Pleiotropic genes are:

The correct answer is Option (3) → Genes with multiple phenotypic expression

Usually one gene controls one trait. In pleiotropy, a single gene has more than one phenotypic effect, usually because it affects a metabolic pathway that leads to several traits. Phenylketonuria is NCERT’s example: one mutated gene causes mental retardation and reduced hair and skin pigmentation.

Option (1) Polygenes — Incorrect. It is the opposite case: several genes control one trait, such as human skin colour.

Option (2) Complementary genes — Incorrect. Complementary genes are two genes that work together to give one trait; they are not single genes with many effects.

Option (3) Genes with multiple phenotypic expression — Correct. Pleiotropic genes are single genes that show multiple phenotypic expressions. In phenylketonuria, one gene mutation causes mental retardation and reduced hair and skin pigmentation.

Option (4) Genes with single phenotypic expression — Incorrect. That is an ordinary gene, the opposite 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). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.”