CBSE Class 12 Biology – Hardy–Weinberg Principle (NCERT) Important Facts

These are the highest-yield NCERT facts for CBSE Board Exams (MCQs, 1 Mark, 2 Marks, Assertion-Reason, and Case-Based Questions).

1. Hardy–Weinberg Principle

Proposed by:

  • G.H. Hardy
  • Wilhelm Weinberg

Definition

The allele frequencies in a population remain constant from generation to generation if no evolutionary forces act on the population.

This state is called Genetic Equilibrium.

2. Gene Pool

Gene pool =

Total collection of all genes and their alleles present in a population.

3. Genetic Equilibrium

When allele frequencies remain constant from one generation to the next, the population is said to be in Hardy–Weinberg equilibrium.

4. Hardy–Weinberg Equation

If:

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

Then,

Allele Frequency

p + q = 1

Genotype Frequency

p² + 2pq + q² = 1

Where:

  • p² = AA (Homozygous dominant)
  • 2pq = Aa (Heterozygous)
  • q² = aa (Homozygous recessive)

Board Tip: Memorize both equations exactly as given in NCERT.

5. Importance

  • Helps determine whether evolution is occurring in a population.
  • If observed genotype frequencies differ from expected values, it indicates evolutionary change.

6. Factors Affecting Hardy–Weinberg Equilibrium

There are five major factors:

  1. Gene Migration (Gene Flow)
  2. Genetic Drift
  3. Mutation
  4. Genetic Recombination
  5. Natural Selection

7. Gene Flow (Gene Migration)

  • Movement of individuals (or their genes) from one population to another.
  • Introduces new alleles into the receiving population.
  • Changes allele frequencies in both populations.

8. Genetic Drift

  • Random changes in allele frequencies due to chance.
  • More significant in small populations.
  • Can eventually lead to speciation.

9. Founder Effect

Occurs when:

  • A small group separates from a larger population.
  • This small group establishes a new population.
  • Its allele frequencies differ from the original population due to genetic drift.

NCERT Point: The original drifted population becomes the founders, and this phenomenon is called the Founder Effect.

10. Mutation

  • Sudden, random, heritable changes in DNA.
  • Produces new alleles.
  • Alters allele frequencies and contributes to evolution.

11. Genetic Recombination

Occurs during gamete formation (meiosis).

Creates:

  • New combinations of genes.
  • Genetic variation.

12. Natural Selection

Natural selection:

  • Acts on heritable variations.
  • Individuals with favourable traits survive and reproduce more.
  • Changes allele frequencies over generations.

13. Types of Natural Selection

A. Stabilising Selection

  • Favours average (mean) individuals.
  • Reduces extreme variations.
  • Mean phenotype is maintained.

Example: Average birth weight in humans.

B. Directional Selection

  • Favours one extreme phenotype.
  • Population mean shifts toward that extreme.

Example: Evolution of antibiotic-resistant bacteria.

C. Disruptive Selection

  • Favours both extreme phenotypes.
  • Intermediate phenotypes are selected against.
  • Can lead to speciation.

Flow Chart

Variation
⬇
Mutation / Recombination / Gene Flow / Genetic Drift
⬇
Change in Allele Frequency
⬇
Natural Selection
⬇
Evolution
⬇
Speciation

One-Mark Questions

Q1. What is the Hardy–Weinberg Principle?
Answer: Allele frequencies remain constant from generation to generation in the absence of evolutionary forces.

Q2. Define gene pool.
Answer: The total collection of all genes and their alleles in a population.

Q3. Write the Hardy–Weinberg equation.
Answer: p² + 2pq + q² = 1.

Q4. What does p represent?
Answer: Frequency of the dominant allele (A).

Q5. What does q² represent?
Answer: Frequency of homozygous recessive genotype (aa).

Q6. Name the five factors affecting Hardy–Weinberg equilibrium.
Answer: Gene flow, genetic drift, mutation, genetic recombination, and natural selection.

Q7. What is genetic drift?
Answer: Random changes in allele frequencies due to chance, especially in small populations.

Q8. What is the founder effect?
Answer: A form of genetic drift in which a small group establishes a new population with different allele frequencies.

NCERT Keywords for Board Exams

  • Hardy–Weinberg Principle
  • Genetic Equilibrium
  • Gene Pool
  • Allele Frequency
  • Genotype Frequency
  • Gene Flow
  • Gene Migration
  • Genetic Drift
  • Founder Effect
  • Mutation
  • Genetic Recombination
  • Natural Selection
  • Stabilising Selection
  • Directional Selection
  • Disruptive Selection

Most Important Equations

  • Allele frequencies: p + q = 1
  • Genotype frequencies: p² + 2pq + q² = 1

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