In Part A, you looked at a single genetic cross involving two parents of genotype Rr. Imagine now that instead of a single mating, you consider all the matings that occur in a population, and all the offspring that are produced.

The figure at right shows a population of flowers with two alleles for color, a red allele CR and a white allele CW . The allele frequencies across the entire population are 80% CR and 20% CW . In other words:

The frequency of the CR allele, called p, is 0.8.

The frequency of the CW allele, called q, is 0.2.

If the population is not evolving, then the population is said to be in Hardy-Weinberg equilibrium. In this case, the Hardy-Weinberg principle tells us that offspring inherit alleles as if they were drawn from the gene pool at random. This means that:

The proportion of individuals with genotype CRCR is expected to be p2.

The proportion of individuals with genotype CRCW is expected to be 2pq.

The proportion of individuals with genotype CWCW is expected to be q2.

Furthermore, if a population is in Hardy-Weinberg equilibrium, the allele frequencies (p and q) and the genotype frequencies stay the same from one generation to the next.