Up to one in twelve men inherit red–green color blindness
Color vision deficiency ranges from a mild shade mix-up to seeing no color at all. The usual culprit is a sex-linked quirk in retinal cone cells. Congenital red–green forms, including protan and deutan types, affect as many as one in twelve men and far fewer women.
Colour vision rests on three classes of cone cell in the retina, and colour blindness is what happens when one or more of them is missing or misbehaves. Most cases are inherited through a sex-linked gene, which is why the commonest form, congenital red–green deficiency, touches about 8% of men but only about 0.5% of women. Severity runs from barely noticeable to no colour perception at all, and it is a popular mistake to think every colour-blind person sees in greyscale. Nearly all are affected only along the red–green axis.
That red–green group splits by which cone is faulty. Deutans, about 6% of men, have trouble with the medium-wavelength cone; protans, about 2%, with the long-wavelength cone, and for them reds also look much dimmer. Each type has a so-called neutral point, a cyan wavelength near 492 or 498 nanometres that looks identical to white. Blue–yellow deficiency, tied to the short-wavelength cone, is far rarer, affecting under 0.01% of people, and is more often acquired than inherited.
Grading follows a scale. Anomalous trichromats still use three cones and may barely notice a problem. Dichromats can match any colour with just two primaries and usually know something is off. Monochromats have a single colour channel and see only brightness; in rod monochromacy the retina has no cones at all, which also makes ordinary daylight hard to bear. A separate brain condition, cerebral achromatopsia, strips colour from perception even when the eyes work.
Often the first clue is a child painting an object the wrong colour. Plate tests such as the Ishihara usually confirm it. Most causes have no cure, though gene therapy is being studied for severe forms, and depending on the country, colour-blind people can be barred from flying aircraft, driving trains, policing, firefighting or military service.
Source: Color blindness
