MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1c Population Genetics Hardy Weinberg

What this quiz covers

This quiz focuses on 1c Population Genetics Hardy Weinberg, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

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.

Question 1

1. Genetic drift in a large population causing consistent excess of homozygotes each generation
2. Directional selection favoring heterozygotes, which increases homozygosity
3. High migration introducing new alleles, which increases homozygosity
4. Inbreeding (nonrandom mating) increasing homozygosity without necessarily changing allele frequencies immediately (correct answer)

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to inbreeding. The Hardy-Weinberg principle states that random mating, no selection, no mutation, no migration, and large size preserve genotype ratios. In this wolf program, pairing relatives increases homozygotes without major allele changes. Choice D correctly identifies inbreeding as the cause, violating random mating. Choice B is incorrect because favoring heterozygotes would decrease, not increase, homozygosity. A transferable check for Hardy-Weinberg assumptions is to calculate inbreeding coefficients; homozygote excess suggests nonrandom mating. Pedigree analysis can quantify inbreeding levels.

Question 2

1. Mutation that increases heterozygosity, thereby causing a heterozygote deficit
2. Genetic drift in a large population producing a stable heterozygote deficit
3. No migration ensuring the population must be in Hardy-Weinberg equilibrium
4. Nonrandom mating (assortative mating) increasing homozygosity relative to $2pq$ (correct answer)

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to nonrandom mating. The Hardy-Weinberg principle states that random mating, no selection, no mutation, no migration, and large size maintain genotype expectations. In this pigeon population, preferential mating based on plumage linked to genotypes causes heterozygote deficit. Choice D correctly identifies assortative mating as the cause, violating the random mating assumption. Choice B is incorrect because drift in large populations is minimal and not consistent. A transferable check for Hardy-Weinberg assumptions is to assess mate choice; homozygote excess suggests assortative mating. Behavioral data can link mating to genotypes.

Question 3

1. Allele frequencies in the focal population may shift toward those of the source population due to gene flow (correct answer)
2. Allele frequencies must remain constant because selection is absent
3. Genetic drift will dominate over migration in a large population, causing random fixation
4. Hardy-Weinberg equilibrium guarantees genotype frequencies remain unchanged even with migration

Explanation: This question tests the ability to apply the Hardy-Weinberg principle to predict changes in allele frequencies when one of its assumptions is violated. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, allele and genotype frequencies remain constant across generations. In this scenario, the bird population initially meets Hardy-Weinberg expectations, but consistent immigration from a nearby population with different allele frequencies introduces gene flow, violating the no-migration assumption. The correct prediction, choice A, follows the principle because gene flow can alter allele frequencies in the focal population, shifting them toward those of the source population. A common distractor, choice D, fails by misconstruing that Hardy-Weinberg equilibrium persists despite migration, ignoring that migration prevents equilibrium by changing allele frequencies. To recognize Hardy-Weinberg assumptions in practice, check for evidence of gene flow, such as immigration data, which would invalidate predictions of constant frequencies. Additionally, confirm other assumptions like large population size and no selection are met to isolate the violating factor.

Question 4

1. Genetic drift due to small effective population size and sampling effects during reproduction (correct answer)
2. Directional selection favoring K in all enclosures, producing identical allele-frequency trajectories
3. High gene flow among enclosures equalizing allele frequencies
4. Random mating preventing any allele-frequency differences among enclosures

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to genetic drift in small populations. The Hardy-Weinberg principle states that in a very large, randomly mating population with no selection, mutation, or migration, allele frequencies remain stable. In this rodent colony, small founding sizes lead to differing allele frequencies across replicates despite identical conditions. Choice A correctly identifies genetic drift due to small size as the cause, violating the large population assumption. Choice B is incorrect because directional selection would produce consistent, not variable, trajectories across replicates. A transferable check for Hardy-Weinberg assumptions is to compare allele frequencies across replicate populations; high variance indicates drift. Small effective sizes amplify random sampling effects.

Question 5

1. No mutation maintaining Hardy-Weinberg equilibrium regardless of reproductive differences
2. Random mating increasing nn frequency above Hardy-Weinberg expectations
3. Genetic drift producing consistent nn deficiency in a large population
4. Selection against nn via reduced reproductive success, changing genotype representation among breeders (correct answer)

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to fecundity selection. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, genotype frequencies match allele-based expectations. In this mammal population, nn has reduced reproductive success, leading to underrepresentation in breeders. Choice D correctly identifies selection against nn as the cause, violating the no-selection assumption. Choice B is incorrect because random mating does not increase homozygote frequencies above q². A transferable check for Hardy-Weinberg assumptions is to compare frequencies across reproductive stages; deficits in breeders suggest fecundity selection. Linking traits to fitness can identify selective mechanisms.

Question 6

1. Stable environment across replicate chemostats
2. Large population size that minimizes sampling error
3. Absence of selection on the marker locus
4. Recurrent mutation introducing new G alleles each generation (correct answer)

Explanation: This question tests conditions that disrupt Hardy-Weinberg equilibrium, specifically mutation. The Hardy-Weinberg principle states that in a large population with random union of gametes, no selection, no migration, and no mutation, allele and genotype frequencies remain stable. In this bacterial chemostat, recurrent mutation from g to G introduces new alleles, potentially shifting frequencies despite large size and no selection. Choice D correctly identifies recurrent mutation as the disrupting factor, violating the no-mutation assumption. Choice B is incorrect because large population size supports equilibrium by reducing drift, not disrupting it. A transferable check for Hardy-Weinberg assumptions is to sequence for new variants; unexpected allele introductions suggest mutation. Monitoring frequency changes without other forces can isolate mutational effects.

Question 7

1. Random mating that directly increases heterozygote frequency above $2pq$
2. Genetic drift in a large population causing predictable heterozygote excess
3. No migration ensuring genotype frequencies cannot deviate from $p^2:2pq:q^2$
4. Sexual selection increasing reproductive contribution of Zz individuals, altering genotype frequencies among adults (correct answer)

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to sexual selection. The Hardy-Weinberg principle states that random mating, no selection (including sexual), no mutation, no migration, and large size maintain equilibria. In this butterfly population, Zz higher mating success causes adult heterozygote excess. Choice D correctly identifies sexual selection as the cause, violating assumptions. Choice B is incorrect because drift in large populations is not predictable. A transferable check for Hardy-Weinberg assumptions is to evaluate reproductive success; heterozygote excess in adults suggests sexual selection. Comparing larval and adult frequencies reveals biases.

Question 8

1. If the fitness difference persists, $q(u)$ is expected to decrease across generations (correct answer)
2. Because germination is similar, allele frequencies must remain constant
3. Random mating will increase the frequency of u because it is recessive
4. Genetic drift will dominate in a large population, producing predictable directional change

Explanation: This question tests predictions about allele frequencies under selection in Hardy-Weinberg context. The Hardy-Weinberg principle states that no selection, random mating, no mutation, no migration, and large size keep allele frequencies constant. In this plant scenario, uu has lower seed production, leading to decreasing q(u). Choice A correctly predicts q(u) decrease if fitness differences persist, consistent with selection violation. Choice B is incorrect because similar germination does not prevent overall fitness effects on frequencies. A transferable check for Hardy-Weinberg assumptions is to measure fitness components; directional changes indicate selection. Tracking alleles over time reveals selection strength.

Question 9

1. Very large population size maintained each generation
2. Ongoing gene flow from an external stock with different allele frequencies (correct answer)
3. Random mating within the cage population
4. No detectable differences in survival among genotypes

Explanation: This question tests understanding of conditions that disrupt Hardy-Weinberg equilibrium, specifically migration. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, allele and genotype frequencies remain constant. In this fruit fly laboratory setup, new individuals with higher p(D) are introduced each generation, potentially shifting allele frequencies despite random mating and no selection. Choice B correctly identifies ongoing gene flow from an external stock as the disrupting factor, violating the no-migration assumption. Choice A is incorrect because large population size actually supports equilibrium by minimizing drift, misconstruing size as a disruptor. A transferable check for Hardy-Weinberg assumptions is to assess external influences like migration by comparing allele frequencies before and after potential influx events. Consistent shifts aligned with source populations indicate gene flow disrupting equilibrium.

Question 10

1. Allele frequencies are expected to remain approximately $p=0.7$ and $q=0.3$ (correct answer)
2. Heterozygotes are expected to disappear because alleles segregate during meiosis
3. The h allele is expected to increase because it is rarer and thus favored
4. Genotype frequencies must change each generation even if allele frequencies remain constant

Explanation: This question tests predictions under Hardy-Weinberg equilibrium when assumptions hold. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, allele frequencies remain constant, and genotypes match p², 2pq, q². In this deer population, current genotypes match expectations, suggesting equilibrium. Choice A correctly predicts stable allele frequencies of p=0.7 and q=0.3 in the next generation if assumptions continue. Choice B is incorrect because heterozygotes persist via random mating, not disappear due to segregation, misconstruing meiotic effects. A transferable check for Hardy-Weinberg assumptions is to verify if observed genotypes fit expected ratios using allele frequencies. Stability over generations confirms equilibrium conditions.

Question 11

1. Absence of selection implies genotype frequencies cannot change after a new allele appears
2. Random mating guarantees the new allele will rapidly fix in the population
3. Large population size ensures the new allele must be eliminated by drift
4. Because mutation occurred, allele frequencies can change and the population is not strictly in Hardy-Weinberg equilibrium (correct answer)

Explanation: This question tests predictions immediately after a mutation event using Hardy-Weinberg reasoning. The Hardy-Weinberg principle states that without mutation, random mating, no selection, no migration, and large size, allele frequencies remain constant. In this mouse population, a new allele arises by mutation, introducing a change. Choice D correctly notes that mutation violates assumptions, allowing allele-frequency changes and disrupting equilibrium. Choice B is incorrect because random mating does not guarantee fixation of new alleles. A transferable check for Hardy-Weinberg assumptions is to screen for rare variants; their appearance suggests mutation. Immediate post-mutation genotyping can show initial frequency shifts.

Question 12

1. Selection is effectively absent at the W/w locus, so genotype frequencies can remain at $p^2:2pq:q^2$ under random mating (correct answer)
2. Any mortality event necessarily violates Hardy-Weinberg equilibrium at all loci
3. Genetic drift is prevented only by mutation, so stable frequencies imply high mutation rates
4. Hardy-Weinberg equilibrium requires allele frequencies to change each generation to maintain $2pq$

Explanation: This question tests why equilibrium can persist despite external mortality using Hardy-Weinberg reasoning. The Hardy-Weinberg principle states that no selection at the locus, random mating, no mutation, no migration, and large size maintain frequencies. In this insect scenario, genotype-independent pesticide allows continued equilibrium. Choice A correctly explains absent locus-specific selection permits matching expectations. Choice B is incorrect because non-locus-specific mortality does not violate assumptions. A transferable check for Hardy-Weinberg assumptions is to test locus-specific fitness; neutral loci remain in equilibrium. Post-event genotyping confirms stability.

Question 13

1. Repeated population bottlenecks from small daily inocula, increasing genetic drift (correct answer)
2. Negligible mutation rates that preserve allele frequencies
3. Absence of selection on the locus
4. Random sampling that enforces random mating and thus prevents drift

Explanation: This question tests conditions disrupting Hardy-Weinberg equilibrium, specifically small effective size from bottlenecks. The Hardy-Weinberg principle states that very large size, random union, no selection, no mutation, and no migration preserve frequencies. In this yeast setup, small daily inocula create bottlenecks, enhancing drift. Choice A correctly identifies repeated bottlenecks as disruptive, violating large size assumption. Choice B is incorrect because negligible mutation supports, not disrupts, equilibrium. A transferable check for Hardy-Weinberg assumptions is to assess sampling procedures; bottlenecks amplify drift. Monitoring frequency variance over transfers detects disruptions.

Question 14

1. Stabilizing selection maintaining both alleles at equal frequency regardless of population size
2. Random mating causing directional change in allele frequencies
3. Absence of migration forcing allele frequencies to fluctuate more
4. Genetic drift due to small population size following habitat restriction (correct answer)

Explanation: This question tests understanding of allele-frequency changes due to genetic drift in small populations. The Hardy-Weinberg principle states that very large size, random mating, no selection, no mutation, and no migration stabilize frequencies. In this flower population, post-landslide small size causes noticeable changes. Choice D correctly identifies drift due to small size as the cause, violating large population assumption. Choice B is incorrect because random mating does not cause directional changes. A transferable check for Hardy-Weinberg assumptions is to estimate effective population size; small Ne predicts drift. Year-to-year variance quantifies drift effects.

Question 15

1. Nonrandom mating driven by spatial structure, increasing homozygosity relative to $2pq$ (correct answer)
2. Genetic drift in a large population producing consistent heterozygote deficits
3. Overdominant selection that reduces heterozygotes below $2pq$
4. No mutation ensuring adults must match Hardy-Weinberg expectations regardless of mating patterns

Explanation: This question tests the understanding of how violations of Hardy-Weinberg assumptions, particularly nonrandom mating, lead to deviations in genotype frequencies. The Hardy-Weinberg principle assumes that allele and genotype frequencies remain stable in a population with random mating, no selection, no mutation, no migration, and large size, predicting genotype frequencies as p², 2pq, and q². Here, the wildflower population shows a heterozygote deficit in adults compared to expectations from seed allele frequencies, with spatial clustering and neighbor pollination indicating nonrandom mating. Choice A is correct because this spatial structure promotes assortative mating, increasing homozygosity and reducing heterozygotes below 2pq in the observed adults. Choice C fails by incorrectly suggesting overdominant selection reduces heterozygotes, when in fact overdominance (heterozygote advantage) would increase heterozygotes above expectations, misapplying selection concepts. A transferable check for Hardy-Weinberg assumptions involves examining mating patterns for randomness, such as assessing spatial distribution or mate choice behaviors. If nonrandom mating is evident, expect deviations like heterozygote deficits due to inbreeding or population substructure.

Question 16

1. Genetic drift in a very large population causing deterministic loss of the a allele
2. Random mating that increases heterozygosity above Hardy-Weinberg expectations
3. Nonrandom survival of aa individuals due to predation, altering genotype frequencies after zygote formation (correct answer)
4. Absence of migration maintaining constant allele frequencies and thus enforcing Hardy-Weinberg equilibrium

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to natural selection. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, allele frequencies remain constant, and genotype frequencies conform to p², 2pq, and q². In this coastal lizard scenario, the introduction of a raptor that preferentially preys on aa individuals leads to lower-than-expected aa frequencies in adults compared to hatchlings. Choice C correctly identifies nonrandom survival due to predation as the factor violating the no-selection assumption, altering genotype frequencies post-zygote formation. Choice B is incorrect because random mating actually maintains heterozygosity at 2pq, not increases it above expectations, misconstruing the role of mating in equilibrium. A transferable check for Hardy-Weinberg assumptions is to calculate expected genotype frequencies from observed allele frequencies and test for significant deviations using chi-square analysis. Persistent discrepancies in specific genotypes often signal selection, while random fluctuations may indicate drift in small populations.

Question 17

1. Viability selection against ff between birth and adulthood (correct answer)
2. Random mating increases the frequency of ff above $q^2$
3. No migration ensures adult genotype frequencies must equal newborn frequencies
4. Genetic drift in a very large population produces consistent ff deficiency each year

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to viability selection. The Hardy-Weinberg principle states that in a large, randomly mating population with no selection, mutation, or migration, genotype frequencies match expectations from allele frequencies. In this human cohort, ff individuals are underrepresented in adults due to childhood mortality from cardiomyopathy. Choice A correctly identifies viability selection against ff as the cause, violating the no-selection assumption. Choice B is incorrect because random mating maintains, rather than increases, homozygote frequencies at q², misconstruing mating effects. A transferable check for Hardy-Weinberg assumptions is to compare genotype frequencies across life stages; deficits in adults versus newborns indicate selection. Consistent patterns linked to traits suggest specific selective pressures.

Question 18

1. Random mating within the entire habitat producing fewer heterozygotes than $2pq$
2. Increased mutation rate converting M to m, which necessarily reduces heterozygosity
3. Reduced gene flow creating subpopulations with different allele frequencies, yielding an apparent heterozygote deficit (correct answer)
4. Genetic drift in a very large population causing identical allele-frequency shifts on both sides

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to reduced gene flow and substructure. The Hardy-Weinberg principle states that in a single, randomly mating population with no selection, mutation, or migration, and large size, genotypes conform to expectations. In this grasshopper scenario, a highway reduces mixing, creating subpopulations and heterozygote deficit in pooled samples. Choice C correctly identifies reduced gene flow as the cause, violating the single population assumption. Choice B is incorrect because increased mutation would not necessarily reduce heterozygosity and lacks directionality. A transferable check for Hardy-Weinberg assumptions is to evaluate barriers to mating; deficits in pooled but not subgroup data indicate substructure. Tracking migration rates can predict such deviations.

Question 19

1. Hardy-Weinberg equilibrium predicts genotype frequencies can change while allele frequencies remain constant
2. Because mating is random, allele frequencies must remain constant despite selection
3. Genetic drift will deterministically drive e to fixation in a large population
4. If selection continues, $p(E)$ is expected to increase across generations (correct answer)

Explanation: This question tests predictions about allele frequencies under selection using Hardy-Weinberg reasoning. The Hardy-Weinberg principle states that without selection, random mating, no mutation, no migration, and large population size, allele frequencies stay constant while genotypes reach equilibrium in one generation. In this plant population, herbicide application favors EE survival, leading to increasing EE proportions over time. Choice D correctly predicts that p(E) will increase across generations due to selection, consistent with violations of the no-selection assumption. Choice B is incorrect because random mating does not prevent allele-frequency changes under selection, misconstruing mating's role in preserving frequencies. A transferable check for Hardy-Weinberg assumptions is to track allele frequencies over generations; directional changes suggest selection. Comparing genotype frequencies before and after selective events can reveal fitness differences.

Question 20

1. A selective advantage of B that steadily increases $p(B)$ each generation
2. Genetic drift due to small effective population size following the founder event (correct answer)
3. High migration from the mainland that buffers allele-frequency fluctuations
4. Random mating that prevents any change in allele frequencies over time

Explanation: This question tests understanding of deviations from Hardy-Weinberg equilibrium due to genetic drift in small populations. The Hardy-Weinberg principle states that in a very large, randomly mating population with no selection, mutation, or migration, allele and genotype frequencies remain stable across generations. In this island bird population founded by only 18 individuals, allele frequencies fluctuate irregularly without directional trend, despite random mating and no environmental changes. Choice B correctly identifies genetic drift due to small effective population size as the cause, violating the large population assumption and causing random allele-frequency changes. Choice A is incorrect because it implies directional selection, but the fluctuations lack consistency, misconstruing drift as selection. A transferable check for Hardy-Weinberg assumptions is to monitor allele frequencies over generations; stability suggests equilibrium, while random changes in small populations indicate drift. Comparing variance in allele frequencies across replicates can help distinguish drift from deterministic forces like selection.