Hardy-Weinberg Principle and Allele Frequencies

This lesson covers the Hardy-Weinberg principle and how it is used to calculate allele and genotype frequencies in populations, as required by the Edexcel A-Level Biology specification (9BI0, Topic 8).

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What Is the Hardy-Weinberg Principle?

The Hardy-Weinberg principle states that in a large, randomly mating population with no evolutionary forces acting upon it, allele frequencies and genotype frequencies will remain constant from generation to generation.

This principle provides a mathematical model of a population in genetic equilibrium — a baseline against which real populations can be compared.

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Hardy-Weinberg Equations

For a gene with two alleles (A and a):

Allele Frequency Equation

$p + q = 1$p+q=1

Where:

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

Genotype Frequency Equation

$p^2 + 2pq + q^2 = 1$p2+2pq+q2=1

Where:

• p² = frequency of homozygous dominant (AA)
• 2pq = frequency of heterozygous (Aa)
• q² = frequency of homozygous recessive (aa)

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Conditions for Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle holds only when ALL of the following conditions are met: