Final answer:
The term for having more than two alleles for a gene in a population is multiple alleles, which leads to a wide array of genetic combinations and phenotypes beyond the basic Mendelian two-allele model, illustrated by human blood types and rabbit coat colors.
Step-by-step explanation:
Any number of alleles greater than two is classified as multiple alleles. Contrary to Mendelian genetics suggesting genes have just two alleles (one dominant, one recessive), multiple alleles can exist at the population level. For instance, in humans, the ABO blood type illustrates this concept with three different alleles: IA, IB, and i. While an individual can only inherit two of these alleles, the population can carry all variations.
The multiplicity of alleles allows for a wider range of genetic variation and hereditary possibilities, enhancing population diversity. It's essential to recognize that while multiple alleles can occur within a population, individual diploid organisms such as humans only possess two alleles for any given gene. Depending on their dominance relationships, these alleles can produce a variety of phenotypes, like the coat color in rabbits or the ABO blood types in humans.