Use the Hardy–Weinberg principle
Calculate allele and genotype frequencies and state the conditions for Hardy–Weinberg equilibrium.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What equation relates allele frequencies for two alleles?
What you will learn
- Use p+q=1 and p²+2pq+q²=1 to calculate frequencies
- State the population conditions required for the model
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Counts | A 11 / B 7 / AB 4 / O 3 |
| Total | 25 invented records |
| Percentages | 44% / 28% / 16% / 12% |
| Graph | Named separated bars, not length intervals |
Build the evolutionary explanation
For two alleles with frequencies p and q, p+q=1. Under Hardy–Weinberg equilibrium, genotype frequencies are p² for homozygote one, 2pq for heterozygotes and q² for homozygote two, summing to one. The model assumes a very large population, random mating, no mutation, no migration and no natural selection. It provides a baseline for detecting whether allele frequencies may be changing.
Interpret data with care
The equations are supplied in the syllabus. Check whether a question gives allele or genotype frequencies and whether values are proportions or percentages. A deviation from expectation suggests one or more assumptions may not hold; it does not identify which condition failed.
Pause and explain
Observed genotype frequencies differ significantly from Hardy–Weinberg expectations. What can be inferred?
Worked example
If q=0.2, calculate p and expected genotype frequencies.
Show the worked solution
- p=1−q=0.8.
- Calculate p², 2pq and q².
Answer p=0.8; p²=0.64, 2pq=0.32 and q²=0.04.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For p=0.7 and q=0.3, calculate expected genotype frequencies.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Use p², 2pq and q².
- Check that the three values sum to one.
p²=0.49, 2pq=0.42 and q²=0.09; they sum to 1.00.
Look for these in your work
- Population process identified
- Cause and consequence linked
A population experiences migration. Which Hardy–Weinberg condition is violated?
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- Migration adds or removes alleles.
- The model assumes no migration.
The no-migration condition is violated, so allele frequencies can change through gene flow.
Look for these in your work
- Evidence interpreted
- Assumptions stated
Observed genotype frequencies differ significantly from Hardy–Weinberg expectations. What can be inferred?
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- At least one model condition may not hold.
- The result alone does not identify which process caused the deviation.
The population may not meet one or more Hardy–Weinberg assumptions; the deviation alone does not establish whether selection, non-random mating, migration, mutation or drift caused it.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Mixing percentage and proportion forms
- Assuming a mismatch identifies the exact evolutionary force
What can you explain now?
What equation relates allele frequencies for two alleles?
Compare with the explanation
p+q=1
The allele frequencies sum to one.
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Make it stick
What do p, q, p², 2pq and q² represent?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.