Pleiotropy refers to a phenomenon where:

The correct answer is Option (1) → A single gene exhibits multiple phenotypic traits

Usually we think of one gene as controlling one trait. In pleiotropy a single gene shows more than one phenotypic effect. This mostly happens because the gene acts on a metabolic pathway that leads to several phenotypes. Phenylketonuria is the book’s example: one mutated gene causes both mental retardation and less pigment in hair and skin.

Option (1) A single gene exhibits multiple phenotypic traits — Correct. This is pleiotropy: one gene, many phenotypic traits.

Option (2) Multiple genes control a single trait — Incorrect. When several genes together control one trait, as in human skin colour, it is polygenic inheritance, the reverse of pleiotropy.

Option (3) Genes segregate independently — Incorrect. Genes for different traits sorting into gametes independently of each other is Mendel’s law of independent assortment. It says nothing about one gene affecting many traits.

Option (4) Only environmental factors determine traits — Incorrect. Traits are controlled by genes. The environment can influence how some of them show up, as the book says for polygenic traits, but it does not decide traits on its own. This is not 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.”