I doubt that a lone gene determines a trait or characteristic. As you suggest, a number of select genes, using information transferred by proteins, express or repress traits. Some of the select may be influenced by other genes that the environment activates, and in this way traits are dependent upon both genetic codes and the physical reality. If we knew that a specific gene, either failing to be expressed or being prematurely expressed, resulted in undesirable circumstances, and we were able to replace that gene, why swap chromosomes?