For a particular gene locus, what changes in allele frequency, if any, occur when there is no selection at that locus? The concept of Hardy-Weinberg equilibrium states that when individuals have the same fitness no matter which alleles they possess, allele frequencies will not change from generation to generation. Imagine there are two alleles, B and b, at a gene locus. In a population we find that 70% of the alleles are B (let us call that frequency p) and 30% are b (frequency q). According to Hardy-Weinberg, the frequencies of the two alleles will remain at 70% B and 30% b every generation. The equilibrium concept also states that there is a specific relationship between allele frequencies and genotype frequencies. The frequency of the BB genotype is p² , (0.7)² , the frequency of the Bb genotype is 2pq, 2 x 0.7 x 0.3 = 0.42, and the frequency of the bb genotype is q², (0.3)² = 0.09. If allele and genotype frequencies do not conform to Hardy-Weinberg expectations, then we know that some process, such as selection, is operating at that locus.