Final answer:
Multiple alleles are more than two forms of a gene within a population, but an individual can only have two alleles for a gene. The ABO blood type system in humans is a classic example of multiple alleles, with three different alleles: IA, IB, and i. Population genetics studies how these alleles spread and vary in a population.
Step-by-step explanation:
Multiple alleles refer to the presence of more than two forms of a gene that dictate a particular trait within a population. While Gregor Mendel suggested that only two alleles exist for a given gene, one dominant and one recessive, we know today that many alleles can exist at the population level. However, any single individual can only carry two alleles for a specific gene, one from each parent. This is a characteristic of diploid organisms like humans.
An example of multiple alleles can be seen in the human ABO blood type system. Here, there are three alleles for the gene that determines blood type: IA, IB, and i. The IA and IB alleles are codominant, and both are dominant over the i allele. Though there are multiple alleles at the population level, each person inherits just two of these alleles. The combination of these two alleles in an individual leads to one of the possible blood types.
Population genetics is the study of how these alleles are distributed within a population, and it gives us insights into the genetic diversity and evolutionary processes of that population.