In the context of Hardy-Weinberg equilibrium, what does allele frequency refer to in a population?

• A. The proportion of individuals in the population that are heterozygous for a gene (e.g., $2pq$).
• B. The proportion of all copies of a given gene that are a particular allele (e.g., the frequency of allele $A$ is $p$ and the frequency of allele $a$ is $q$, with $p+q=1$).
• C. The probability that two randomly chosen individuals will have the same genotype.
• D. The number of different alleles present at a locus in the population (allelic richness).

Answer: (B)