The Hardy-Weinberg principle is used in biology to predict allele frequencies and genotype frequencies in a population. It helps us decide whether a population is changing genetically or staying in equilibrium.
The key formulae are:
p + q = 1
p² + 2pq + q² = 1
Where:
For a population to remain in Hardy-Weinberg equilibrium, these conditions must apply:
In real populations, these conditions are rarely perfectly met. The principle is mainly used as a model for comparison.
In a population of flowers, 9% show the recessive phenotype (white flowers). White flowers must have the genotype aa, so this is the frequency of q².
| Information | Value | Meaning |
|---|---|---|
| q² | 0.09 | Frequency of homozygous recessive genotype (aa) |
| q | 0.3 | Frequency of recessive allele (a) |
| p | 0.7 | Frequency of dominant allele (A) |
| p² | 0.49 | Frequency of AA |
| 2pq | 0.42 | Frequency of Aa |
Step 1: Identify the recessive phenotype frequency, so q² = 0.09.
Step 2: Take the square root to find q.
q = √0.09 = 0.3
Step 3: Use p + q = 1.
p = 1 - 0.3 = 0.7
Step 4: Find genotype frequencies.
p² = 0.7² = 0.49
2pq = 2 × 0.7 × 0.3 = 0.42
Conclusion: In this population, the dominant allele frequency is 0.7 and the recessive allele frequency is 0.3. The expected genotype frequencies are 49% AA, 42% Aa, and 9% aa.
Most Hardy-Weinberg questions give you the proportion with the recessive phenotype. Start by treating that as q², then find q, then p, and finally calculate the genotype frequencies if needed.
Solve the Hardy-Weinberg question shown below.