What phenomenon occurs when a single gene influences multiple traits?

The phenomenon that occurs when a single gene influences multiple traits is known as pleiotropy. In pleiotropy, one gene can produce effects on various, seemingly unrelated traits. This is an important concept in genetics because it highlights how the action of a single gene can have far-reaching consequences on an organism's phenotype.

For example, in humans, the gene responsible for Marfan syndrome affects connective tissue, which can lead to a variety of traits including tall stature, long limbs, and issues with heart valves and vision. This illustrates how one gene can impact multiple physical characteristics.

In contrast, polygenic inheritance refers to the situation where multiple genes contribute to a single trait, like height or skin color, which does not capture the definition of a single gene affecting multiple traits. Similarly, epistasis involves interactions between two or more genes where one gene can mask or modify the expression of another, and codominance refers to a situation where both alleles in a genotype are fully expressed in the phenotype, such as in ABO blood types. Neither of these options pertain to the influence of a single gene on multiple traits, reinforcing the concept of pleiotropy.