Genotype frequencies in AP Biology

Genotype frequencies are the proportions of each genotype (like AA, Aa, aa) in a population. In AP Bio, you use them to calculate allele frequencies and to test whether a population is in Hardy-Weinberg equilibrium under topic 7.5.

What are genotype frequencies?

Genotype frequencies tell you what fraction of a population carries each genotype. If a wildflower population is 0.16 AA, 0.48 Aa, and 0.36 aa, those three numbers are the genotype frequencies, and they always add up to 1.0.

Under the Hardy-Weinberg model (EK 7.5.A.2), you can pull allele frequencies straight out of genotype frequencies. Count every allele: each homozygote contributes two of the same allele, each heterozygote contributes one of each. That's where the equation p² + 2pq + q² = 1 comes from. The p² term is the AA frequency, 2pq is the Aa frequency, and q² is the aa frequency. The big idea: if genotype frequencies stay the same generation after generation, the population is in equilibrium and not evolving. If they shift, something on the Hardy-Weinberg checklist got violated and evolution is happening.

Why genotype frequencies matter in AP® Biology

This lives in Unit 7 (Natural Selection), topic 7.5, and it's the engine behind learning objective AP Bio 7.5.A: describing the conditions under which allele and genotype frequencies change. Hardy-Weinberg equilibrium is a null hypothesis (EK 7.5.A.1), meaning it's the 'nothing is happening' baseline you compare real data against. The five conditions (large population, no migration, no new mutations, random mating, no natural selection) are never fully met in nature, but that's the point. When observed genotype frequencies drift away from the predicted ones, you have evidence that a real evolutionary force is at work. That connects directly to the unit's theme of evolution as measurable change in a population over time.

How genotype frequencies connect across the course

Genetic Drift (Unit 7)

Genetic drift is random change in allele frequencies, and it hits small populations hardest. After a drought wipes out most of a beetle population, the surviving genotype frequencies can shift purely by chance, violating the 'large population size' condition.

Gene Flow (Unit 7)

Gene flow is migration of alleles in or out of a population. New individuals bring new genotypes, so allowing migration directly changes genotype frequencies and breaks the 'no migration' Hardy-Weinberg condition.

Mutation (Units 6-7)

Mutation creates brand-new alleles, which adds genotypes that weren't there before. Even a rare new allele nudges genotype frequencies, which is why 'no new mutations' is on the equilibrium checklist.

Null Hypothesis (Unit 7)

Hardy-Weinberg equilibrium IS a null hypothesis. The expected genotype frequencies are your 'no evolution' prediction, and you test real data against them to see if a population is actually evolving.

Are genotype frequencies on the AP® Biology exam?

Expect MCQs that hand you genotype frequencies and ask you to interpret them. One common stem gives values like AA = 0.16, Aa = 0.48, aa = 0.36 and asks whether the population is in equilibrium (it is here, since 0.4² = 0.16). Another classic gives before-and-after frequencies, like beetles going from 0.36/0.48/0.16 to 0.49/0.42/0.09 after a drought, and asks what caused the shift (natural selection against the aa phenotype). You may also see a question where someone calculates p² + 2pq + q² = 0.96 instead of 1.0, and you explain the error (the genotype frequencies must sum to 1, so a value below 1 means a math or rounding mistake). Self-pollination questions test the 'random mating' condition. On FRQs, you may need to calculate allele frequencies from genotype frequencies and then argue which Hardy-Weinberg condition was violated, using the data as evidence.

Genotype frequencies vs allele frequencies

Genotype frequencies are the proportions of each genotype (AA, Aa, aa), while allele frequencies are the proportions of each allele (A, p, and a, q). You get allele frequencies FROM genotype frequencies: p = frequency of AA + half the frequency of Aa. Both must sum to 1, but they count different things.

Key things to remember about genotype frequencies

• Genotype frequencies are the proportions of each genotype in a population, and they always add up to 1.0.

• You can calculate allele frequencies from genotype frequencies, which is exactly what EK 7.5.A.2 expects you to do.

• In the Hardy-Weinberg equation, p² is the AA frequency, 2pq is the Aa frequency, and q² is the aa frequency.

• If genotype frequencies stay constant across generations, the population is in equilibrium and not evolving.

• A shift in genotype frequencies means one of the five Hardy-Weinberg conditions was violated, so evolution is occurring.

• Hardy-Weinberg equilibrium works as a null hypothesis: it's the 'no evolution' baseline you compare real data against.

Frequently asked questions about genotype frequencies

What are genotype frequencies in AP Bio?

They're the proportions of each genotype (like AA, Aa, and aa) in a population, and they sum to 1.0. In topic 7.5 you use them to calculate allele frequencies and to test whether a population is in Hardy-Weinberg equilibrium.

Do changing genotype frequencies always mean a population is evolving?

Yes. If observed genotype frequencies shift across generations, at least one Hardy-Weinberg condition was broken, which is the definition of evolution at the population level. A stable set of frequencies means the population is at equilibrium and not evolving.

What's the difference between genotype frequencies and allele frequencies?

Genotype frequencies count whole genotypes (AA, Aa, aa), while allele frequencies count individual alleles (A and a). You derive allele frequencies from genotype frequencies: p equals the AA frequency plus half the Aa frequency, and both totals add up to 1.

How do I get allele frequencies from genotype frequencies?

Add the homozygous frequency to half the heterozygous frequency. For example, if AA = 0.16 and Aa = 0.48, then p = 0.16 + 0.24 = 0.40, and q = 1 minus p = 0.60.

Why should genotype frequencies add up to 1?

Because every individual has exactly one genotype, so the proportions must cover the whole population. If a student calculates p² + 2pq + q² = 0.96 instead of 1.0, that's usually a rounding or math error, since the frequencies have to total 1.