Correct Option: D

When a single gene has an effect on the expression of two or more phenotypic traits, it is said to have a pleiotropic effect on the traits. For example, testosterone controls the development of what are referred to as secondary sexual characteristics, but it also relates to behavioral traits like aggression. Thus, a gene that controls the levels of testosterone would have a pleiotropic effect on the expression of many secondary sexual traits which are morphological, as well as behavioral.