CHAPTER 26 QUESTIONS
2. In directional selection, explain the meaning of the word "directional".
"directional" means that a certain trait or phenotype with increased survivability is favored over another and over generations the entire population has more of that phenotype. A graph shows a complete shift from one phenotype to another.
E13: What is DNA fingerprinting? How can it be used in human identification?
-DNA fingerprinting is a method of identification based on the properties of DNA. Minisatellites and microsatellite sequences are variable with regard to size in natural populations. This variation can be seen when DNA fragments are subjected to gel electrophoresis. Within a population, any two individuals (except for identical twins) will display a different pattern of DNA fragments, which is called their DNA fingerprint-
C12: What is Darwinian fitness? What types of characteristics can promote high fitness values? Give several examples.
-Darwinian fitness is the relative likelihood that one genotype will contribute to the gene pool of the next generation compared to other genotypes. It is basically the measure of reproductive success. Characteristics that promote survival, ability to attract a mate, or an enhanced fertility would be expected to promote Darwinian fitness. Examples are the thick fur of a polar bear, which helps it to survive in a cold climate
E8: In the Grant's study of the medium ground finch, do you think the pattern of natural selection was directional, stabilizing, disruptive, or balancing? Explain your answer. If the environment remained dry indefinitely (for many years), what do you think would be the long-term outcome?
-Directional selection, because the results of the experiment showed that the environments that produce larger seeds will select for birds with larger beaks. If the environment remained dry, the long-term effect would be smaller seed size, and therefore the birds would have smaller beak depth-
C16: In the term genetic drift, what is drifting? Why is this an appropriate term to describe this phenomenon?
-Genetic drift is the phenomenon in which allele frequencies change in a population due to random fluctuations. The frequencies of alleles found in gametes that unite to form zygotes vary from generation to generation. It is an appropriate term because the word drift implies a random process. Nevertheless, drift can be directional-
C3: What is a genetic polymorphism? What is the source of genetic variation?
-Genetic polymorphism is used to describe a gene that commonly exists as two or more alleles in a population. Mutation is the ultimate source of genetic variation-
C8: In a population, the frequencies of two alleles are B= 0.67 and b= 0.33. The genotype frequencies are BB= 0.50, Bb= 0.37, and bb= 0.13. Do these number suggest inbreeding. Explain why or why not.
-If we apply the Hardy-Weinberg equation- BB = (0.67) 2 = 0.45, or 45% Bb = 2(0.67)(0.33) = 0.44, or 44% bb = (0.33) 2 = 0.11, or 11% The actual data show a higher percentage of homozygotes (compare 45% with 50% and 11% with 13%) and a lower percentage of heterozygotes (compare 44% with 37%) than expected. Therefore, these data would be consistent with inbreeding, which increases the percentage of homozygotes and decreases the percentage of heterozygotes-
C19: Describe what happens to allele frequencies as a result of the bottleneck effect. Discuss the relevance of this effect with regard to species that are approaching extinction.
-Initial bottleneck may be greatly influenced by genetic drift because the surviving members may have allele frequencies that differ from those of the original populations. Allele frequencies are expected to drift substantially during the generations when the population size is small and in extreme cases alleles may be eliminated. Eventually the population will regain its original size however, the new population has less genetic variation that the original large population Species that are approaching extinction also face a bottleneck as their numbers decrease. The loss of genetic diversity may make it even more difficult for the species to rebound-
E12: Here are traditional DNA fingerprints of five people: a child, mother, and three potential fathers. Which males can be ruled out as being the father? Explain your answer. If one of the males could be the father, explain the general strategy for calculating the likelihood that he could match the offspring's DNA fingerprint by chance alone.
-Male 2 is the father because he is responsible for paternal bands that the mother and fathers 1 and 3 cannot pass on to the offspring. He also has the most bands in common with the offspring than the other males do. Geneticists can calculate the likelihood that the matching bands between the offspring and a prospective gather could occur just as a matter of random chance. To do so, they must analyze the frequency of each band in a reference population (e.g. Caucasians living in the United States-
C10: What evolutionary factors can cause allele frequencies to change and possibly lead to a genetic polymorphism? Discuss the relative importance of each type of process.
-New allelic variation is the introduction of a new genetic variation into a population is one essential aspect of microevolution. This can be caused by random mutations, but they are rate and do not act as a major factor in promoting widespread changes in a population. Mutations are important because they have to potential to bring variation into a gene pool-
C23: Does inbreeding affect allele frequencies? Why or why not? How does it affect genotype frequencies? With regard to rare recessive diseases, what are the consequences of inbreeding in human populations?
-No, it has no effect on allele frequencies when there are no other forces present because it does not favor the transmission of one allele over another. It increases the likelihood of homozygosity. In human populations, it increases the frequency of individuals who are homozygous for rare recessive diseases-
C13: What is the intuitive meaning of the mean fitness of a population? How does its value change in response to natural selection?
-The mean fitness of a population is an equation used to calculate the expected genotype and allele frequencies after one generation of directional selection. Its value changes as natural selection increases-
C5: The term polymorphism can refer to both genes and traits. Explain the meaning of a polymorphic gene and a polymorphic trait. If a gene is polymorphic, does the trait that the gene affects also have to be polymorphic? Explain why or why not.
-When a trait is polymorphic, this means that different individuals show phenotypic variation with regard to the trait. When a gene is polymorphic, it exists in two or more alleles. At the molecular level, alleles of a given gene have different DNA sequences. These differences could be very slight (e.g., a single-base change) or they could involve significant additions or deletions. Different alleles may cause differences in phenotype. However, alleles do not always cause differences in phenotype. A single-base substitution in a gene may not affect the amino acid sequence of the encoded polypeptide (e.g., the base change may affect the wobble base) or a single-base change might alter the amino acid sequence, but not in a way that alters the protein's function. Therefore, gene polymorphism does not always result in phenotypic polymorphism-
C1: What is the gene pool? How is the gene pool described in a quantitative way?
-a gene pool is all the alleles of every gene in a population (generation-per-generation basis- individuals of one generation). Population genetics study the genetic variation within gene pools and how such variation changes from one generation to the next. If a gene is monomorphic, the allele frequency is close to 100%. If it is polymorphic, each allele has a frequency that is between 1 and 99%. The sum of all the allele frequencies for a particular gene will add up to 100%-
S1: The phenotype frequency of people who cannot taste PTC is approximately 0.3. The inability to taste this bitter substance is due to a recessive allele. If we assumer there are only two alleles in the population (namely, taster T, and nontasters, t) and that the population is in Hardy-Weinberg equilibrium, calculate the frequencies of these two alleles.
-p = allele frequency of the taster, q = allele frequency of the nontaster allele. The frequency of nontasters is 0.3. This is the frequency of the genotype tt, which in this case is equal to q^2. q^2 = 0.3to determine the frequency q of the nontaster allele, we take the square root of both sides of this equation-q= 0.55With this value, we can calculate the frequency p of the taster allele-P= 1-q= 1-0.55 = 0.45
E6: Resistance to the poison warfarin is a genetically determined trait in rats. Homozygotes carrying the resistance allele (WW) have a lower fitness because they suffer from vitamin K deficiency, but heterozygotes (Ww) do not. However heterozygotes are still resistance to warfarin. In an area where warfarin is applied, the heterozygote has a survival advantage. Due to warfarin resistance, the heterozygote is also more fit than the normal homozygote (ww). If the relative fitness values for Ww, WW, and ww individuals are 1.0, 0.37, and 0.19, respectively, in areas where warfarin is applied, calculate the allele frequencies at equilibrium. How would this equilibrium be affected if the rates were no longer exposed to warfarin?
-sAA+saar- The selection coefficients aresww = 1 - 0.19 = 0.81sWW = 1 - 0.37 = 0.63If the rats are not exposed to warfarin, the equilibrium will no longer exist, and natural selection will tendto eliminate the warfarin-resistance allele because the homozygotes are vitamin K deficient-
E15: What would you expect to be the minimum percentage of matching peaks in an automated DNA fingerprint for the following pairs of individuals? A. Mother and son: B. Sister and brother: C. Uncle and niece: D. Grandfather and grandson:
A. 50% B. 50% C. 25% D. 25%
E3: In a large herd of 5438 sheep, 76 animals have yellow fat, compared with the rest of the members of the herd, which have white fat. Yellow fat is inherited as a recessive trait. This herd is assumed to be in Hardy-Weinberg equilibrium. A. What are the frequencies of the white and yellow fat alleles in this population? B. Approximately how many sheep with white fat are heterozygous carriers of the yellow allele?
A. Allelic frequencies: F(w)= 2(76) / 2(5438) = 0.0139 for yellow fat = q^2; q= .117=.12 p+q=1; p+.12 = 1 p= 1-0.12= 0.88 B. Heterozygous carrier= 2pq= 2(0.88)(0.12)= 0.21 * 5438 = 1148 sheep will be heterozygous
C22: Two populations of antelope are separated by a mountain range. The antelope are known to occasionally migrate from one population to the other. Migration can occur in either direction. Explain how migration affects the following phenomena: A. Genetic diversity in the two populations B. Allele frequencies in the two populations C. Genetic drift in the two populations
A. Migration will increase the genetic diversity in both populations. A random mutation could occur in one population to create a new allele. This new allele could be introduced into the other population via migration. B. The allele frequencies between the two populations will tend to be similar to each other, due to the intermixing of their alleles. C. Genetic drift depends on population size. When two populations intermix, this has the effect of increasing the overall population size. In a sense, the two smaller populations behave somewhat like one big population. Therefore, the effects of genetic drift are lessened when the individuals in two populations can migrate. The net effect is that allele loss and allele fixation are less likely to occur due to genetic drift-
1. Which of the following is a factor that does not promote widespread changes in allele or genotype frequencies? A. New mutation B. Natural selection C. genetic drift D. Migration E. Nonrandom mating
A. New mutation
C20: With regard to genetic drift, are the following statements true or false? Is a statement is false, explain why. A. Over the long run, genetic drift leads to allele fixation or loss. B. When a new mutation occurs within a population, genetic drift is more likely to cause the loss of the new allele rather than the fixation of the new allele. C. Genetic drift promotes genetic diversity in large populations D. Genetic drift is more significant in small populations
A. TRUE B. TRUE C. FALSE, it causes allele loss or fixation, which results in less diversity. D. TRUE
S3: The Hardy-Weinberg equation provides a way to predict genotype frequency based on allele frequency. In the case of mammals, males are hemizygous for X-linked genes, whereas females have two copies. Among males, the frequency of an X-linked trait equals the frequency of males with that trait. For example, is an allele frequency for an X-linked disease-causing allele was 5%, then 5% of all males would be affected with the disorder. Female genotype frequencies are computed using the Hardy-Weinberg equation. As a specific example, let's consider the human X-linked trait known as hemophilia A. In human populations, the allele frequency of the hemophelia A allele is approximately 1 in 10,000, or 0.0001. The other allele for this gene is the normal allele. Males can be affected or unaffected, whereas females can be affected, unaffected carriers, or unaffected noncarriers. A. What are the allele frequencies for the mutant and normal allele in the human population? B. Among males, What is the frequency of affected individuals? C. Among females, what is the frequency of affected individuals and heterozygous carriers? D. Within a population of 100,000 people, what is the expected number of affected males? In this same population, what is the expected number of carrier females?
A. X^H normal allele, frequency= 0.9999= p X^h hemophilia allele, frequency= 0.0001 = q B. X^hY genotype frequency of affected males = q = 0.0001 C. X^hX^h genotype frequency of affected females = q^2 = (0.0001)^2 = 0.00000001 X^HX^H genotype frequency of carrier females = 2pq = (0.9999)(0.0001) = 0.0002 D. We assume this population is composed of 50% males and 50% females Number of affected males = 50000 * 0.0001 = 5 Number of carrier females = 50000 * 0.0002 = 10
C15: Do the following examples describe directional, disruptive, balancing, or stabilizing selection? A. Polymorphisms in snail color and banding pattern as described in Figure 26.12 B. Thick fur among mammals exposed to cold climates C. Birth weight in humans D. Sturdy stems and leaves among plants exposed to windy climates
A. disruptive selection B. directional selection C. stabilizing selection D. directional selection
C4: State for each of the following whether it is an example of an allele, genotype, and/or phenotype frequency: A. Approximately 1 in 2500 people of Northern European descent is born with cystic fibrosis. B. The percentage of carriers of the sickle cell allele in West Africa is approximately 13%. C. The number of new mutations for achondroplasia, a genetic disorder, is approximately 5 x 10-5.
A. phenotype frequency/ genotype frequency B. genotype frequency C. allelic frequency
C6: Cystic fibrosis (CF) is a recessive autosomal trait. In certain populations of Northern European descent, the number of people born with this disorder is about 1 inn every 2500. Assuming Hardy-Weinberg equilibrium for this trait: A. What are the frequencies for the normal and CF alleles? B. What are the genotype frequencies of homozygous normal, heterozygous, and homozygous affected individuals? C. Assuming random mating, what is the probability that two phenotypically unaffected heterozygous carriers will choose each other as mates?
ANS: A. The genotype frequency for the CF homozygote is 1/2,500, or 0.004. This would equal q^2. The allele frequency is the square root of this value, which equals 0.02. The frequency of the corresponding normal allele equals 1 - 0.02 = 0.98. B. The frequency for the CF homozygote is 0.004; for the unaffected homozygote, (098)2 = 0.96; and for the heterozygote, 2(0.98)(0.02), which equals 0.039. C. If a person is known to be a heterozygous carrier, the chances that this particular person will happen to choose another as a mate is equal to the frequency of heterozygous carriers in the population, which equals 0.039, or 3.9%. The chances that two randomly chosen individuals will choose each other as mates equals 0.039 × 0.039 = 0.0015, or 0.15%.
S5: Lets suppose that pigmentation in a species of insect is controlled by a single gen existing in two alleles, D for dark and d for light. The heterozygote Dd is intermediate in color. In a heterogeneous environment, the allele frequencies are D = 0.7 and d = 0.3. this polymorphism is maintained because the environment contains some dimply lit forested areas and some sunny fields. During a hurricane, a group of 1000insects is blown to a completely sunny area. In this environment, the fitness values are DD = 0.3, Dd = 0.7, and dd = 1.0. Calculate the allele frequencies in the next generation.
ANS: 1. Calculate the mean fitness of the population... P2wAA + 2pqwAa + q2waa = w P^2= (0.7)^2 , wDD= 0.3, 2pq= 2(0.7)(0.3), wDd= 0.7, q^2= (0.3)^2, wdd= 1.0 W= (0.7)^2(0.3) + 2(0.7)(0.3)(0.7) + (0.3)^2(1.0) = 0.147 +0.294 + .09= 0.531 After one generation... Allele frequency od D: pD= p^2*wDD/w + pq wDd/ w = (0.7)^2(0.3) / 0.531 + (0.7)(0.3)(0.7) / 0.531= 0.276 + 0.2768 = 0.55 Allele frequency of d: qd= q2wdd/w + pq wDd/ w = (0.3)^2(1.0)/0.531 + (0.7)(0.3)(0.7) / 0.531 = 0.16981+ 0.2768 = 0.44716 = 0.45
E9: A recessive lethal allele has achieved a frequency of 0.22 due to genetic drift in a very small population. Based on natural selection, how would you expect the allele frequencies to change in the next three generations? Now: Your calculation can assume that genetic drift is not altering allele frequencies in either direction.
ANS: As natural selection increases, the allele frequency will increase as well. Let's assume that the relative fitness values are 1.0 for the dominant homozygote and the heterozygote and 0 for the recessive homozygote. The first thing we need to do is to calculate the mean fitness of the population. (0.78)^2+2(0.78)(0.22)= w = 0.95 The allele frequency (p) in the next generation equals... p= P^2WAA/w + pqWAa/w = 0.82 q = 1 - p = 0.18 We would follow the same general strategy for the second and third generations as well. For the second generation, the mean fitness of the population now equals 0.97. Using the preceding equation, the allele frequency of A in the second generation approximately equals 0.85. The frequency of the recessive allele in the second generation would equal about 0.15 and the mean fitness would now be approximately 0.98. The allele frequency of A in the third generation would be approximately 0.87. The frequency of the recessive allele would be about 0.13.
S6: An important application of DNA fingerprinting is relationship testing. Persons who are related genetically have some bands or peaks in common. The number they share depends on the closeness of the genetic relationship. For example, an offspring is expected to receive half of his or her minisatellites from one parent and the rest from the other. This diagram shows here schematically illustrates a traditional DNA fingerprint of an offspring, mother and two potential fathers. In paternity testing the offspring's DNA fingerprint is first compared with that of the mother. The bands that the offspring have in common with the mother are depicted in purple. The bands that are not similar between the offspring and the mother must have been inherited from the father. These bands are depicted in red. Which male could be the father?
ANS: Male 2 does not have as many of the paternal bands, therefore he can be excluded as being the father of this child. However, male 1 has all of the paternal bands. He is very likely to be the father. Geneticists can calculate the likelihood that the matching bands between the offspring and a prospective gather could occur just as a matter of random chance. To do so, they must analyze the frequency of each band in a reference population (e.g. Caucasians living in the United States). For example, let's suppose that DNA fingerprinting analyzed 40 bands. Of these, 20 bands matched with the mother and 20 bands matched with the prospective father. If the probability of each of these bands in a reference population was 1/4, the likelihood of such a match occurring be random chance would be (1/4)^20, or roughly 1 in 1 trillion. Therefore, a match between two samples is rarely a matter of random chance.
C14: Describe the similarities and differences among directional, balancing, disruptive, and stabilizing selection.
ANS: Similarities between these patterns are that they all favor one or more phenotypes because of their reproductive advantage. The patterns are different with regard to whether a single phenotype or multiple phenotypes are favored, and whether the phenotype that is favored is in the middle of the phenotypic range or at one or both extremes. Directional selection is natural selection that favors individuals that are more likely to survive and reproduce in a particular environment. Balancing selection is a pattern of natural selection that favor the maintenance of two or more alleles in a population. Disruptive selection is a pattern of natural selection that favors the survival of two or more different phenotypes. Stabilizing selection is a pattern of natural selection in which extreme phenotypes for a trait are selected against, and those individuals with intermediate phenotypes have the highest fitness values.
C7: For a gene existing in two alleles, what are the allele frequencies when the heterozygote frequency is at the maximum value, assuming a Hardy-Weinberg equilibrium? What if there are three alleles?
ANS: The allele frequency for the two alleles when the heterozygote frequency is at its maximum, the frequency will be 0.5 for both alleles. If there are 3 alleles, then the frequencies would be 0.33 (1/3).
4. Explain how sickle-cell anemia is an example of balancing selection. Your response should include the term "heterozygote advantage".
Balancing selection favors the maintenance of two or more alleles in a population. In heterozygote advantage, a heterozygote has a higher fitness than a homozygote. For example, in sickle-cell anemia, which is a homozygote, the heterozygote contains the allele that when in homozygous state causes sickle-cell anemia. It is chosen over the other allele that causes malaria when homozygous.
6. In general, why does stabilizing selection decrease genetic diversity?
Because it eliminates alleles that cause a greater variation in phenotypes.
C9: The ability to roll your tongue is inherited as a recessive trait. The frequency of the rolling allele is approximately 0.6, and the dominant (non-rolling) allele is 0.4. What is the frequency of individuals who can roll their tongues?
Because this is a recessive trait, only the homozygotes for the rolling allele will be able to roll their tongues. If p equals the rolling allele and q equals the nonrolling allele, the Hardy-Weinberg equation predicts that the frequency of homozygotes who can roll their tongues would be p^2. In this case, p^2 = (0.6)^2= 0.36, or 36%
1. What is "F"? What is the equation to calculate it?
F is the inbreeding coefficient. F = Ʃ (1/2)n(1 + FA)
1. What are two effects that can cause genetic drift? Distinguish between them.
Genetic drift can be caused by the bottleneck effect or the founder effect. The bottleneck effect is when a population can be reduces dramatically in size by events such as earthquakes, floods, drought, or human destruction of habitat. Such events may randomly eliminate most of the members of the population without regard to its genetic composition. The found effect occurs when a group of individuals separates from a large population and establishes a colony in a new location. Key difference between bottleneck and founder effect is that the founder effect involves migration.
4. As inbreeding increases, what happens (numerically) to the frequency of homozygous and heterozygous genotypes?
In natural populations, inbreeding lowers the mean fitness of the population if homozygous offspring have a lower fitness value. As population shrinks, inbreeding becomes more likely because individuals have fewer potential mates from which to choose. Inbreeding produces homozygotes that are less fit, thereby decreasing the reproductive success of the population INBREEDING DEPRESSION
2. What is happening at a genetic bottleneck? What is the effect of genetic drift during the bottleneck?
Initial bottleneck may be greatly influenced by genetic drift because the surviving members may have allele frequencies that differ from those of the original populations. Allele frequencies are expected to drift substantially during the generations when the population size is small and in extreme cases: alleles may be eliminated. Eventually the population will regain its original size however, the new population has less genetic variation that the original large population.
2. Explain the phrase "inbreeding coefficient of the common ancestor".
It is basically measuring the degree of relatedness within a pedigree.
1. What are the four mechanisms that can alter existing genetic variation? Which one of these mechanisms doesn't result in changes to allele frequency?
Natural selection, genetic drift, migration, and nonrandom mating. Nonrandom mating does not change allelic frequencies.
1. Are polymorphisms common or rare in natural populations?
Polymorphism is rare in natural populations. Most are monomorphism.
2. What is a "conglomerate population"?
Population that has new members due to migration. Composed of members of both recipient and donor populations.
2. What is a "SNP"? SNPs represent _____% of all of variations in DNA sequences that occur among humans.
Single-nucleotide polymorphism. 90%
1. What three population have to be considered when determining the effects of migration?
The original donor population, the original recipient population, and the population that has new members due to migration
3. What is the function of a selection coefficient? How is it calculated?
The selection coefficient allows for the measurement of the degree to which a genotype is selected against. It is calculated by this equation: s= 1-w
5. Does disruptive selection favor polymorphisms? Explain the rationale for your response.
Yes, heterogenous environments favors the maintenance of two or more alleles, which leads to polymorphism is the species does not migrate.
3. Can a pedigree have more than one common ancestor? Explain the basis for your response.
Yes, there can be more than one inbreeding path.
3. A gene exists in two alleles designated D and d. If 48 copies of this gene are the D allele and 152 are the d allele, what is the allele frequency of D? a. 0.24 b. 0.32 c. 0.38 d. 0.76
a. 0.24
2. Within a population, darkly colored rats are more likely to survive than more lightly colored individuals. This scenario is likely to result in... a. directional selection b. stabilizing selection c. disruptive selection d. balancing selection
a. directional selection
3. DNA fingerprinting analyzes the DNA from individuals based on the occurrence of ______ in their genomes. a. repetitive sequences b. abnormalities in chromosome structure c. specific genes d. viral insertions
a. repetitive sequences
2. In a natural population, most genes are... a. polymorphic b. monomorphic c. recessive d. both a and c
b. monomorphic
1. Darwinian fitness is a measure of... a. survival b. reproductive success c. heterozygosity of the gene pool d. polymorphisms in a population
b. reproductive success
2. Which of the following types of genetic drift involve the migration of a population from one location to another? a. the bottleneck effect b. the founder effect c. both a and b d. none of the above
b. the founder effect
1. A gene pool is a. all of the genes in a single individual b. all of the genes in the gametes from a single individual c. all of the genes in a population of individuals d. the random mixing of genes during sequela reproduction
c. all of the genes in a population of individuals
3. A population occupies a diverse environment in which the fitness of some genotypes is higher in one environment while the fitness of other individuals is higher in another environment. This scenario is likely to result in... a. directional selection b. stabilizing selection c. disruptive selection d. balancing selection
c. disruptive selection
1. Genetic drift is... a. a change in allele frequencies due to random fluctuations b. likely to result in allele loss or fixation over the long run c. more pronounced in smaller populations d. all of the above
d. all of the above
4. A gene exists in two alleles, and the heterozygote has the higher fitness. This scenario is likely to result in... a. directional selection b. stabilizing selection c. disruptive selection d. balancing selection
d. balancing selection
1. Gene flow depends on... a. migration b. the ability of migrant alleles to be passed to subsequent generations c. genetic drift d. both a and b.
d. both a and b.
1. Inbreeding refers to mating between individuals that are... a. homozygous b. heterozygous c. part of the same genetic linkage d. both a and c.
d. both a and c.