Population Genetics and Hardy-Weinberg Study Pack
Kibin's free study pack on Population Genetics and Hardy-Weinberg includes a 5-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Population Genetics and Hardy-Weinberg Study Guide
Master the Hardy-Weinberg equilibrium equation (p² + 2pq + q² = 1), allele frequency calculations, and the five conditions required for a stable gene pool.
Key Takeaways
- •The Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant across generations when five specific conditions are met: no mutation, no gene flow, no genetic drift, random mating, and no natural selection.
- •Allele frequencies in a gene pool are expressed as p (dominant allele) and q (recessive allele), where p + q = 1 by definition.
- •Genotype frequencies under Hardy-Weinberg equilibrium are predicted by the expansion of (p + q)² = p² + 2pq + q², representing homozygous dominant, heterozygous, and homozygous recessive individuals, respectively.
- •Violations of Hardy-Weinberg conditions — such as small population size causing genetic drift, or directional natural selection — are detectable by comparing observed genotype frequencies to Hardy-Weinberg predictions.
- •Genetic drift has the largest effect in small populations, and can cause rare alleles to be lost entirely or fixed at 100% frequency by chance alone.
- •Gene flow, the movement of alleles between populations, tends to homogenize allele frequencies across populations and can counteract the divergence produced by genetic drift or local selection.
- •Hardy-Weinberg equilibrium serves as a null model in population genetics: deviations from its predictions signal that one or more evolutionary forces are actively changing the gene pool.
The Gene Pool and Allele Frequencies
Population genetics focuses on the collective genetic makeup of a group of interbreeding individuals — the gene pool — rather than on any single organism's genome. Understanding how to quantify allele and genotype frequencies is the foundation for analyzing evolutionary change.
Defining the Gene Pool
- •A gene pool consists of all copies of every allele carried by every individual in a population at a given time.
- •For a single gene locus with two alleles, every individual contributes exactly two allele copies to the gene pool.
- •Population genetics tracks how the proportions of alleles shift — or stay stable — over successive generations.
Calculating Allele Frequencies
- •The frequency of an allele is calculated by dividing the number of copies of that allele by the total number of allele copies for that locus in the population.
- •For a diploid population of N individuals, the total number of allele copies at one locus is 2N.
- •By convention, p denotes the frequency of one allele (often the dominant form) and q denotes the frequency of the alternative allele; because these are the only two options, p + q must equal 1.
Genotype Frequencies vs. Allele Frequencies
- •Genotype frequency is the proportion of individuals carrying a specific combination of alleles (e.g., AA, Aa, or aa), while allele frequency refers to the proportion of individual allele copies.
- •Knowing genotype frequencies allows calculation of allele frequencies, but allele frequencies alone do not uniquely determine genotype frequencies unless equilibrium assumptions are applied.
Hardy-Weinberg Equilibrium: The Null Model of Population Genetics
The Hardy-Weinberg principle, formulated independently by Godfrey Hardy and Wilhelm Weinberg in 1908, establishes a mathematical baseline predicting what genotype frequencies would look like in a population experiencing no evolutionary change. It functions as a null hypothesis — a reference point against which real populations are compared.
The Five Conditions Required for Equilibrium
- •No mutation: allele frequencies are not altered by the introduction of new alleles through mutation.
- •No gene flow: no individuals (and therefore no alleles) move into or out of the population.
- •No genetic drift: the population must be infinitely large so that random sampling events do not shift allele frequencies.
- •Random mating: individuals pair without any preference related to genotype, so every combination of alleles is equally likely.
- •No natural selection: all genotypes survive and reproduce with equal success, so differential fitness does not alter allele frequencies.
The Hardy-Weinberg Equations
- •The allele frequency equation p + q = 1 ensures that all allele frequencies at a locus sum to 100%.
- •The genotype frequency equation p² + 2pq + q² = 1 predicts the expected frequency of homozygous dominant (p²), heterozygous (2pq), and homozygous recessive (q²) genotypes.
- •This equation is derived by expanding (p + q)², which mathematically models random combination of alleles during sexual reproduction.
Using Hardy-Weinberg as a Diagnostic Tool
- •If a population's observed genotype frequencies match the values predicted by p² + 2pq + q², the population is in Hardy-Weinberg equilibrium for that locus.
- •A significant departure from predicted frequencies indicates that at least one evolutionary force is acting on the population, though the equations alone do not identify which force is responsible.
- •Researchers commonly use chi-square tests to determine whether observed deviations from Hardy-Weinberg predictions are statistically significant.
Evolutionary Forces That Violate Equilibrium
Real populations almost never satisfy all five Hardy-Weinberg conditions simultaneously, meaning evolutionary forces are nearly always acting to change allele frequencies. Each force operates through a distinct mechanism.
Natural Selection
- •Natural selection changes allele frequencies when different genotypes have different survival rates, reproductive success, or both — collectively called fitness.
- •Directional selection consistently favors one allele, causing its frequency to increase over generations at the expense of the less-fit allele.
- •Balancing selection, such as heterozygote advantage (where the Aa genotype has higher fitness than either AA or aa), can maintain both alleles in the population at stable intermediate frequencies — a phenomenon seen with the sickle-cell allele in malaria-endemic regions.
Genetic Drift
- •Genetic drift refers to random fluctuations in allele frequencies caused by chance events in reproduction, independent of whether an allele is advantageous or harmful.
- •Drift is most powerful in small populations; in a population of only a few dozen individuals, an allele can disappear entirely (be lost) or reach 100% frequency (be fixed) in just a few generations purely by chance.
- •Two specific forms of genetic drift are the bottleneck effect, which occurs when a population is drastically reduced in size by a catastrophic event, and the founder effect, which occurs when a small subset of a population colonizes a new habitat and carries only a fraction of the original gene pool's diversity.
Gene Flow
- •Gene flow occurs when individuals migrate between populations and introduce alleles that were previously absent or present at different frequencies.
- •Gene flow tends to reduce genetic differences between populations by homogenizing allele frequencies across geographic boundaries.
- •High levels of gene flow can prevent local populations from adapting to their specific environments because locally disadvantageous alleles are continuously reintroduced from other populations.
Mutation
- •Mutation is the ultimate source of all new alleles and therefore the ultimate source of genetic variation in a population.
- •Mutation rates are generally very low (on the order of 10⁻⁴ to 10⁻⁸ per locus per generation), so mutation alone changes allele frequencies extremely slowly compared to selection, drift, or gene flow.
Non-Random Mating
- •Non-random mating changes genotype frequencies without necessarily changing allele frequencies.
- •Inbreeding — mating between genetically related individuals — increases the frequency of homozygous genotypes and decreases heterozygosity, which can expose recessive deleterious alleles in the homozygous state.
- •Assortative mating, where individuals preferentially mate with phenotypically similar partners, also shifts genotype frequencies away from Hardy-Weinberg predictions.
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Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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Question 1 of 25
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In a population of 200 diploid individuals, how many total allele copies exist at a single gene locus?
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Concept 1 of 5
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Gene Pool and Allele Frequencies
Explain what a gene pool is and how allele frequencies are calculated within it. Why do we use p and q, and what does the relationship p + q = 1 tell us about a population?
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