AP Biology · Exam guide
AP Biology Hardy-Weinberg Problems: Allele Frequencies Step by Step
Hardy-Weinberg questions give you one number and ask for another. The two equations and one habit, starting from the recessive phenotype, solve almost all of them. This guide gives the routine, a checked example, and the conditions that explain when the model fails.
The two equations
| Equation | Meaning |
|---|---|
| p + q = 1 | The two allele frequencies add to one. p is the dominant allele, and q is the recessive allele. |
| p^2 + 2pq + q^2 = 1 | The genotype frequencies: homozygous dominant, heterozygous, and homozygous recessive. |
A worked example
In a population in Hardy-Weinberg equilibrium, 16 percent of individuals show a recessive trait. Find the frequency of each allele and the percentage of heterozygous carriers. In a population of 500, how many carriers are there?
- q^2 = 0.16, so q = 0.40.
- p = 1 - 0.40 = 0.60.
- Carriers: 2pq = 2(0.60)(0.40) = 0.48, or 48 percent.
- In a population of 500, 0.48 x 500 = 240 carriers.
It treats the recessive phenotype frequency as the allele frequency. The phenotype frequency is q squared.
It starts from q squared and takes the square root.
The five conditions
- A very large population, so chance changes (genetic drift) are small.
- No gene flow in or out.
- No mutation.
- Random mating.
- No natural selection.
If a question asks why a population is not in equilibrium, name the condition that is broken and give the evidence for it in the situation.
Common mistakes
- Treating the recessive phenotype frequency as q.
- Forgetting to take the square root.
- Using the dominant phenotype frequency as p squared. Dominant individuals include carriers.
- Leaving out the calculation of the number of individuals when the question asks for a count.
Frequently asked questions
Why start from the recessive phenotype?
Individuals who show the recessive trait have a known genotype, so their frequency is q squared.
Is the dominant phenotype frequency p squared?
No. It is p squared plus 2pq.
What if the population is small?
Then genetic drift is likely and the model may not hold.
Do I need to know the equations?
They are given on the exam equation sheet, but you need to know what each term means.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every unit in the AP Biology course ends with a quiz of multiple-choice and short free-response questions, with a writing space that scores against the four-point rubric and a model response to compare against. These free lessons also build the skills the free-response section tests:
See the AP Biology course Free lessons and lesson list
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Multiple choice
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