What does pleiotropy refer to in genetics?

Pleiotropy is a key concept in genetics that describes a situation in which one gene influences multiple phenotypic traits. This is significant because a single gene can have various effects on an organism’s overall biology, leading to multiple observable characteristics. For instance, a mutation in a single gene can affect not only one feature but potentially several traits across different systems, which can be observed in various genetic conditions. This is a critical aspect of understanding how genes interact and contribute to the complexity of an organism’s phenotype.

The other options focus on different genetic principles. The first option describes polygenicity, where multiple genes contribute to a single trait rather than one gene affecting multiple traits. The third option refers to gene mutations that lead specifically to disease but does not encompass the broader definition of pleiotropy. The fourth choice discusses the inheritance of traits, which relates to Mendelian genetics but is outside the specific definition of pleiotropy. Thus, "one gene controls the expression of several phenotypic traits" accurately captures the essence of pleiotropy.