Final answer:
One person having two different alleles and the presence of another different allele in the population exemplifies polymorphism, which is core to population genetics, especially shown in the multiple alleles of the human ABO blood type system.
Step-by-step explanation:
If one person in a population has two different alleles of a particular gene, and it's known that other people in the population have yet another different allele of this gene, then this is an example of polymorphism. Polymorphism is a situation in which a gene has two or more alleles in a population at frequencies greater than 1 percent. This concept is fundamental in the field of population genetics, where scientists study how varying allele and genotypic frequencies are influenced by selective forces within a population.
An example of polymorphism is the ABO blood type system in humans, where the alleles IA, IB, and i code for different blood-type carbohydrates on the surface of red blood cells. These alleles demonstrate multiple alleles existing at the population level, although any one individual can only carry two of these alleles. This diversity of alleles contributes to the genetic variation seen in populations.