The phenomenon described in the question is called Epistasis. In genetics, epistasis occurs when the expression of one gene is suppressed or masked by the presence of another gene at a different locus. This interaction affects the phenotypic outcome in a way that one gene can override or alter the expression of another gene.

To clarify the other options:
- Pleiotropy refers to a single gene influencing multiple, seemingly unrelated phenotypic traits.
- Codominance is a situation where both alleles in a heterozygote are fully expressed, resulting in a phenotype that simultaneously shows characteristics of both alleles.
- Polygenic Inheritance involves multiple genes contributing additively to a single trait, such as height or skin color, without one gene masking the other.

Thus, the key point is that in epistasis, one gene's expression can mask or suppress another gene, which distinguishes it from the other genetic phenomena.

Reference: Griffiths, A.J.F., Wessler, S.R., Carroll, S.B., et al. Introduction to Genetic Analysis, 11th Edition, Chapter 5, Page 180.