Revision summary
Hardy and Weinberg showed that p², 2pq, and q² persist under stated assumptions. Assumptions are large size, random mating, and no mutation, migration, or selection. Equilibrium is the null against which evolution is inferred. Forensics and counselling use it to get genotype frequencies. Sickle-cell selection and isolate inbreeding are standard anthropological departures. Real populations only approximate the model.
Model answer
Introduction
Genetic equilibrium is the Hardy–Weinberg state in which allele and genotype frequencies stay stable across generations. It is a null model, and it holds only when a short list of assumptions is met.
Body
The Hardy–Weinberg statement
- For two alleles, p and q, random mating produces genotypes p², 2pq, and q², a relation named for G. H. Hardy and Wilhelm Weinberg in 1908.
- If the assumptions hold, those frequencies recur, and allele frequencies do not drift by the mating system itself.
Assumptions
- The population is indefinitely large, so chance genetic drift is negligible.
- Mating is random with respect to the locus, with no inbreeding or assortative mating that would raise homozygotes.
- There is no mutation at rates that would shift p and q in the study window.
- There is no migration that imports a different allele frequency.
- There is no selection for or against genotypes at that locus.
- Generations are discrete in the simple teaching model, and the locus is autosomal in the basic two-allele form; sex-linked and multiple-allele extensions need extra algebra.
Importance
- Equilibrium is the null hypothesis of population genetics: departure diagnoses drift, inbreeding, migration, or selection, which is why R. C. Lewontin and anthropological genetics use it on blood groups and now on SNPs.
- Forensic match probabilities and genetic counselling use Hardy–Weinberg expectations to translate allele frequencies into genotype frequencies.
- Sickle-cell heterozygote advantage in malaria belts is taught as a selection break of equilibrium, not as a failure of Mendelism.
- Inbreeding in small tribal isolates raises homozygosity above 2pq expectations, which is an anthropological use of the same test.
- Conservation genetics of small populations uses the opposite of the infinite-population assumption to explain loss of diversity.
What it is not
- No real human population meets every assumption forever.
- The model is still necessary as a measuring stick.
Flow diagram
flowchart TD A[Large random mating] --> H[Hardy-Weinberg] N[No mutation migration selection] --> H H --> Nul[Null model] B[Departure] --> F[Drift inbreeding selection]
Conclusion
Hardy–Weinberg equilibrium needs a large, randomly mating population without mutation, migration, or selection at the locus. Its importance is as a null model for detecting evolutionary forces and for translating allele frequencies into expected genotypes.
Quick related
Students also ask
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Balanced and transient genetic polymorphism
Next question on this syllabus topic (2022 · Q5(a)). View answer →
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Does equilibrium mean no evolution ever?
It means no change from those forces at that locus in that window. Other loci can be under selection at the same time.
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Why teach a model that is never fully true?
Because departure from it is how drift, inbreeding, and selection become visible in data.
PYQ trend
When UPSC asked this
Related PYQs from other years, newest first. Open a question to read it.
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2022 · Q5(a) · Anthropology GS 1 · 10 marks
Balanced and transient genetic polymorphism -
2015 · Q8(b) · Anthropology GS 1 · 15 marks
Define genetic polymorphism. Give details of its types with suitable examples
More from this topic
Q5(a) · UPSC Mains 2022 · Anthropology GS 1 · 10 marks
Balanced and transient genetic polymorphism
Concept of genetic polymorphism and selection, Mendelian population, Hardy-Weinberg law
Polymorphism is lasting allelic diversity at one locus. Balanced polymorphism keeps alleles, classically by heterozygote advantage. Sickle-cell and malaria is the human type case. Transient polymorphism is a passing stage while one allele replaces another. Lactase persistence is a human illustration of a sweep still in motion.
Q8(b) · UPSC Mains 2015 · Anthropology GS 1 · 15 marks
Define genetic polymorphism. Give details of its types with suitable examples
Concept of genetic polymorphism and selection, Mendelian population, Hardy-Weinberg law
Polymorphism is two or more common alleles at a locus. Balanced types include sickle cell and malaria. ABO, Rh, and HLA are serological polymorphisms. SNPs and STRs are DNA types used in medicine and forensics. Most human genetic variation is polymorphic within populations.
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