Hardy-Weinberg Equilibrium: Population Genetics

What is Hardy-Weinberg Equilibrium?

The Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant from generation to generation — provided five conditions are met.

Learning Goals:

1. State the five conditions for Hardy-Weinberg equilibrium.
2. Calculate allele frequencies from phenotype data.
3. Identify which conditions are violated in real populations.

Five Conditions

1. No mutation — Alleles don't change.
2. Random mating — No mate preference for genotype.
3. No natural selection — All genotypes equally fit.
4. Large population — No genetic drift.
5. No migration — No gene flow in or out.

If any condition is violated, the population evolves (allele frequencies change).

Worked Examples

Example 1: Basic Calculation

In a population of 500, 20 individuals have the recessive phenotype (aa).

1. $q^2 = 20/500 = 0.04$
2. $q = \sqrt{0.04} = 0.2$
3. $p = 1 - 0.2 = 0.8$
4. Heterozygous frequency: $2pq = 2(0.8)(0.2) = 0.32 = 32\%$
5. 160 individuals are carriers (Aa).

Example 2: Cystic Fibrosis

CF affects ~1 in 2500 people. $q^2 = 1/2500 = 0.0004$ → $q = 0.02$ → $p = 0.98$ → Carrier frequency: $2pq = 2(0.98)(0.02) = 0.0392 ≈ 1 \text{ in } 25$

Common Mistakes

1. Forgetting to square-root — $q^2$ is the recessive phenotype frequency; $q$ is the allele frequency. Square root first!
2. Using phenotype ratios as allele frequencies — Only $q^2$ can be directly observed (recessive phenotype).
3. Assuming populations are in equilibrium — In reality, no population perfectly meets all five conditions.

Exam Tips

• Start with the recessive phenotype because it's the only genotype you can identify directly from the phenotype.
• Show all steps: $q^2 → q → p → 2pq → p^2$.
• You must state which HW condition is violated when explaining evolution.

Related Topics

• Mendelian Genetics — Individual-level inheritance patterns.
• Pedigree Analysis — Tracking alleles through families.