Phenylketonuria is an example of:

The correct answer is Option (4) → Pleiotropy

Option (4) Pleiotropy — Correct. In phenylketonuria a single gene mutation stops the enzyme that converts phenylalanine to tyrosine. This one gene change has many effects (mental retardation, less hair and skin pigment), so it is an example of pleiotropy.

NCERT Extract:
Pleiotropy – “We have so far seen the effect of a gene on a single phenotype or trait. There are however instances where a single gene can exhibit multiple phenotypic expression. Such a gene is called a pleiotropic gene. 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).”