In Hardy-Weinberg equilibrium, what do p and q denote?

• A. Allele frequencies
• B. Genotype frequencies
• C. Phenotype frequencies
• D. Mutation rates

Answer: (A)

p and q are the frequencies of the two alleles at a single gene locus in a population. They tell you how common each version of the gene is across all gene copies. In Hardy-Weinberg equilibrium, these allele frequencies stay constant from generation to generation (no evolution), and the genotype frequencies follow p^2 for the first homozygote, 2pq for the heterozygote, and q^2 for the second homozygote. The sum p + q = 1 because every allele bit in the population is one of the two options. For example, if p = 0.7 and q = 0.3, the predicted genotype frequencies are 0.49, 0.42, and 0.09, respectively. So p and q specifically denote allele frequencies, not genotype or phenotype frequencies or mutation rates.