What is the phenomenon called when a single gene has multiple phenotypic effects?

Pleiotropy

The correct answer is Option (1) –Pleiotropy

Pleiotropy: The term pleiotropy is derived from the Greek words pleio, which means "many," and tropic, which means "affecting." When a single gene can exhibit multiple phenotypic expression. Such a gene is called a pleiotropic gene.
Pleiotropy should not be confused with polygenic traits, in which multiple genes converge to result in a single phenotype. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes. An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.