Hardy-Weinberg equilibrium — what factors disturb allele frequencies

Question

State the Hardy-Weinberg principle. What are the five conditions required for genetic equilibrium, and what factors can disturb allele frequencies in a population?

(NCERT Class 12, commonly asked in NEET)

Solution — Step by Step

The Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary forces. This genetic equilibrium is the null hypothesis of population genetics — deviations from it indicate evolution is occurring.

For a gene with two alleles (p = frequency of dominant allele, q = frequency of recessive allele):

p + q = 1

p^2 + 2pq + q^2 = 1

Where $p^2$ = frequency of homozygous dominant, $2pq$ = frequency of heterozygotes, $q^2$ = frequency of homozygous recessive.

Hardy-Weinberg equilibrium holds only when ALL five conditions are met:

No mutation: Alleles are not being created or converted.
Random mating: Individuals mate without preference for genotype (no assortative mating).
No natural selection: All genotypes have equal fitness (no survival/reproductive advantage).
Large population size: No genetic drift (random fluctuations are negligible in large populations).
No gene flow: No migration of individuals into or out of the population.

In reality, none of these conditions is perfectly met — which is why populations evolve.

When any of the five conditions is violated, allele frequencies change:

Gene flow (migration): New alleles enter or leave the population, shifting frequencies.
Genetic drift: Random changes in allele frequency, especially significant in small populations. Includes the founder effect and bottleneck effect.
Mutation: Introduces new alleles or alters existing ones, though the rate is usually slow.
Natural selection: Favoured alleles increase in frequency; deleterious alleles decrease.
Non-random mating: Changes genotype frequencies (though not necessarily allele frequencies directly). Sexual selection is a form of this.

Why This Works

The Hardy-Weinberg equation is essentially a binomial expansion: $(p + q)^2 = p^2 + 2pq + q^2$ . If gametes combine randomly, the probability of each genotype follows this distribution. The equation gives us a mathematical baseline — any deviation tells us something interesting is happening evolutionarily.

The principle is powerful because it tells us what to expect when evolution is NOT happening. By comparing observed genotype frequencies to Hardy-Weinberg predictions, we can detect which evolutionary forces are at work.

NEET numerical shortcut: if a question gives you the frequency of the recessive phenotype (homozygous recessive), that equals $q^2$ . Take the square root to get $q$ , then $p = 1 - q$ . From there, calculate carrier frequency ( $2pq$ ) or any other genotype frequency.

Common Mistake

Students often confuse allele frequency with genotype frequency. Allele frequency ( $p$ and $q$ ) is the proportion of a specific allele in the gene pool. Genotype frequency ( $p^2$ , $2pq$ , $q^2$ ) is the proportion of individuals with a specific genotype. The Hardy-Weinberg equation connects the two, but they are not the same thing.

Another frequent error: listing only natural selection as a cause of evolution. While selection is the most directional force, genetic drift can be equally powerful in small populations and is a common NEET distractor. All five factors can disrupt equilibrium.

Question

Solution — Step by Step

Why This Works

Common Mistake

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