When scientists cloned a housefly, they changed a single gene of the housefly  The cloned houseflies did not have ultraviolet vision because they lacked the eye cells that make the houseflies have ultraviolet vision  At the same time, the flies cloned in the conventional way (without changing the gene) had normal vision
The scientists thus showed that the housefly, which does not have ultraviolet vision, must have some kind of defect or damage in this gene
The housefly doesn't need other genes to make UV vision cells.