent disadvantage to recessive alleles; the Hardy-Weinberg model illustrates that allele frequencies do not change unless evolution is occurring.

    17. HINT: Many cultures encourage this practice, despite health concerns about it. a. Incorrect. Children that are inbred are more homozygous. b. Correct. This is a universally observed phenomenon. c. Incorrect. Children that are inbred contain fewer alleles and are more homozygous. d. Incorrect. Children that are inbred are at higher risk of spontaneous abortion.

    18. HINT: Plant breeders often create hybrids that are superior to nonhybrids but are sterile, so that the growers cannot replicate the seed. a. Incorrect. Deleterious homozygous recessive genotypes from either parent are masked as heterozygotes in the hybrids. b. Incorrect. Generally, a breeder's intention in creating a hybrid is to combine the best traits of two different parents in the final product. c. Correct. Hybrids tend to have a lower probability of deleterious mutations due to their heterozygous genotype. d. Incorrect. The theory of overdominance, which says that the heterozygote is often superior to either homozygote, may be based on the fact that heterozygous loci have two forms of the gene product available.

    19. HINT: There are only two alleles in the human MN blood group system. False. The sum of the allele frequencies must equal 1, so the frequency of the N allele must be 0.35. Web Tutorial 25.1, Population Genetics

    20. HINT: The frequency of homozygous recessives equals the number of affected individuals divided by the population. a. Incorrect. This value represents the frequency of homozygous recessive individuals (q2) in the population. Web Tutorial 25.1, Population Genetics b. Incorrect. This value represents the frequency of the recessive allele q in the population. Web Tutorial 25.1, Population Genetics c. Correct. If q2 = 0.0005, then q = 0.022 and p = 1 – q = 0.978. The heterozygote frequency is 2pq, or 2 (978) (0.022) = 0.043. Web Tutorial 25.1, Population Genetics d. Incorrect. This value represents the frequency of homozygous dominant individuals (p2) in the population. Web Tutorial 25.1, Population Genetics

    21. HINT: Which individuals are the most fit in this population? a. Correct. Birds with an intermediate beak phenotype are at a disadvantage in this population and will be selected against because they are less fit. Web Tutorial 25.1, Population Genetics b. Incorrect. Directional selection occurs when a phenotype at one extreme is more fit than the others; in this population, the phenotypes at both extremes are more fit than the intermediate phenotype. Web Tutorial 25.1, Population Genetics c. Incorrect. Stabilizing selection occurs when the intermediate phenotype is more fit than the phenotypes at either extreme; in this population, the phenotypes at both extremes are more fit than the intermediate phenotype. Web Tutorial 25.1, Population Genetics d. Incorrect. The selective agent in this population is food, since only certain individuals can crack and eat seeds efficiently. Web Tutorial 25.1, Population Genetics