The phenomenon of pleiotropism refers to

The phenomenon of pleiotropism refers to a single gene affecting multiple phenotypic expressions. This occurs because genes do not act in isolation but can have cascading effects on many different biological 
 processes.
(1) When more than two genes collectively influence a single character or trait, it is referred to as polygenic inheritance or polygenic traits.

(2) When there are multiple alleles of a single gene involved in controlling a single crossover event, it is referred to as multiple allelism or multiple alleles. Multiple allelism occurs when there are more than two alternative forms of a gene (alleles) present in a population.

(3) When there is presence of two alleles, each of the two genes controlling a single trait, it is called gene interaction. There are various types of gene interactions, including epistasis, complementary gene interaction, duplicate genes etc.