Genetics Quiz: Pleiotropy And Polygenic Inheritance
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Pleiotropy And Polygenic InheritanceQuestion 1 of 20

A team of plant breeders implements a selective breeding program for a species of tomato. They successfully increase the average fruit size, a polygenic trait, over several generations. However, they observe that the lines with the largest fruits consistently show increased susceptibility to a common fungal pathogen. What is the most likely genetic explanation for this unintended negative correlation?

The selection for large fruit size increased the frequency of alleles that have a pleiotropic, negative effect on fungal resistance.
The genes controlling fruit size and fungal resistance are located on different chromosomes.
The intensive selection process caused new mutations to arise in genes responsible for pathogen defense.
The alleles for small fruit size are epistatic to the alleles for fungal resistance.
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Genetics Quiz

Genetics Quiz: Pleiotropy And Polygenic Inheritance

Practice Pleiotropy And Polygenic Inheritance in Genetics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on Pleiotropy And Polygenic Inheritance, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A team of plant breeders implements a selective breeding program for a species of tomato. They successfully increase the average fruit size, a polygenic trait, over several generations. However, they observe that the lines with the largest fruits consistently show increased susceptibility to a common fungal pathogen. What is the most likely genetic explanation for this unintended negative correlation?

  1. The selection for large fruit size increased the frequency of alleles that have a pleiotropic, negative effect on fungal resistance. (correct answer)
  2. The genes controlling fruit size and fungal resistance are located on different chromosomes.
  3. The intensive selection process caused new mutations to arise in genes responsible for pathogen defense.
  4. The alleles for small fruit size are epistatic to the alleles for fungal resistance.
Explanation: When you encounter questions about selective breeding that produces unexpected negative correlations between traits, think about the genetic mechanisms that could link seemingly unrelated characteristics. The observation that selecting for larger fruit size consistently increases fungal susceptibility suggests these traits are genetically linked. The most likely explanation is pleiotropy - when single genes or alleles affect multiple, seemingly unrelated traits. In this case, the alleles that increase fruit size also have the unintended effect of reducing pathogen resistance. As breeders selected for plants with larger fruits, they unknowingly increased the frequency of these pleiotropic alleles in their breeding population, inadvertently selecting against fungal resistance at the same time. Let's examine why the other options don't fit: Option B incorrectly suggests that genes on different chromosomes can't be correlated - but pleiotropy can create such correlations regardless of chromosome location. Option C proposes that selection caused new mutations, but the consistent pattern across multiple lines suggests an existing genetic relationship rather than random new mutations. Option D misapplies epistasis (where one gene masks another's expression) - this scenario doesn't involve gene masking but rather single alleles affecting multiple traits. The correct answer is A because it identifies pleiotropy as the mechanism creating this trade-off. Study tip: Remember that pleiotropy is a common explanation when selective breeding for one trait consistently affects another trait. This concept frequently appears on genetics exams when discussing unintended consequences of artificial selection.

Question 2

A pleiotropic allele in a fish population increases the number of vertebrae, which enhances swimming speed, but also leads to a slight reduction in immune system function. Under conditions of high predation but low disease prevalence, this allele increases in frequency. Which evolutionary concept best explains the persistence and spread of this allele despite its negative effect?

  1. Genetic drift
  2. Purifying selection
  3. Fitness trade-off (correct answer)
  4. Frequency-dependent selection
Explanation: The correct answer is C. This scenario is a direct example of a fitness trade-off, which often arises from antagonistic pleiotropy. The allele has a positive effect on one component of fitness (survival from predation) and a negative effect on another (immune function). The net selective effect of the allele depends on the environment. In an environment where the benefit (predator evasion) outweighs the cost (disease susceptibility), the allele will be favored by natural selection and increase in frequency.
  • A is incorrect because the allele frequency is changing due to selection based on its phenotypic effects, not due to random chance (genetic drift).
  • B is incorrect because purifying (or negative) selection is the process that removes deleterious alleles. If it were the only force acting, the allele would be selected against due to its negative impact on immunity.
  • D is incorrect because frequency-dependent selection occurs when an allele's fitness effect depends on its frequency in the population. The scenario does not provide any information to suggest this is the case.

Question 3

Height in humans is a classic example of a polygenic trait, with contributions from hundreds of genes and significant environmental influence. Given this understanding, which of the following observations from a large-scale study of human height would be the most surprising or unexpected?

  1. The heights of individuals in the study population form a bell-shaped, continuous distribution.
  2. Adult offspring's height is positively correlated with the average height of their parents.
  3. The population can be sorted into exactly five distinct, non-overlapping height categories. (correct answer)
  4. Genome-wide association studies (GWAS) identify over 700 different genetic loci associated with height.
Explanation: The correct answer is C. A primary characteristic of a polygenic trait is continuous variation, where phenotypes fall along a spectrum rather than in discrete categories. Finding a small number of distinct, non-overlapping height classes would be highly unexpected and would suggest a much simpler mode of inheritance (e.g., Mendelian inheritance with one or two genes), contradicting the known polygenic nature of height.
  • A is incorrect. A normal (bell-shaped) distribution is the expected pattern for a quantitative, polygenic trait in a large population.
  • B is incorrect. Because height is heritable, a correlation between parental and offspring height is expected.
  • D is incorrect. The finding that hundreds or even thousands of genetic loci are associated with height is a key piece of modern evidence confirming its highly polygenic nature. This would be expected, not surprising.

Question 4

In a species of wheat, grain color is a polygenic trait determined by 3 unlinked gene pairs (A/a, B/b, C/c), where the capital alleles are additive and contribute equally to red pigment, and lowercase alleles contribute no pigment (white). A cross is made between a true-breeding red-grained plant (AABBCC) and a true-breeding white-grained plant (aabbcc). If the F1 generation (AaBbCc) is self-crossed, what proportion of the F2 offspring is expected to have a phenotype identical to the F1 parents?

  1. 1/64
  2. 6/64
  3. 15/64
  4. 20/64 (correct answer)
Explanation: The correct answer is D. The F1 parents have the genotype AaBbCc, which means they have 3 additive (capital) alleles. The question asks for the proportion of F2 offspring that also have exactly 3 additive alleles. This is a binomial probability problem. For each of the 6 alleles in the genotype, the probability of it being a capital allele is 1/2. We can use the binomial expansion or combinations to find the answer. The number of ways to get exactly 3 capital alleles out of 6 total alleles is given by the combination formula C(n, k) = n! / (k!(n-k)!), where n=6 and k=3. C(6, 3) = (6 * 5 * 4) / (3 * 2 * 1) = 20. There are 26=642^6 = 64 possible genotypes in total. Therefore, the proportion is 20/64.
  • A is incorrect because 1/64 is the proportion of offspring with an extreme phenotype (AABBCC or aabbcc), having 6 or 0 additive alleles.
  • B is incorrect because 6/64 is the proportion of offspring with 1 or 5 additive alleles (C(6,1) = 6; C(6,5) = 6).
  • C is incorrect because 15/64 is the proportion of offspring with 2 or 4 additive alleles (C(6,2) = 15; C(6,4) = 15).

Question 5

In a type of gourd, fruit weight is a polygenic trait. A cross between a true-breeding 2-lb gourd and a true-breeding 10-lb gourd produces F1 offspring that are all 6 lbs. When the F1 are self-crossed, the F2 generation shows a wide range of weights, with the original 2-lb and 10-lb phenotypes each appearing at a frequency of approximately 1/256. Assuming all controlling genes are unlinked and contribute equally and additively, how many gene pairs are involved in determining fruit weight?

  1. 2
  2. 4 (correct answer)
  3. 6
  4. 8
Explanation: The correct answer is B. For a polygenic trait, the proportion of F2 offspring that resemble one of the extreme parental phenotypes is given by the formula (1/4)n(1/4)^n, where n is the number of gene pairs controlling the trait. The problem states that this frequency is 1/256. We need to solve the equation (1/4)n=1/256(1/4)^n = 1/256. Since 41=44^1=4, 42=164^2=16, 43=644^3=64, and 44=2564^4=256, the value of n must be 4. Therefore, 4 gene pairs are involved.
  • A is incorrect. If 2 gene pairs were involved, the frequency of an extreme phenotype would be (1/4)2=1/16(1/4)^2 = 1/16.
  • C is incorrect. If 6 gene pairs were involved, the frequency would be (1/4)6=1/4096(1/4)^6 = 1/4096.
  • D is incorrect. A common mistake is to use the formula (1/2)n(1/2)^n or to solve 2n=2562^n = 256, which gives n=8. This is incorrect because the probability of inheriting the specific two alleles for a homozygous genotype (e.g., AA or aa) from heterozygous parents is 1/4, not 1/2.

Question 6

In a study of bone health in mice, researchers conducted a genome-wide scan and identified a significant Quantitative Trait Locus (QTL) on chromosome 5 associated with variation in femur length. In a separate analysis of the same mice, they found that a QTL for serum phosphate level also mapped to the exact same location on chromosome 5. Subsequent fine-mapping and sequencing revealed that a single gene, Enpp1, within this locus has variants that affect both traits. This discovery provides strong evidence for which genetic principle?

  1. Epistasis
  2. Linkage
  3. Polygenic inheritance
  4. Pleiotropy (correct answer)
Explanation: The correct answer is D. The research process described moves from a broad association (a QTL) to a specific cause. The final finding—that variants in a single gene (Enpp1) are responsible for variation in two distinct traits (femur length and serum phosphate level)—is a textbook example of pleiotropy.
  • A is incorrect. Epistasis is an interaction between different genes. The study narrowed the cause down to a single gene.
  • B is incorrect. The initial co-localization of QTLs could suggest linkage (two different genes located close together). However, the subsequent finding that a single gene is responsible refutes the linkage hypothesis in favor of pleiotropy.
  • C is incorrect. Polygenic inheritance refers to a single trait (like femur length) being influenced by many genes. The question is about the observation that one gene influences multiple traits.

Question 7

In mice, the TYR gene is required for melanin production; the recessive allele c causes albinism. A different gene, KIT, has a dominant allele W that is pleiotropic, causing white fur and deafness. (The W allele is epistatic to other color genes, and the WW genotype is lethal). A cross is made between two mice of genotype CcWw. What proportion of the viable offspring are expected to be deaf and have the potential to produce pigment (i.e., not albino)?

  1. 1/6
  2. 3/8
  3. 1/3
  4. 1/2 (correct answer)
Explanation: The correct answer is D. This multi-step problem involves lethality, pleiotropy, and epistasis. The cross is CcWw x CcWw.
  1. Analyze lethality: The Ww x Ww cross produces 1/4 WW (lethal), 1/2 Ww, and 1/4 ww. Because 1/4 of all possible zygotes are lethal, the total proportion of viable offspring is 3/4. We must normalize our results by this fraction.
  2. Identify the target phenotype: We want offspring that are 'deaf' AND 'not albino'.
    • Deafness is caused by the Ww genotype. The probability of Ww is 1/2.
    • 'Not albino' means having at least one C allele. The Cc x Cc cross gives 3/4 C_ (CC or Cc) and 1/4 cc. The probability of being not albino is 3/4.
  3. Calculate the combined probability: The probability of an offspring being both deaf (Ww) and not albino (C_) is P(Ww) * P(C_) = (1/2) * (3/4) = 3/8.
  4. Correct for lethality: This 3/8 represents the fraction of all zygotes. To find the fraction among viable offspring, we divide by the proportion of viable offspring: (3/8) / (3/4) = (3/8) * (4/3) = 12/24 = 1/2.
  • A is incorrect (1/6). This is 2/12, which could arise from miscounting genotypes (e.g., only counting CCWw).
  • B is incorrect (3/8). This is a common error where the final probability is not corrected for the lethal WW genotype.
  • C is incorrect (1/3). This is 4/12, which could arise from miscounting genotypes (e.g., only counting CcWw).

Question 8

A geneticist studies a particular mutation in mice that results in both a kinked tail and deafness. One hypothesis is pleiotropy, where one gene affects both traits. An alternative hypothesis is that two different, tightly linked genes are involved. Which finding would provide the most definitive evidence for pleiotropy?

  1. The two traits are always inherited together through multiple generations of crosses.
  2. No recombinant individuals (e.g., kinky tail with normal hearing) are found after analyzing 1,000 F2 offspring.
  3. A single base-pair substitution in the coding sequence of one gene is identified and is shown to be sufficient to cause both phenotypes. (correct answer)
  4. The F1 generation from a cross between a mutant mouse and a wild-type mouse expresses both the kinky tail and deafness.
Explanation: The correct answer is C. Identifying a single molecular change (like a point mutation) within a single gene that is causally linked to both distinct phenotypes provides the most direct and definitive evidence for pleiotropy. It demonstrates that a single genetic product or its regulation is responsible for the multiple effects, making the two-gene hypothesis extremely unlikely.
  • A is incorrect because perfect co-inheritance is expected for pleiotropy, but it is also expected for two genes that are very tightly linked.
  • B is incorrect because while failing to find recombinants in a large sample provides strong statistical support for pleiotropy, it cannot definitively rule out the possibility of extremely tight linkage (e.g., a genetic distance less than 0.1 cM). It is strong evidence, but not as definitive as a direct molecular cause.
  • D is incorrect because this observation only indicates that the mutation(s) are dominant. It does not help distinguish between a single pleiotropic gene and two linked genes with dominant mutations.

Question 9

In a hypothetical plant, stem length is a polygenic trait controlled by two unlinked loci (A/a and B/b) with additive alleles (AABB=tallest, aabb=shortest). A separate, unlinked gene (C/c) controls flower color, where C (purple) is dominant to c (white). The C allele is pleiotropic and essential for embryo development; the cc genotype is lethal. An F1 plant with genotype AaBbCc is self-pollinated. What fraction of the viable F2 progeny will have an intermediate stem length and purple flowers?

  1. 1/4
  2. 3/8 (correct answer)
  3. 1/2
  4. 9/16
Explanation: The correct answer is B. This is a two-step problem. First, analyze the flower color/viability gene. A Cc x Cc cross yields 1/4 CC, 1/2 Cc, and 1/4 cc. Since cc is lethal, the viable offspring are only the CC and Cc individuals, which make up 3/4 of the total zygotes. Among the viable offspring, all have at least one C allele, so 100% of them have purple flowers. Second, analyze the polygenic stem length trait. The cross is AaBb x AaBb. An intermediate stem length corresponds to having 2 additive alleles. The genotypes for this are AAbb, AaBb, and aaBB. The probabilities are: P(AAbb) = (1/4)(1/4) = 1/16; P(AaBb) = (1/2)(1/2) = 4/16; P(aaBB) = (1/4)*(1/4) = 1/16. The total probability of an intermediate stem length is 1/16 + 4/16 + 1/16 = 6/16 = 3/8. Since all viable offspring are purple, the fraction with intermediate stem length and purple flowers is simply the fraction with intermediate stem length, which is 3/8.
  • A is incorrect. 1/4 is the probability of the AaBb genotype alone, neglecting the other intermediate genotypes.
  • C is incorrect. This might result from miscalculation of the probabilities.
  • D is incorrect. 9/16 is the proportion of offspring showing dominant phenotypes for two traits in a standard dihybrid cross without lethal alleles.

Question 10

Sickle-cell anemia is a classic example of pleiotropy, where a single mutation in the β-globin gene leads to a wide range of health problems, including anemia, pain crises, organ damage, and increased resistance to malaria. Which statement best explains the molecular and cellular basis for these diverse pleiotropic effects?

  1. The β-globin gene is located near many other genes, and the mutation disrupts the regulation of these adjacent genes.
  2. The mutant β-globin protein is a transcription factor that regulates the expression of many different genes throughout the body.
  3. The primary defect in the hemoglobin protein structure causes red blood cells to deform, which then leads to a cascade of secondary circulatory and physiological problems. (correct answer)
  4. The single nucleotide substitution creates multiple different protein products through alternative splicing, each affecting a different organ system.
Explanation: The correct answer is C. This accurately describes the causal chain of pleiotropy in sickle-cell anemia. The single mutation causes a change in the hemoglobin protein (molecular level), which leads to the sickling of red blood cells under low-oxygen conditions (cellular level). Because red blood cells circulate throughout the body, their abnormal shape and function lead to a wide range of systemic problems like blockages, anemia, and organ damage (organismal level). This cascade of effects originating from a single primary defect is the essence of pleiotropy.
  • A is incorrect. This describes a position effect, where gene expression is altered by its chromosomal location. This is not the mechanism for sickle-cell anemia.
  • B is incorrect. β-globin is a structural component of hemoglobin and is not a transcription factor. This suggests a plausible but factually incorrect molecular mechanism.
  • D is incorrect. The sickle-cell mutation is a missense mutation that causes a single amino acid substitution; it does not introduce new splice sites or lead to the production of multiple protein products.

Question 11

In a type of gourd, fruit weight is a polygenic trait. A cross between a true-breeding 2-lb gourd and a true-breeding 10-lb gourd produces F1 offspring that are all 6 lbs. When the F1 are self-crossed, the F2 generation shows a wide range of weights, with the original 2-lb and 10-lb phenotypes each appearing at a frequency of approximately 1/256. Assuming all controlling genes are unlinked and contribute equally and additively, how many gene pairs are involved in determining fruit weight?

  1. 2
  2. 4 (correct answer)
  3. 6
  4. 8
Explanation: The correct answer is B. For a polygenic trait, the proportion of F2 offspring that resemble one of the extreme parental phenotypes is given by the formula (1/4)n(1/4)^n, where n is the number of gene pairs controlling the trait. The problem states that this frequency is 1/256. We need to solve the equation (1/4)n=1/256(1/4)^n = 1/256. Since 41=44^1=4, 42=164^2=16, 43=644^3=64, and 44=2564^4=256, the value of n must be 4. Therefore, 4 gene pairs are involved.
  • A is incorrect. If 2 gene pairs were involved, the frequency of an extreme phenotype would be (1/4)2=1/16(1/4)^2 = 1/16.
  • C is incorrect. If 6 gene pairs were involved, the frequency would be (1/4)6=1/4096(1/4)^6 = 1/4096.
  • D is incorrect. A common mistake is to use the formula (1/2)n(1/2)^n or to solve 2n=2562^n = 256, which gives n=8. This is incorrect because the probability of inheriting the specific two alleles for a homozygous genotype (e.g., AA or aa) from heterozygous parents is 1/4, not 1/2.

Question 12

Marfan syndrome is an autosomal dominant disorder characterized by a suite of traits including disproportionately long limbs, arachnodactyly (long, thin fingers), and cardiovascular defects such as aortic aneurysm. All affected individuals in pedigrees trace back to a mutation in a single gene, FBN1. Which of the following concepts is best exemplified by the diverse and variable symptoms of Marfan syndrome?

  1. Polygenic inheritance with a major QTL
  2. Pleiotropy with variable expressivity (correct answer)
  3. Incomplete penetrance of a dominant allele
  4. Genetic heterogeneity
Explanation: The correct answer is B. The fact that a single gene mutation (FBN1) causes a wide range of different symptoms (skeletal, cardiovascular) is a clear example of pleiotropy. The fact that the severity and specific combination of symptoms can differ among affected individuals illustrates variable expressivity. Together, these two concepts fully describe the clinical presentation.
  • A is incorrect because polygenic inheritance involves many genes contributing to one trait, whereas Marfan syndrome is a single-gene disorder.
  • C is incorrect because incomplete penetrance means that some individuals with the disease-causing genotype show no symptoms at all. The question focuses on the variety of symptoms in those who are affected.
  • D is incorrect because genetic heterogeneity means that mutations in different genes can cause the same disorder. The stem specifies that Marfan syndrome is caused by mutations in a single gene, FBN1.

Question 13

In certain mosquito populations, a single allele at the gste2 locus confers resistance to the insecticide DDT. However, mosquitoes homozygous for this resistance allele also exhibit slower development and reduced fecundity compared to susceptible mosquitoes in a DDT-free environment. This phenomenon, where a single gene has both a beneficial and a detrimental effect on fitness, is best described as:

  1. Epistasis
  2. Genetic drift
  3. Antagonistic pleiotropy (correct answer)
  4. Heterozygote advantage
Explanation: The correct answer is C. The scenario describes an allele with two opposing effects on fitness depending on the context: it is beneficial in the presence of DDT but detrimental (in terms of development and fecundity) in its absence. This is a classic example of antagonistic pleiotropy, where a single gene influences multiple traits in ways that have opposite effects on fitness.
  • A is incorrect because epistasis is an interaction between different genes, where one gene masks the effect of another. This scenario involves multiple effects of a single gene.
  • B is incorrect because genetic drift refers to random fluctuations in allele frequencies, whereas the scenario implies a selective pressure.
  • D is incorrect because heterozygote advantage (overdominance) describes a situation where the heterozygote has higher fitness than both homozygotes. The stem describes the effects of a specific allele in the homozygous state, not a comparison of heterozygote vs. homozygote fitness.

Question 14

A study on corn height, a known polygenic trait, examines genetically identical clones planted in two different experimental fields. Field 1 receives optimal water and nutrients, while Field 2 is subjected to drought conditions. The corn in Field 1 grows to an average height of 8 feet with a narrow distribution, while the corn in Field 2 averages 5 feet with a much wider distribution of heights. What concept do these results best illustrate?

  1. Pleiotropy, as the genes for height also affect drought response.
  2. The norm of reaction for a polygenic trait. (correct answer)
  3. Epistasis, where drought-related genes mask the effect of height genes.
  4. The principle of independent assortment for height-related alleles.
Explanation: The correct answer is B. The norm of reaction describes the pattern of phenotypic expression of a single genotype across a range of environments. Since the corn plants are genetically identical clones, the differences in average height and distribution of heights between the two fields are due to the different environmental conditions. This experiment directly demonstrates the norm of reaction for the genotype controlling height.
  • A is incorrect. The experiment does not provide evidence that the same genes control both height and drought response (pleiotropy); it shows that the expression of height genes is affected by the environment.
  • C is incorrect. Epistasis is a gene-gene interaction. The primary interaction described here is between the plant's genotype and the external environment.
  • D is incorrect. The principle of independent assortment describes how different genes are inherited during meiosis. It is not relevant to explaining how a single genotype is expressed phenotypically in different environments.

Question 15

A geneticist is studying the inheritance of scale coloration in a species of lizard. Observation of many families reveals two key patterns: 1) Offspring phenotypes often appear to be a blend of the parental phenotypes. 2) Crosses between lizards of intermediate coloration produce a wide spectrum of colors in their offspring, including some that are much darker or lighter than either parent. Which genetic model is most consistent with these observations, and why?

  1. Multiple alleles, because more than two allelic versions for a single gene exist in the population, creating diverse combinations.
  2. Pleiotropy, because a single gene is likely affecting multiple pigment-production pathways simultaneously.
  3. Polygenic inheritance, because the blending and appearance of extreme phenotypes in offspring are characteristic of the additive effects of multiple genes. (correct answer)
  4. Incomplete dominance, because the blended appearance of offspring phenotypes points to a heterozygote being intermediate to the two homozygotes.
Explanation: The correct answer is C. Both observations are classic hallmarks of polygenic inheritance. The apparent 'blending' (observation 1) results from offspring inheriting a mix of additive alleles from their parents. The production of a wide spectrum of phenotypes, including offspring that are more extreme than either parent (observation 2, known as transgressive segregation), is explained by the recombination of alleles at multiple loci. Parents with intermediate phenotypes can carry hidden recessive alleles that, when combined in offspring, can produce more extreme phenotypes.
  • A is incorrect. A multiple allele system produces a finite number of discrete phenotypes, not a continuous spectrum.
  • B is incorrect. Pleiotropy describes one gene with multiple effects, not multiple genes affecting a single trait like coloration.
  • D is incorrect. While incomplete dominance at a single locus can produce an intermediate F1 phenotype, a cross of these F1s would only yield three distinct phenotypic classes in the F2, not a wide spectrum or phenotypes more extreme than the original grandparents. It cannot explain transgressive segregation.

Question 16

A newly discovered autosomal dominant condition in humans, "Syndrome X," is caused by a single mutation in the FBN2 gene. Affected individuals exhibit a wide array of seemingly unrelated symptoms, including elongated limbs, cardiovascular abnormalities, and dislocation of the eye lens. Which genetic principle is best illustrated by this single gene influencing multiple phenotypic systems?

  1. Polygenic inheritance
  2. Pleiotropy (correct answer)
  3. Incomplete penetrance
  4. Linkage disequilibrium
Explanation: The correct answer is B. Pleiotropy is the phenomenon where a single gene influences multiple, often seemingly unrelated, phenotypic traits. The scenario describes a single gene mutation (FBN2) causing a variety of symptoms across different body systems (skeletal, cardiovascular, ocular), which is a classic example of pleiotropy.
  • A is incorrect because polygenic inheritance is when a single trait is influenced by multiple genes, which is the reverse of the situation described.
  • C is incorrect because incomplete penetrance refers to cases where an individual has the disease-causing genotype but does not express the phenotype. The stem focuses on the variety of symptoms in affected individuals, not the lack of symptoms.
  • D is incorrect because linkage disequilibrium refers to the non-random association of alleles at two or more different loci, which pertains to the inheritance of multiple genes, not the multiple effects of a single gene.

Question 17

In a species of wheat, grain color is a polygenic trait determined by 3 unlinked gene pairs (A/a, B/b, C/c), where the capital alleles are additive and contribute equally to red pigment, and lowercase alleles contribute no pigment (white). A cross is made between a true-breeding red-grained plant (AABBCC) and a true-breeding white-grained plant (aabbcc). If the F1 generation (AaBbCc) is self-crossed, what proportion of the F2 offspring is expected to have a phenotype identical to the F1 parents?

  1. 1/64
  2. 6/64
  3. 15/64
  4. 20/64 (correct answer)
Explanation: The correct answer is D. The F1 parents have the genotype AaBbCc, which means they have 3 additive (capital) alleles. The question asks for the proportion of F2 offspring that also have exactly 3 additive alleles. This is a binomial probability problem. For each of the 6 alleles in the genotype, the probability of it being a capital allele is 1/2. We can use the binomial expansion or combinations to find the answer. The number of ways to get exactly 3 capital alleles out of 6 total alleles is given by the combination formula C(n, k) = n! / (k!(n-k)!), where n=6 and k=3. C(6, 3) = (6 * 5 * 4) / (3 * 2 * 1) = 20. There are 26=642^6 = 64 possible genotypes in total. Therefore, the proportion is 20/64.
  • A is incorrect because 1/64 is the proportion of offspring with an extreme phenotype (AABBCC or aabbcc), having 6 or 0 additive alleles.
  • B is incorrect because 6/64 is the proportion of offspring with 1 or 5 additive alleles (C(6,1) = 6; C(6,5) = 6).
  • C is incorrect because 15/64 is the proportion of offspring with 2 or 4 additive alleles (C(6,2) = 15; C(6,4) = 15).

Question 18

In certain mosquito populations, a single allele at the gste2 locus confers resistance to the insecticide DDT. However, mosquitoes homozygous for this resistance allele also exhibit slower development and reduced fecundity compared to susceptible mosquitoes in a DDT-free environment. This phenomenon, where a single gene has both a beneficial and a detrimental effect on fitness, is best described as:

  1. Epistasis
  2. Genetic drift
  3. Antagonistic pleiotropy (correct answer)
  4. Heterozygote advantage
Explanation: The correct answer is C. The scenario describes an allele with two opposing effects on fitness depending on the context: it is beneficial in the presence of DDT but detrimental (in terms of development and fecundity) in its absence. This is a classic example of antagonistic pleiotropy, where a single gene influences multiple traits in ways that have opposite effects on fitness.
  • A is incorrect because epistasis is an interaction between different genes, where one gene masks the effect of another. This scenario involves multiple effects of a single gene.
  • B is incorrect because genetic drift refers to random fluctuations in allele frequencies, whereas the scenario implies a selective pressure.
  • D is incorrect because heterozygote advantage (overdominance) describes a situation where the heterozygote has higher fitness than both homozygotes. The stem describes the effects of a specific allele in the homozygous state, not a comparison of heterozygote vs. homozygote fitness.

Question 19

Sickle-cell anemia is a classic example of pleiotropy, where a single mutation in the β-globin gene leads to a wide range of health problems, including anemia, pain crises, organ damage, and increased resistance to malaria. Which statement best explains the molecular and cellular basis for these diverse pleiotropic effects?

  1. The β-globin gene is located near many other genes, and the mutation disrupts the regulation of these adjacent genes.
  2. The mutant β-globin protein is a transcription factor that regulates the expression of many different genes throughout the body.
  3. The primary defect in the hemoglobin protein structure causes red blood cells to deform, which then leads to a cascade of secondary circulatory and physiological problems. (correct answer)
  4. The single nucleotide substitution creates multiple different protein products through alternative splicing, each affecting a different organ system.
Explanation: The correct answer is C. This accurately describes the causal chain of pleiotropy in sickle-cell anemia. The single mutation causes a change in the hemoglobin protein (molecular level), which leads to the sickling of red blood cells under low-oxygen conditions (cellular level). Because red blood cells circulate throughout the body, their abnormal shape and function lead to a wide range of systemic problems like blockages, anemia, and organ damage (organismal level). This cascade of effects originating from a single primary defect is the essence of pleiotropy.
  • A is incorrect. This describes a position effect, where gene expression is altered by its chromosomal location. This is not the mechanism for sickle-cell anemia.
  • B is incorrect. β-globin is a structural component of hemoglobin and is not a transcription factor. This suggests a plausible but factually incorrect molecular mechanism.
  • D is incorrect. The sickle-cell mutation is a missense mutation that causes a single amino acid substitution; it does not introduce new splice sites or lead to the production of multiple protein products.

Question 20

A study on corn height, a known polygenic trait, examines genetically identical clones planted in two different experimental fields. Field 1 receives optimal water and nutrients, while Field 2 is subjected to drought conditions. The corn in Field 1 grows to an average height of 8 feet with a narrow distribution, while the corn in Field 2 averages 5 feet with a much wider distribution of heights. What concept do these results best illustrate?

  1. Pleiotropy, as the genes for height also affect drought response.
  2. The norm of reaction for a polygenic trait. (correct answer)
  3. Epistasis, where drought-related genes mask the effect of height genes.
  4. The principle of independent assortment for height-related alleles.
Explanation: The correct answer is B. The norm of reaction describes the pattern of phenotypic expression of a single genotype across a range of environments. Since the corn plants are genetically identical clones, the differences in average height and distribution of heights between the two fields are due to the different environmental conditions. This experiment directly demonstrates the norm of reaction for the genotype controlling height.
  • A is incorrect. The experiment does not provide evidence that the same genes control both height and drought response (pleiotropy); it shows that the expression of height genes is affected by the environment.
  • C is incorrect. Epistasis is a gene-gene interaction. The primary interaction described here is between the plant's genotype and the external environment.
  • D is incorrect. The principle of independent assortment describes how different genes are inherited during meiosis. It is not relevant to explaining how a single genotype is expressed phenotypically in different environments.