ACT Science: Use Hardy-Weinberg Equilibrium to Predict Allele Frequency

Hardy-Weinberg equilibrium: p²+2pq+q²=1. Here, p=frequency of dominant allele, q=frequency of recessive allele (p+q=1). The terms represent: p²=homozygous dominant individuals (AA), 2pq=heterozygous individuals (Aa), q²=homozygous recessive individuals (aa). If you know one frequency (e.g., q²=0.04, meaning 4% have the recessive phenotype), you can find all others. q=√0.04=0.2. p=1-0.2=0.8. p²=0.64 (64% homozygous dominant). 2pq=0.32 (32% heterozygous). The equation always balances: 0.64+0.32+0.04=1.00.

Use Hardy-Weinberg when a passage states a population's allele or phenotype frequency and asks you to predict genotype distributions. It's a mechanical calculation, not a logic puzzle.

Mistake 1: Confusing allele frequency with phenotype frequency. If 4% show the recessive phenotype, that's q², not q. You must take the square root. Mistake 2: Forgetting that p+q must equal 1. If p=0.7, then q=0.3, not something else. Mistake 3: Misidentifying which term corresponds to which genotype. Remember: p² is AA, q² is aa, 2pq is Aa. Write out the equation and label each term before substituting numbers.

Verification: Always check that p²+2pq+q²=1.00 after calculating all three terms. If it doesn't equal 1, you made a computational error.

Scenario: 1% of butterflies are white (recessive). What fraction are heterozygous (carrier) for the white allele? Solution: q²=0.01, so q=0.1, p=0.9. Heterozygotes: 2pq=2(0.9)(0.1)=0.18 or 18%. Check: p²=0.81, 2pq=0.18, q²=0.01. Sum: 0.81+0.18+0.01=1.00. Correct. Complete this calculation twice without looking, then redo it with different numbers until it's automatic.

Practice variation: If 9% of a population is homozygous recessive for a trait, find the percentage that are homozygous dominant and heterozygous. Solution: q²=0.09, q=0.3, p=0.7, p²=0.49 (49%), 2pq=0.42 (42%).

ACT Science tests Hardy-Weinberg because it's the foundation of population genetics and evolutionary biology. Understanding allele frequencies predicts how traits spread through generations. Expect 1-2 Hardy-Weinberg questions per Science section, and the equation method solves them both reliably.

Spend 20 minutes this week solving 5-6 Hardy-Weinberg problems with varying starting information. By test day, you'll solve these in under one minute each and collect points that feel like a gift.