Opsins are proteins that are found in the light-sensing cells in the human eye. Different opsin proteins are sensitive to different colors of light. The instructions for creating the type of opsin sensitive to red light are found on a gene known as OPN1LW, which is on the X chromosome. A genetic mutation in the OPN1LW gene can lead to a form of color blindness that affects a person's ability to detect red light. However, women have two copies of the X chromosome, and they have normal color vision if only one copy contains the mutation. Why do women who carry one copy of the mutated gene still have normal color vision?