Final answer:
The sum of p and q is 1 because p represents the frequency of the dominant allele and q represents the frequency of the recessive allele. These two alleles make up all of the alleles for that locus found in the population.
Step-by-step explanation:
Population geneticists represent different alleles as variables in their mathematical models. The variable p typically represents the frequency of the dominant allele, while q represents the frequency of the recessive allele. If these are the only two possible alleles for a given trait in the population, p + q = 1.
If we know the allele frequencies p and q, we can predict the frequencies of different genotypes. The frequency of homozygous dominant individuals is p², the frequency of heterozygous individuals is 2pq, and the frequency of homozygous recessive individuals is q². Together, these genotype frequencies sum to one: p² + 2pq + q² = 1.