Population genetics Practice Questions
20 free Population genetics practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
In a population at Hardy-Weinberg equilibrium, the frequency of a recessive disease is 1 in 10,000 individuals. What is the approximate carrier frequency?
- A 1/100
- B 1/50
- C 1/25
- D 1/10
Correct answer: 1/50
If q-squared = 1/10,000, then q = 1/100 and p is approximately 1. The carrier (heterozygote) frequency is 2pq = 2(1)(1/100), which is approximately 1/50.
Which of the following conditions must be met for a population to maintain Hardy-Weinberg equilibrium?
- A A persistently high mutation rate across generations
- B Non-random assortative mating between individuals
- C Large population size that minimizes drift
- D Strong genetic drift acting on a small population
Correct answer: Large population size that minimizes drift
Hardy-Weinberg equilibrium requires a large population size to minimize random fluctuations in allele frequency. Assortative mating and genetic drift both violate equilibrium assumptions, and mutation rates must remain negligible.
A rare autosomal recessive disorder affects 1 in 40,000 individuals. What is the approximate allele frequency of the mutant allele?
- A 1/200
- B 1/100
- C 1/400
- D 1/20
Correct answer: 1/200
If q² = 1/40,000, q = 1/200. This is the mutant allele frequency in the population. The value reflects the square root of the disease prevalence.
Inbreeding in a population is most likely to increase the frequency of which of the following?
- A A higher proportion of heterozygous carriers
- B Autosomal recessive disorders
- C A greater number of de novo germline mutations
- D X-linked dominant disorders in the population
Correct answer: Autosomal recessive disorders
Inbreeding increases homozygosity, including for recessive alleles, leading to increased expression of autosomal recessive disorders. It decreases heterozygosity and does not directly affect mutation rates or X-linked dominant conditions.
A population experiences a sudden reduction in size due to natural disaster. Which population genetics concept best describes the resulting change?
- A Founder effect
- B Genetic drift
- C Gene flow
- D Heterozygote advantage
Correct answer: Genetic drift
Genetic drift is a random change in allele frequencies, most prominent in small populations. A sudden reduction magnifies drift. The founder effect is a specific subtype involving migration.
A small group leaves a large population and establishes a new isolated community. After generations, a previously rare allele becomes more common. Which effect explains this?
- A Gene flow
- B Founder effect
- C Genetic drift
- D Directional selection
Correct answer: Founder effect
The founder effect occurs when a small group establishes a new population with different allele frequencies from the original. Rare alleles may become common. This is distinct from gene flow, which mixes populations.
If p = 0.7 in a Hardy-Weinberg population, what is the expected heterozygote frequency?
- A 0.21
- B 0.42
- C 0.14
- D 0.49
Correct answer: 0.42
Heterozygote frequency is 2pq = 2(0.7)(0.3) = 0.42. This assumes population stability and random mating. The other values correspond to homozygous frequencies or incorrect calculations.
Which of the following violates the Hardy-Weinberg assumption of random mating?
- A Large population size
- B Selection against heterozygotes
- C Consanguinity
- D Low mutation rates
Correct answer: Consanguinity
Consanguinity (inbreeding) is a form of non-random mating that increases homozygosity. Random mating is essential for Hardy-Weinberg equilibrium. Selection and mutation violate different assumptions.
Migration of individuals between populations results in which genetic effect?
- A Genetic drift
- B Gene flow
- C Founder effect
- D Fixation of alleles
Correct answer: Gene flow
Gene flow refers to the exchange of alleles between populations, reducing genetic differences. It counteracts drift. The founder effect involves isolation, not migration into a population.
A heterozygote advantage helps maintain alleles in a population by:
- A Increasing homozygous recessive individuals
- B Decreasing mutation rates
- C Providing greater fitness to heterozygotes
- D Preventing gene flow
Correct answer: Providing greater fitness to heterozygotes
Heterozygote advantage occurs when heterozygotes have increased fitness, maintaining both alleles. Sickle cell trait is a classic example. It does not decrease mutation or gene flow.
If a population has allele frequencies p = 0.4 and q = 0.6, what is the expected frequency of homozygous recessive individuals?
- A 0.12
- B 0.16
- C 0.36
- D 0.64
Correct answer: 0.36
Homozygous recessive frequency is q² = 0.6² = 0.36. Hardy-Weinberg allows these predictions under equilibrium conditions. The other values represent heterozygotes or p².
Which factor is most likely to increase genetic variation within a population?
- A Genetic drift acting on allele frequencies
- B Inbreeding within the population
- C Mutation introducing new alleles
- D Stabilizing selection on the phenotype
Correct answer: Mutation introducing new alleles
Mutation introduces new alleles and is the ultimate source of genetic variation. Drift and inbreeding tend to reduce variation, and stabilizing selection removes phenotypic extremes, lowering diversity.
In a recessive disease, the carrier frequency is found to be approximately 1/25. What is the approximate disease prevalence?
- A 1/2500
- B 1/625
- C 1/125
- D 1/50
Correct answer: 1/2500
The carrier frequency 2pq is approximately 1/25. With p approximately 1, solving gives q approximately 1/50, so disease prevalence q-squared is approximately 1/2500.
Directional selection acts on a population by:
- A Favoring intermediate phenotypes
- B Increasing allele frequency at one extreme
- C Maintaining multiple alleles via balancing selection
- D Randomly changing allele frequencies
Correct answer: Increasing allele frequency at one extreme
Directional selection favors one extreme phenotype, shifting allele frequencies in a particular direction. Stabilizing selection favors intermediates. Drift changes alleles randomly.
A population in which one allele becomes 100% prevalent has undergone:
- A Fixation
- B Mutation
- C Gene flow
- D Balanced polymorphism
Correct answer: Fixation
Fixation occurs when an allele reaches 100% frequency. It commonly results from drift or strong selection. Balanced polymorphism maintains multiple alleles instead.
Which situation exemplifies genetic drift?
- A A new allele spreads due to high reproductive fitness
- B A small group of survivors repopulates after a catastrophe
- C Mutations continually generate new alleles
- D Individuals prefer mating with similar phenotypes
Correct answer: A small group of survivors repopulates after a catastrophe
A post-catastrophe small population undergoes random allele fluctuations, characteristic of drift. Fitness-driven changes represent selection. Assortative mating violates random mating.
Which event increases homozygosity within a population?
- A Gene flow
- B Inbreeding
- C Mutation
- D Directional selection
Correct answer: Inbreeding
Inbreeding promotes homozygosity by increasing the chance of inheriting identical alleles. Mutation introduces new alleles, increasing variation. Gene flow mixes populations.
In Hardy-Weinberg equilibrium, which term represents the frequency of heterozygotes?
- A p²
- B q²
- C p–q
- D 2pq
Correct answer: 2pq
Heterozygotes are represented by 2pq in the Hardy-Weinberg equation. Homozygous frequencies are p² and q². p–q does not appear in the equation.
Which factor most strongly disrupts Hardy-Weinberg equilibrium by introducing new alleles?
- A Migration
- B Random mating
- C Large population size
- D No natural selection
Correct answer: Migration
Migration (gene flow) introduces new alleles into a population. Random mating and large population size support equilibrium. Absence of selection also maintains equilibrium.
A new population is formed by 10 individuals who carry an unusually high frequency of a recessive allele. After several generations, the allele remains common. Which mechanism explains this?
- A Genetic drift
- B Stabilizing selection
- C Founder effect
- D Mutation
Correct answer: Founder effect
The founder effect occurs when a new population begins with a small group that carries different allele frequencies than the original population. This can make rare alleles common. Drift may amplify the effect but is not the primary explanation here.