Genetics Quiz
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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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?
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.
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.
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?
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.
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?
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.
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?
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.
Which of the following statements most accurately distinguishes the typical population-level consequence of polygenic inheritance from that of pleiotropy?
Explanation: When you encounter questions comparing polygenic inheritance and pleiotropy, focus on understanding what each concept explains at the population level. Both involve multiple genes affecting traits, but they describe fundamentally different genetic phenomena. Polygenic inheritance occurs when multiple genes contribute to a single trait, creating the continuous variation you see in characteristics like height, skin color, or blood pressure. Each gene adds a small effect, and when you plot these traits in a population, you get a bell-shaped distribution rather than discrete categories. Pleiotropy, conversely, describes when one gene affects multiple different traits simultaneously. For example, a single gene mutation might affect both bone density and hearing ability. Answer A correctly captures this distinction: polygenic inheritance creates continuous variation for single traits, while pleiotropy involves single genes with multiple effects that may or may not show continuous distributions. Answer B reverses the concepts entirely—pleiotropy doesn't typically create continuous distributions for single traits, and polygenic inheritance specifically produces continuous rather than discrete variation. Answer C incorrectly suggests pleiotropy only occurs in lab settings, when it's actually common in natural populations (think of genetic syndromes affecting multiple body systems). Answer D confuses these concepts with gene-environment interactions, and incorrectly implies pleiotropy is environmentally independent while polygenic traits involve environmental effects. Remember this key distinction: polygenic = multiple genes → one trait with continuous variation; pleiotropy = one gene → multiple traits. This fundamental difference drives their different population-level consequences.
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?
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 (2^6 = 64) possible genotypes in total. Therefore, the proportion is 20/64.
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?
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), 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). Since (4^1=4), (4^2=16), (4^3=64), and (4^4=256), the value of n must be 4. Therefore, 4 gene pairs are involved.
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?
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.
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)?
Explanation: The correct answer is D. This multi-step problem involves lethality, pleiotropy, and epistasis. The cross is CcWw x CcWw.
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?
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.
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?
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.
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?
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.
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?
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), 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). Since (4^1=4), (4^2=16), (4^3=64), and (4^4=256), the value of n must be 4. Therefore, 4 gene pairs are involved.
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?
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.
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:
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 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?
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 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?
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 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?
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.
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?
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 (2^6 = 64) possible genotypes in total. Therefore, the proportion is 20/64.
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:
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.
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?
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.