Explain Hardy Weinberg Theorem

Long Note for Class 12 Biology (Chapter 24 Evolution – 5–8 Marks Question) (RTS/HED/Punjab Board Pattern)

Definition / Statement

The Hardy-Weinberg theorem (1908) states that: In a large, randomly mating population, the allele frequencies and genotype frequencies remain constant from generation to generation provided no evolutionary forces (mutation, natural selection, genetic drift, gene flow or migration) act on the population.

This is also called Hardy-Weinberg equilibrium. It describes a non-evolving population.

Hardy-Weinberg Equation

Let:

$p$ = frequency of dominant allele (A)
$q$ = frequency of recessive allele (a)

Then: $p + q = 1$ (allele frequency equation)

Genotype frequencies in the population: $p^2 + 2pq + q^2 = 1$

Where:

$p^2$ = frequency of homozygous dominant (AA)
$2pq$ = frequency of heterozygous (Aa)
$q^2$ = frequency of homozygous recessive (aa)

Assumptions (Conditions) for Hardy-Weinberg Equilibrium

For gene frequencies to remain constant, the following five conditions must be met:

No.	Condition	Explanation
1	Large population size	Prevents random fluctuations (genetic drift)
2	Random mating	No preference for any genotype
3	No mutation	No new alleles introduced
4	No natural selection	All genotypes have equal survival & reproduction
5	No gene flow (migration)	No alleles enter or leave the population

If any one condition is violated, the population evolves (gene frequency changes).

Significance / Importance

Acts as a null hypothesis (baseline) to detect whether evolution is occurring.
Helps calculate allele and genotype frequencies in a population.
Used in population genetics and to study genetic disorders.
Explains why recessive alleles are not eliminated from the population.

Example (Simple Calculation)

In a population, frequency of recessive allele (q) = 0.3 Then p = 1 – 0.3 = 0.7

Genotype frequencies:

AA = $p^2$ = 0.49
Aa = $2pq$ = 0.42
aa = $q^2$ = 0.09

Key Exam Lines:

“Hardy-Weinberg principle states that allele frequencies remain constant in the absence of evolutionary forces.”
“The equation is $p^2 + 2pq + q^2 = 1$ .”
“It is used as a mathematical model to measure evolution.”