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Home/Anthropology/Mains PYQ/2014/Paper I/Human Genetics & Human Biology
UPSC MainsAnthropology Optional Paper I 2014 20 Marks Model Answer Available

Factors Affecting Gene Frequencies — 2014 Paper I

Question · 2014 · Paper I · 20 Marks

Discuss the factors affecting gene frequencies among human populations.

Model Answer

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Approach

  • Demand of Question: Explain the major evolutionary forces that alter allele frequencies in human populations.
  • Structuring the Response: Begin with Hardy-Weinberg equilibrium as the baseline and then discuss mutation, selection, gene flow, genetic drift and mating structure with human examples.
  • Key Dimensions to Cover: Mutation, natural selection, migration, genetic drift, founder effect, bottleneck, non-random mating, population subdivision and human adaptation.

Model Answer

Introduction: Gene frequency or allele frequency refers to the proportion of a particular allele at a locus within a population. Under Hardy-Weinberg equilibrium, allele frequencies remain constant in an infinitely large, randomly mating population without mutation, migration or natural selection. Human populations, however, rarely satisfy all these conditions.

Thus:

Mutation + Selection + Gene flow + Genetic drift + Population structure → Change in gene frequencies

  1. Mutation

Mutation is the ultimate source of new genetic variation.

It may involve:

  • nucleotide substitution;
  • insertion or deletion;
  • chromosomal changes.

Mutation rates are generally low, so mutation alone changes allele frequencies slowly, but it supplies the variation on which other evolutionary forces act.

  1. Natural selection

Natural selection changes gene frequencies when different genotypes differ in survival or reproductive success.

A classic human example is the HbS allele in malaria-endemic areas.

Individuals with HbA/HbS generally possess greater protection against severe Plasmodium falciparum malaria than HbA/HbA individuals, while HbS/HbS causes sickle-cell disease.

Thus:

Malaria environment → Heterozygote advantage → Maintenance of HbS

This is an example of balanced polymorphism.

Other examples include:

  • lactase persistence in dairying populations;
  • genetic adaptations to high altitude.
  1. Gene flow or migration

Migration introduces alleles from one population into another.

Thus:

Population A → Migrants → Population B → Altered allele frequencies

Gene flow generally:

  • increases variation within populations;
  • reduces genetic differentiation between populations.

Human migrations, intermarriage and urbanization have therefore been important forces in population history.

  1. Genetic drift

Genetic drift refers to random changes in allele frequencies, especially in small populations.

Unlike natural selection, drift is non-adaptive.

Its effects include:

  • random loss of alleles;
  • fixation of alleles;
  • increased differentiation between small populations.
  1. Founder effect

When a small group establishes a new population, its gene frequencies may differ substantially from the parent population.

Thus:

Small founder group → Unrepresentative allele frequencies → New population

Rare alleles may become relatively common simply because they were present among the founders.

  1. Population bottleneck

A severe temporary reduction in population size due to:

  • epidemic;
  • famine;
  • migration;
  • environmental disaster

can randomly alter allele frequencies.

The surviving population may therefore contain only part of the original genetic variation.

  1. Non-random mating

Human mating is frequently influenced by:

  • caste;
  • religion;
  • ethnicity;
  • geography;
  • kinship rules.

Endogamy limits gene flow between groups and can increase genetic differentiation.

Inbreeding itself primarily alters genotype frequencies, increasing homozygosity, but over generations it may interact with selection and drift to influence allele frequencies.

  1. Population size

Small populations are much more strongly influenced by genetic drift.

Large populations tend to maintain allele frequencies more consistently unless affected by:

  • strong selection;
  • migration;
  • mutation.
  1. Population subdivision and isolation

Geographical or social isolation restricts gene flow.

Examples include:

  • island populations;
  • geographically isolated tribes;
  • endogamous caste groups.

Isolation allows drift and local selection to produce distinctive allele-frequency patterns.

  1. Cultural practices

Human evolution is distinctive because culture can alter biological selection.

For example:

Dairying → Milk consumption → Selection for lactase persistence

Similarly, settlement, agriculture and diet have modified exposure to:

  • pathogens;
  • nutritional environments;
  • reproductive pressures.

Hence human gene frequencies are also influenced by gene-culture coevolution.

Conclusion

Gene frequencies among human populations are shaped principally by mutation, natural selection, gene flow and genetic drift, while mating patterns, population size, isolation and culture influence the intensity of these processes. Human genetic diversity is therefore the outcome of continuous interaction between evolutionary forces, demographic history and cultural behaviour.

2026 Canalization and catch-up growth. 2026 Genetic polymorphism and natural selection. 2026 Numerical chromosomal aberrations can be due to abnormalities in autosomes as well as sex chromosomes. Elaborate in terms of the effects these have on human body, giving suitable examples. 2026 Distinguishing features of nuclear and mitochondrial DNA.

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