From the Scott Freeman textbook Biological Science 4th Edition: https://www.amazon.com/Biological-Science-4th-Scott-Freeman/dp/0321598202,
Multiple allelism is defined as "the existence of more than two alleles of the same gene within a population."
Polymorphism is defined as "the occurrence of more than two distinct phenotypes of a trait in a population."
I don't really understand the difference - unless it means that two alleles may not result in two different phenotypes?
Additionally, another definition of polymorphism also in the textbook was "the existence of more than one allele at a certain genetic locus."
What does this mean? I was under the impression that all genes had at least two alleles.