Revision summary
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.
Model answer
Introduction
Genetic polymorphism is the stable presence of two or more alleles at a locus, each at a frequency too high to be only recurrent mutation. A common teaching cut-off is 1 percent.
Body
Definition
- Ford used polymorphism for discrete forms in a population. In humans it now covers blood groups, enzymes, and DNA sites.
- It is not a rare disease allele. It is a normal variation that population genetics can count.
Types and examples
- Balanced polymorphism: heterozygote advantage. Sickle haemoglobin in malaria belts, shown by Allison, and thalassaemia in parts of India and the Mediterranean.
- Transient polymorphism: a rising advantageous allele not yet fixed, or a falling one.
- Chromosomal polymorphism: inversions in Drosophila were the classic; humans show copy-number and inversion variants too.
- Protein / serological: ABO, Rh, HLA, G6PD.
- DNA polymorphism: SNPs, STRs used in forensics, Alu insertions.
- Restricted vs widespread: PTC tasting, lactase persistence as gene–culture polymorphism.
Meaning
- Polymorphism feeds Harris enzyme work and Lewontin’s point that most variation is within groups.
Flow diagram
flowchart TD GP[Genetic polymorphism] --> BAL[Balanced sickle] GP --> DNA[SNP STR] GP --> SER[ABO Rh HLA]
Conclusion
Genetic polymorphism is common allelic variation. Types run from balanced sickle-cell systems to SNPs and STRs, and they are the raw material of human diversity.
Quick related
Students also ask
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What do you understand by Immunogenetics? Explain with suitable examples
Next question in the 2015 paper (Q8). View answer →
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Is a 0.1 percent disease allele a polymorphism?
Usually no. The teaching definition wants appreciable frequency in the population.
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Is polymorphism the same as polytypism?
Polytypism is between-population difference. Polymorphism is variation within a breeding population.
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