The area at the back of the eye, called the retina, is sensitive to light and colour. It contains specialized cells, called cones, which respond to colour. There are three types of cone cells. One responds best to red light, one to green light, and one to blue light. When a specific type of cone cell doesn't work properly, a person will have trouble seeing the colour that particular cone cell responds to. For example, a person with red colour blindness has a defect in red cone cells.
Most colour blindness is inherited, although some cases are caused by an injury or disease of the retina or optic nerve, the nerve that takes information from the eye to the brain. People inherit colour blindness as a result of a defect on the gene(s) for colour located on the X chromosome.
Men inherit colour blindness 10 times as often as women do. Colour blindness "shows up" in men because they have only one X chromosome. Since women inherit two X chromosomes, a healthy gene on one X chromosome can override the unhealthy gene on the other. A woman can still have the unhealthy gene; it just doesn't always show up. She can, however, pass the gene to her children. A person who doesn't have a genetic condition like colour blindness but who can pass it to her children is called a "carrier."