In a population of 500 beetles, 320 are green (dominant) and 180 are brown (recessive). Assuming Hardy-Weinberg equilibrium, what is the approximate frequency of the dominant allele G?

The frequency of homozygous recessive individuals $q^2 = 180/500 = 0.36$, so the frequency of the recessive allele $q = \sqrt{0.36} = 0.60$. Since allele frequencies must sum to 1, the frequency of the dominant allele $p = 1 - q = 1 - 0.60 = 0.40$. Common errors include confusing p and q or using the phenotype frequency directly as the allele frequency.