Summary
Predicts that allele and genotype frequencies stay constant in an idealised population, following p squared plus 2pq plus q squared.
In depth
Under no selection, mutation, migration, drift or nonrandom mating, allele frequencies (p + q = 1) and genotype frequencies (p² + 2pq + q² = 1) remain constant. Deviations indicate evolution is occurring.
It is used to estimate carrier frequencies: if 1 in 2500 are affected by a recessive disease, q² = 0.0004, q = 0.02, and carriers 2pq are about 4 percent.
Common mistake: the principle describes genotype frequencies, not individuals' traits.
Applying it: if 1 in 2500 people have cystic fibrosis (q squared = 0.0004), then q = 0.02, p = 0.98, and carriers number 2pq, about 0.039 or roughly 1 in 25. The assumptions are large population, random mating, no mutation, no migration and no selection.
It serves as a null model: deviations from expected genotype frequencies suggest evolution or non-random mating, genotyping errors or population structure. Forensic and clinical genetics use it to estimate frequencies and carrier risks, and a chi-square test checks fit.
Key idea: the principle shows that sexual reproduction alone does not change allele frequencies; something else must.
Frequently asked questions
What is Hardy-Weinberg Principle?
Predicts that allele and genotype frequencies stay constant in an idealised population, following p squared plus 2pq plus q squared.
Which area of biology covers Hardy-Weinberg Principle, and how advanced is it?
Hardy-Weinberg Principle is one of the 54 topics in Genetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Genetic Variation, Homozygous and Heterozygous, Dominant and Recessive.
What should I learn before Hardy-Weinberg Principle?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Hardy-Weinberg Principle. Intermediate topics assume the Beginner vocabulary.
What topics are related to Hardy-Weinberg Principle?
The closest topics to Hardy-Weinberg Principle are Genetic Variation, Homozygous and Heterozygous, Dominant and Recessive, Allele. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Hardy-Weinberg Principle?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Hardy-Weinberg Principle. To practise the whole of Genetics, use the Genetics quiz.
How is Hardy-Weinberg Principle different from Genetic Variation?
Hardy-Weinberg Principle: Predicts that allele and genotype frequencies stay constant in an idealised population, following p squared plus 2pq plus q squared. Genetic Variation: Differences in DNA among individuals, arising from mutation, recombination and sexual reproduction. Read both side by side and note what each one explains.
What is a common mistake about Hardy-Weinberg Principle?
The principle describes genotype frequencies, not individuals' traits.
How should I study Hardy-Weinberg Principle step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Genetic Variation and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.