A bit of background: the light we see moves through the eye until it’s projected onto the retina, which consists of a thin layer of light-sensitive nerve cells called photoreceptors. The photons are absorbed by pigments in these cells, then converted into electro-chemical signals and sent to the brain.
There are two types of these photoreceptor cells: rods and cones. Our retina contains approximately 125 million rods, which are monochromatic so don’t perceive colour, but there are 6 million cones, which are sensitive to a wide range of brightness. There are three types of cones, sensitive to short, medium and long wavelengths of light respectively—blue cones, green cones and red cones.
Cones therefore provide the eye’s sensitivity to colour, and are responsible for sending colour information to the brain. This means that the 8% of males and 0.5% of females who are colour blind either don’t have a particular kind of cone, or one kind of cone might be weak.
There has been research focused on artificially replacing these cones in order to restore or add colour vision. For example, one study was able to take dichromat mice (with two cones) and breed them into trichromats (three cones), thereby improving their range of colour vision.
Another study worked with dichromat squirrel monkeys born red-green colourblind, which are far closer to human eyes and brains (in terms of colour vision). Using gene therapy that injected the missing photopigment gene into the monkeys’ retinas, researchers then demonstrated that the treated monkeys were successfully able to discriminate blue-green from red-violet—thus becoming trichromats.
So, since human genes were used, there’s definitely potential to treat human colour-blindness!