In a population, if the proportion of individuals who have sickle-cell disease is 0.25, and the population is assumed to be at Hardy-Weinberg equilibrium, what is the expected frequency of the HbS allele? (Hint: What is the genotype of people with sickle-cell disease, and how is that genotype represented in the Hardy-Weinberg equation?) a. 0.125
b. 0.25
c. 0.5
d. 0.75
