Final answer:
The phenomenon where a single gene affects multiple traits in an individual is known as pleiotropy, which differs from epistasis, where the expression of one gene is masked or modified by another.
Step-by-step explanation:
The phenomenon of a gene which has multiple phenotypic effects on an individual is referred to as pleiotropy. Unlike epistasis, where one gene masks or modifies the expression of another, pleiotropy involves a single gene influencing multiple traits or aspects of an organism's phenotype.
An example is the gene responsible for Marfan syndrome in humans, which can affect the skeletal system, eyes, and cardiovascular health concurrently. This is distinct from polygenic inheritance, where characteristics like skin color or height are determined by the collective influence of several genes acting together.