The correct option is option (a) the frequency of the recessive allele in a population.
Hardy- Weinberg equation is a mathematical equation that helps in the calculation of the genetic variation in a population at equilibrium. It was proposed by G.H. Hardy and Wilhelm Weinberg independently in the year 1908. It states that the alleles (two alternative forms of gene) and the genotype (genetic composition) frequencies in a population will remain constant through generations when the evolutionary forces like the mutation, genetic drift, natural selection, gene flow and other forces are absent.
The equation is p2 +2pq + q2 = 1 [square of p + 2 x pq + square of q = 1], where ‘p’ represents the frequency of the dominant allele in the population, ‘q’ represents the frequency of the recessive allele in a population and 2pq represents the frequency of the heterozygous genotype. Therefore, p2 represents the frequency of homozygous dominant genotype and the q2 represents the frequency of homozygous recessive genotype.
Thus, the letter q in the Hardy-Weinberg equation represents the frequency of the recessive allele in a population.