Chapter 21 Homework Questions & Answers
How can gene flow improve adaptation of population?
Beneficial alleles are transferred to a new population
How did Mendel's model of inheritance help explain Darwin's theory of natural selection?
By proposing a mechanism of how organisms could transmit discrete, heritable units to offspring
How can genetic variation at the whole gene level (gene variability) be quantified?
By the average percentage of loci that are heterozygous
Which of the following describes a mechanism for generating more genes?
Duplication
Which statement about the beak size of finches on the island of Daphne Major during prolonged drought is true?
Each bird's survival was strongly influenced by the depth and strength of its beak as the drought persisted Explanation: Birds with larger/deeper beaks survived and the ones with small beaks died
Evolution of a gene
can happen whenever any of the conditions for Hardy-Weinberg equilibrium are not met for that particular gene
You are maintaining a small population of fruit flies in the laboratory by transferring the flies to a new culture bottle after each generation. After several generations, you notice the viability of the flies has decreased greatly. Recognizing that small population size is likely to be linked to decreased viability, the best way to reverse this trend is to
cross your flies with flies from another lab
What is the frequency of the A1A2 genotype in a population composed of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals?
0.4 Calculation: 80 A1A2 individuals / (20 + 80 + 100) total individuals = 0.4
The frequency of a disease is 30/1000. This is a disease that follows a dominant/recessive pattern of inheritance on one gene (where the only affected individuals are all homozygous recessive). What is the frequency of the disease genotype q2?
3% q^2 = 30/1000 = .03 times 100 or 3%
In a population of 200 seagulls, dark brown feathers are dominant to having only white feathers. If 160 of the 200 seagulls have brown feathers, what is the frequency of seagulls that are homozygous dominant?
31% q^2 = 40/200 = .2 (square root it) q = .45 p = 1-q (1-.45) p= .55 p^2 = (.55)^2 = 0.30 or 31%
Sparrows with average-sized wings survive severe storms better than those with longer or shorter wings, illustrating _________.
stabilizing selection
Whenever diploid populations are in Hardy-Weinberg equilibrium at a particular locus,
the allele's frequency should not change from one generation to the next, but its representation in homozygous and heterozygous genotypes may change.
The evolutionary effects of genetic drift are greatest when __________.
the population size is small
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β-hemoglobin alleles in the human gene pool. Again, if this population were in equilibrium, and if the sickle-cell allele is recessive, what proportion of the population should be susceptible (likely harmed) to sickle-cell disease under typical conditions?
0.04 homozygous recessive: q^2: (0.2)^2 = 0.04
In the year 2500, five male space colonists and five female space colonists (all unrelated to each other) settle on an uninhabited Earthlike planet in the Andromeda galaxy. The colonists and their offspring randomly mate for generations. All 10 of the original colonists had free earlobes, and 2 were heterozygous for that trait. The allele for free earlobes is dominant to the allele for attached earlobes. Which of these is closest to the allele frequency in the founding population?
0.1 a, 0.9 A
A fruit fly population has a gene with two alleles, A1A1 and A2A2. Tests show that 85% of the gametes produced in the population contain the A1A1 allele. If the population is in Hardy-Weinberg equilibrium, what proportion of the flies carries both A1A1 and A2A2?
0.255 A1 = p = 85% = 0.85 A2 = q = 15% or 0.15 2(0.85)(0.15) = 0.255
Consider a population of wildflowers in which the frequency of the red allele CR is p = 0.7. What is the frequency of the white allele (CW ) in this population?
0.3 p + q = 1, (p)0.7-1 = (q)0.3
What is the frequency of the A1 allele in a population composed of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals?
0.3 Calculation: The frequency of the A1 allele is p = (number of A1 alleles) / (total of all alleles) = [(2 ( 20) + 80] / [(2 × 20) + (2 × 80) + (2 × 100)] = 0.3.
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β-hemoglobin alleles in the human gene pool. Although selection is clearly present, if the ideal equilibrium of alleles existed, what should be the proportion of heterozygous individuals in populations who live here?
0.32 q = 0.2 p = 1-0.2 = 0.8 2(0.8)(0.2) = 0.32
A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa), the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants. What is the estimated frequency of allele A in the gene pool?
0.50 q = a (q^2 = 0.25 (square rooted) = 0.5) p = 1 - q (1-0.5 = 0.5) p = A = 0.5
A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa), the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants. What proportion of the population is probably heterozygous (Aa) for this trait?
0.50 q^2 = 0.25 (square rooted) q = 0.5 p = 1-0.5 = 0.5 2(0.5)(0.5) = 0.50
In peas, a gene controls flower color such that R = purple and r = white. In an isolated pea patch, there are 36 purple-flowering plants and 64 white-flowering plants. Assuming Hardy-Weinberg equilibrium, what is the value of q for this population?
0.80 q^2 = 64/36=64 = 0.64 (square root it) q = 0.8
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β hemoglobin alleles in the human gene pool. With respect to the sickle-cell allele, what should be true of the β hemoglobin locus in U.S. populations of African Americans whose ancestors were from equatorial Africa? 1. The average heterozygosity at this locus should be decreasing over time. 2. There is an increasing heterozygote advantage at this locus. 3. Diploidy is helping to preserve the sickle-cell allele at this locus. 4. Frequency-dependent selection is helping to preserve the sickle-cell allele at this locus.
1 and 3
One out of 10,000 babies born in North America is affected by cystic fibrosis, a recessive condition. Assuming that the North American human population is in Hardy-Weinberg equilibrium for this trait, what percentage of the population is heterozygous for this trait? (Remember the equation for a population in Hardy-Weinberg equilibrium: p2 + 2pq + q2 = 1.)
2% 1. q^2: 1/10,000 = 0.0001 (square root it) 2. q= .01 3. p= 1-q (1-.01) = .99 4. 2(.99)(.01) = .0198 or 2%
Phenylketonuria (PKU) is a birth defect that causes an amino acid called phenylalanine to build up in the body. The frequency of individuals born in a population with PKU is q2. There is 1 PKU (q 2) birth per 10,000 births. The frequency of the recessive allele for PKU in this population is q = 0.01. The frequency of the dominant allele in this population is p = 1 −q = 0.99. What is the frequency of carriers (2pq) in this population?
2% 2(0.99)(0.01) = .0198 or 2%
In a population of 200 mice, brown fur is dominant to gray fur. If 120 of the 200 mice have brown fur, what is the frequency of mice that are heterozygous?
46% q^2 = 80/200 =.4 (square root it) q= .63 p = 1-q (1-.63) = .37 2(.37)(.63) = .46 or 46%
If, on average, 46% of the loci in a species' gene pool are heterozygous, then the average homozygosity of the species should be
54% Explanation: 100%-45% = 54%
Which statement about variation is true?
All new alleles are the result of nucleotide variability
Which process in sexual reproduction results in diversity at the nucleotide level?
Chromosome crossover
Examining the figure, which type of selection eliminates extreme frequency of individuals' phenotype?
Examining the figure, which type of selection eliminates extreme frequency of individuals' phenotype?
Which of the following are basic components of the Hardy-Weinberg model?
Frequencies of two alleles in a gene pool before and after many random mating's
Swine are vulnerable to infection by bird flu virus and human flu virus, which can both be present in an individual pig at the same time. When this occurs, it is possible for genes from bird flu virus and human flu virus to be combined. If the human flu virus contributes a gene for Tamiflu resistance (Tamiflu is an antiviral drug) to the new virus, and if the new virus is introduced to an environment lacking Tamiflu, then what is most likely to occur?
If the Tamiflu-resistance gene involves a cost, it will experience directional selection leading to reduction in its frequency
Why is the Hardy-Weinberg model useful?
It allows you to calculate genotype frequencies of a population and compare them to what was expected under the assumptions
Which of the following statements best summarizes evolution as it is viewed today?
It is the differential survival and reproduction of the most-fit phenotypes
Which of the following is true of microevolution?
It is the kind of evolution that can change the relative abundance of a particular allele in a gene pool over the course of generations Explanation: Microevolution is a generation-to-generation change in allele frequency
Which of the following is correct about genetic variation?
It must be present in a population before natural selection can act upon the population Explanation: variation is a prerequire for evolution by natural selection
It is estimated by a wide body of scientists that in recent decades we have begun another mass extinction event on Earth. Amphibians in particular have been in decline, sometimes due to climate change, and they therefore provide a possible early sign of dramatic biodiversity change to come. With respect to climate change and natural selection acting upon a population of one species of frog in a tropical rain forest, mutations of DNA sequences of the frog genome that are inherited by offspring
May have no effect at all, be beneficial, or be harmful
Which of the following evolutionary forces could create new genetic information in a population?
Mutation
Which of the following consistently improves the degree to which organisms are well suited for life in their environment?
Natural selection
When using the Hardy-Weinberg equilibrium model, we assume that
Natural selection is not occurring
HIV's genome of RNA includes the code for reverse transcriptase (RT), an enzyme that acts early in infection to synthesize a DNA genome off an RNA template. The HIV genome also codes for protease (PR), an enzyme that acts later in infection by cutting long viral polyproteins into smaller, functional proteins. Both RT and PR represent potential targets for antiretroviral drugs. Drugs called nucleoside analogs (NA) act against RT, whereas drugs called protease inhibitors (PI) act against PR. Every HIV particle contains two RNA molecules. If two genes from one RNA molecule become detached and then, as a unit, get attached to one end of the other RNA molecule within a single HIV particle, which of these is true?
One of the RNA molecules has experienced gene duplication as the result of translocation.
Recent research investigating variation in human height have found that a variety of genes at different loci, as well as early nutrition, especially that of the mother during gestation (the period of time during development of a fetus) and childhood years, influence height. Given this information, which of the following is correct?
Phenotypes are usually the result of a combination of genes as influenced by the environment
Why don't similar genotypes always produce the same phenotype?
Phenotypes can vary depending upon environment Explanation: Phenotype is a product of BOTH inherited genotype and environmental influences
You are studying three populations of birds. Population A has 10 birds, of which 1 is brown (a recessive trait) and 9 are red. Population B has 100 birds, of which 10 are brown. Population C has 30 birds, and 3 of them are brown. In which population would it be least likely that an accident would significantly alter the frequency of the brown allele?
Population B
According to the Hardy-Weinberg theorem, the frequencies of alleles in a population will remain constant if __________ is the only process acting on the gene pool.
Random mating Explanation: Random sexual reproduction does not change the frequency of alleles in the gene pool
Which of the following evolutionary forces consistently results in adaptive changes in allele frequencies?
Selection
The fruit fly Drosophila melanogaster has an allele that confers resistance to DDT and similar insecticides. Laboratory strains of D. melanogaster have been established from flies collected in the wild in the 1930s (before the widespread use of insecticides) and the 1960s (after 20 years of DDT use). Lab strains established in the 1930s have no alleles for DDT resistance. In lab strains established in the 1960s, the frequency of the DDT-resistance allele is 37%. Which statement is correct?
The evolutionary fitness associated with the heritable trait of DDT resistance changed once DDT use became widespread
What genotype frequencies are expected under Hardy-Weinberg equilibrium for a population with allele frequencies of p = 0.8 and q = 0.2 for a particular gene?
The expected genotype frequencies are 0.64, 0.32, and 0.04 for A1A1, A1A2 , and A2A2 , respectively Calculation: A1A1 genotype is p 2 = (0.8)(0.8) = 0.64; A1A2 genotype is 2pq = 2(0.8)(0.2) = 0.32; A2A2 genotype is q 2 = (0.2)(0.2) = 0.04. To verify your calculations, confirm that the three frequencies add up to one.
Imagine you predict a particular frequency for a genotype, based upon detailed knowledge of that genotype frequency in population "X" of a species. Then, using the Hardy-Weinberg model, you find that the observed genotype frequency in a different population, named "Y" of the same species, is not significantly different from what you predicted/expected based on population "X." Which of the following should you conclude?
The particular gene of interest is not undergoing any mechanisms of evolution.
A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa), the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants. What is the most reasonable conclusion that can be drawn from the fact that the frequency of the recessive trait (aa) has not changed over time?
The two phenotypes are about equally adaptive under laboratory conditions.
Which statement correctly describes the role of chance in evolution?
The ultimate source of new alleles is mutation, random changes in the nucleotide sequences of an organism's DNA Explanation: Genetic variation does arise by chance mutations
When evolution occurred on the Galápagos Islands, the finch beaks
Varied in size across the population and sometimes conferred benefits in feeding, survivorship, and reproduction that were passed down to surviving offspring
Which one of the genetic drift effects would limit natural selection?
alter frequencies in small population
What is the Hardy-Weinberg equation?
an assessment of evolution at a particular locus
In a Hardy-Weinberg population with two alleles, A and a, that are in equilibrium, the frequency of the allele a is 0.3. What is the frequency of individuals that are homozygous for this allele?
asking for (a) allele: 0.09 a = q = 0.3 (0.3)^2 = 0.09 A = p = 0.7
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β-hemoglobin alleles in the human gene pool. The sickle-cell allele is pleiotropic (i.e., it affects more than one phenotypic trait). Specifically, this allele affects oxygen delivery to tissues and affects an individual's susceptibility to malaria. Under conditions of low atmospheric oxygen availability, individuals heterozygous for this allele can experience life-threatening sickle-cell "crises." Such individuals remain less susceptible to malaria. Thus, pleiotropic genes/alleles such as this can help explain why
adaptations are often compromises
In a very large population, a quantitative trait has the distribution pattern shown in the figure. If the curve in the figure shifts to the left or to the right, there is no gene flow, and the population size consequently increases over successive generations. Which of the following is (are) probably occurring?
directional selection and adaptation
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the ββ-hemoglobin alleles in the human gene pool. In the United States, the parasite that causes malaria is not present, but African Americans whose ancestors were from equatorial Africa are present. What should be happening to the sickle-cell allele in the United States, and what should be happening to it in equatorial Africa?
directional selection; disruptive selection
What situation most likely explains the occasional high frequency of certain inherited disorders among human populations established by a small population?
founder effect
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β-hemoglobin alleles in the human gene pool. Considering the overall human population of the U.S. mainland at the time when the slave trade brought large numbers of people from equatorial Africa, what was primarily acting to change the frequency of the sickle-cell allele in the overall U.S. population?
gene flow
over time, the movement of people on Earth has steadily increased. This has altered the course of human evolution by increasing
gene flow
If the nucleotide variability of the locus equals 0% with the point mutation in a locus , what are the gene variability and number of alleles at that locus?
gene variability > (greater than) 0%; number of alleles = 2
Allele frequencies in a gene pool may shift randomly and by chance. What is this random shift called?
genetic drift
In evolution, which driving factor most consistently requires a small population as a precondition for its occurrence?
genetic drift
In the year 2500, five male space colonists and five female space colonists (all unrelated to each other) settle on an uninhabited Earthlike planet in the Andromeda galaxy. The colonists and their offspring randomly mate for generations. All 10 of the original colonists had free earlobes, and 2 were heterozygous for that trait. The allele for free earlobes is dominant to the allele for attached earlobes. If four of the original colonists died before they produced offspring, the ratios of genotypes could be quite different in the subsequent generations. This would be an example of
genetic drift
There are 25 individuals in population 1, all with genotype AA, and there are 40 individuals in population 2, all with genotype aa. Assume that these populations are located far from each other and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation most likely resulted from __________.
genetic drift
If the original finches that flew over to the Galápagos Islands from South America had already been genetically different from the parental population of South American finches, even before adapting to the Galápagos, this would have been an example of
genetic drift and the founder effect
The Hardy-Weinberg equation p2 + 2pq + q2 = 1 allows you to calculate
genotype frequencies
In the formula for determining a population's genotype frequencies, the 2 in the term 2pq is necessary because
heterozygotes can come about in two ways based upon parental gametes.
In the formula for determining a population's genotype frequencies, the pq in the term 2pq is necessary because
heterozygotes have two alleles
Natural selection changes allele frequencies because some __________ survive and reproduce better than others.
individuals
The original source of all genetic variation is __________
mutation Explanation: Mutation is, in itself, very important to evolution because it is the original source of the genetic variation that serves as the raw material for evolution
You are studying three populations of birds. Population A has 10 birds, of which 1 is brown (a recessive trait) and 9 are red. Population B has 100 birds, of which 10 are brown. Population C has 30 birds, and 3 of them are brown. Which population is most likely to be subject to the bottleneck effect?
population A
In the year 2500, five male space colonists and five female space colonists (all unrelated to each other) settle on an uninhabited Earthlike planet in the Andromeda galaxy. The colonists and their offspring randomly mate for generations. All 10 of the original colonists had free earlobes, and 2 were heterozygous for that trait. The allele for free earlobes is dominant to the allele for attached earlobes. If you assume that Hardy-Weinberg equilibrium applies to the population of colonists on this planet, about how many people will have attached earlobes when the planet's population reaches 10,000?
q = .1 q^2 = .1X.1X10000 = 100
A mutation that is neither harmful nor beneficial is termed a
silent mutation
HIV's genome of RNA includes the code for reverse transcriptase (RT), an enzyme that acts early in infection to synthesize a DNA genome off an RNA template. The HIV genome also codes for protease (PR), an enzyme that acts later in infection by cutting long viral polyproteins into smaller, functional proteins. Both RT and PR represent potential targets for antiretroviral drugs. Drugs called nucleoside analogs (NA) act against RT, whereas drugs called protease inhibitors (PI) act against PR. Which of the following represents the treatment option most likely to avoid the production of drug-resistant HIV (assuming no drug interactions or side effects)?
using moderate doses of NA and two different PIs at the same time for several months
What is the founder effect?
when a few individuals become isolated from a larger population and their gene pool differs from that source population
What is a population in biology?
when a group of individuals of the same species live in the same area and interbreed
