Revision summary
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.
Model answer
Introduction
Polymorphism means two or more alleles at one locus stay at frequencies that mutation alone cannot explain. Balanced and transient name two different fates of that variation.
Body
Balanced polymorphism
- Heterozygote advantage holds both alleles. A. C. Allison linked sickle-cell haemoglobin to malaria. AS people resist falciparum; SS is costly; A and S both remain.
- E. B. Ford and R. A. Fisher framed the idea. PTC tasting and some HLA alleles are taught as further human cases, with weaker proof than sickle cell.
- Frequency-dependent selection, as in some immune loci, is another balancing path.
Transient polymorphism
- A new advantageous allele rises and the older one falls. The population is polymorphic only while replacement is unfinished.
- Industrial melanism in Biston betularia, used by Kettlewell, is the animal textbook. In humans, lactase persistence alleles are still spreading in some dairying populations.
Distinction
- Balanced variation is maintained. Transient variation is a stage on the way to fixation or loss.
Flow diagram
flowchart TD P[Polymorphism] --> B[Balanced heterozygote] P --> T[Transient sweep] B --> S[Sickle malaria] T --> L[Lactase rise]
Conclusion
Sickle-cell malaria is balanced polymorphism. A sweeping advantageous allele is transient polymorphism. Both produce diversity, but only one is an equilibrium.
Quick related
Students also ask
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Genetic imprinting in human diseases
Next question in the 2022 paper (Q5). View answer →
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Is all human variation balanced?
No. Most new mutants are lost or, if useful, may sweep. Balance is a special explanation.
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Does balanced mean good for every person?
No. The SS child pays a cost so that AS resistance can persist in a malarial place.
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