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Home/Anthropology/Mains PYQ/2019/Paper I/Human Genetics & Human Biology
UPSC MainsAnthropology Optional Paper I 2019 15 Marks Model Answer Available

Transient and Balanced Genetic Polymorphism — 2019 Paper I

Question · 2019 · Paper I · 15 Marks

Differentiate between transient and balanced genetic polymorphism. Illustrate your answer with suitable examples from human populations.

Model Answer

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Approach

  • Demand of Question: Differentiate transient and balanced polymorphism in terms of allele-frequency dynamics and evolutionary mechanisms.
  • Structuring the Response: Define genetic polymorphism, compare both forms directly, and illustrate balanced polymorphism particularly through sickle-cell and malaria.
  • Key Dimensions to Cover: Allele frequencies, directional selection, equilibrium, heterozygote advantage, HbS, malaria and G6PD variation.

Model Answer

Introduction: Genetic polymorphism refers to the occurrence of two or more genetically determined alternative forms in a population at appreciable frequencies. Depending on the evolutionary forces involved, polymorphism may be transient or balanced.

Transient polymorphism

Transient polymorphism occurs when two or more alleles temporarily coexist because one allele is progressively replacing another under directional selection.

Thus:

Old allele → New advantageous allele increases → Old allele declines → Fixation

Its major characteristics are:

  • allele frequencies are changing directionally;
  • no stable equilibrium is maintained;
  • one allele eventually tends towards fixation;
  • polymorphism is therefore temporary on an evolutionary timescale.

An advantageous mutation spreading through a population may temporarily generate such polymorphism before replacing the earlier allele.

Balanced polymorphism

Balanced polymorphism occurs when two or more alleles are maintained at relatively stable frequencies because natural selection favours their continued coexistence.

Mechanisms include:

  • heterozygote advantage;
  • frequency-dependent selection;
  • environmental heterogeneity;
  • opposing selective forces.

Thus:

Selective advantage of multiple genotypes → Stable allele frequencies → Persistent polymorphism

Major differences

Feature Transient Polymorphism Balanced Polymorphism
Duration Temporary Persistent
Allele frequency Directionally changing Maintained around equilibrium
Main process Directional selection Balancing selection
Long-term outcome One allele may replace another Multiple alleles retained
Adaptive meaning Evolutionary transition Maintenance of variation

Sickle-cell polymorphism

The classic human example of balanced polymorphism is the HbA/HbS system in malaria-endemic regions.

Genotypes show different fitness consequences:

  • HbAA: Normal haemoglobin but relatively greater susceptibility to severe Plasmodium falciparum malaria.
  • HbSS: Sickle-cell disease with substantial fitness costs.
  • HbAS: Sickle-cell trait with significant protection against severe malaria while usually avoiding severe sickle-cell disease.

Therefore:

HbAA — malaria disadvantage
HbAS — highest relative fitness in malarial environment
HbSS — sickle-cell disadvantage

This heterozygote advantage maintains both HbA and HbS alleles in malaria-endemic populations.

Other examples

Variants involving G6PD deficiency and certain thalassaemia alleles also show associations with historical malaria environments, although their population dynamics are more complex than the classic HbS case.

Evolutionary significance

Balanced polymorphism demonstrates that an allele harmful in one genetic condition may persist because it provides an advantage in another.

Hence:

Environment + Genotype → Differential fitness → Maintenance of genetic diversity

Conclusion

Transient polymorphism represents a temporary stage of allele replacement, whereas balanced polymorphism represents the stable maintenance of genetic alternatives through balancing selection. The HbS-malaria relationship is a classic demonstration of how ecological pressures maintain human genetic diversity.

2026 In what manner is our understanding of serogenetics and cytogenetics useful in reproductive biology? What are the major ethical issues involved in reproductive technology? 2026 Why the classifications of Indian populations, based on somatometric and somatoscopic characteristics failed to develop a common understanding of the population diversity? 2026 Canalization and catch-up growth. 2026 Genetic polymorphism and natural selection.

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