What is the difference between multiple alleles and polygenic traits?

Multiple alleles refer to the presence of more than two alleles for a gene while polygenic trait is one that is regulated by alleles of more than one different gene.

Explanation:

When a gene has more than two alternating forms, it is said to be having multiple alleles. These alleles occupy the same position on chromosomes. I^A, I^B and i are the three alleles of the ABO blood type gene. On the other hand, when a trait is controlled by more than one gene, it is said to be a polygenic trait. The alleles of different genes that regulate the polygenic trait are present on the different positions on the chromosomes. For example, human skin color is controlled by more than one gene and is a polygenic trait.