Something that affects the structure or function of an organism is referred to as

Something that affects the structure or function of an organism is referred to as a modifier.

Explanation:

Cells are the fundamental unit of life, grouped through a series of organization. As basic histology explains the primal hierarchy between each organized organism has basic unit called cells. Cells when grouped form tissues, when group of tissues are formed together they make organs, some organisms skip from organs to organs systems to form an existing organism as a whole. Describing how is the cells group is the process of multiplication of cell or the so-called cell division, mitosis. This mechanism of each cell produces another cell that binds together and produces again, repeatedly, to be a larger system called now the tissue as explained earlier. How? Through cell division called, mitosis. Excerpt meiosis –cell division specifically in the sex cells. 

football or basketball players.