Final answer:
The phenomenon where a single gene affects multiple phenotypic traits is called pleiotropy.
Step-by-step explanation:
The ability of a single gene to have multiple phenotypic effects is known as pleiotropy. This concept differs from incomplete dominance, multiple alleles, and epistasis, each of which involves interactions between different genes or between different alleles of the same gene in determining the phenotype of an organism. Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. An example of pleiotropy is the Marfan syndrome, where a single genetic mutation affects multiple systems in the body, including the heart, eyes, and skeletal system.