Colour Blindness in Human beings

Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired. "Color blind" is a term of art; there is no actual blindness but there is a fault in the development of either or both sets of retinal cones that perceive color in light and transmit that information to the optic nerve. The gene that causes color blindness is carried on the X chromosome, making the handicap more common among men (who have just one X chromosome) than among women.

Color blindness can be inherited. It is most commonly inherited from mutations on the X chromosome but the mapping of the human genome has shown there are many causative mutations – mutations capable of causing color blindness originate from at least 19 different chromosomes and 56 different genes.

Types:

There are many types of color blindness. The most common are red–green hereditary photoreceptor disorders, but it is also possible to acquire color blindness through damage to the retina, optic nerve, or higher brain areas. Acquired color blindness is generally unlike the more typical genetic disorders. For example, it is possible to acquire color blindness only in a portion of the visual field but maintain normal color vision elsewhere. Some forms of acquired color blindness are reversible. Transient color blindness also occurs (very rarely) in the aura of some migraine sufferers.

Red-green color blindness

This term combines four different types of color blindness. Protanomaly and protanopia are caused by defective or even missing L-cones (long-wavelengths). In opposite defective or missing M-cones (medium-wavelengths) are the source of deuteranomaly or deuteranopia. The genes encoding the L- and M-cone photopigments are located side by side on the X chromosome. Because of the genes are highly homologous and adjacent to one another, recombinations between them is common and can lead to anomalous pigments.

Blue cone monochromacy

As this type of monochromacy is caused by a complete absence of L- and M-cones, blue cone monochromacy is encoded at the same place as red-green color blindness on the X chromosome.

Blue-yellow color blindness

Tritanomaly and tritanopia which are commonly referred to as blue-yellow color blindness are caused by defective or missing S-cones (short-wavelength). These photopigments are encoded in genes which reside on chromosome 7, an autosomal chromosome. This is why blue-yellow color blindness occures at the same rate on both sexes.

Rod monochromacy

The total loss of color vision is called rod monochromacy or complete achromatopsia. In this case the retina does not have any cone cells at all. It is known to be an autosomal recessive disease and can be provoked by different circumstances. Recent studies show that it can be encoded on chromosome 2 as well as on chromosome 8. Earlier studies assigned chromosome 14 to rod monochromacy but this could not be reconstructed.

Chromosomes Involved in Color Blindness

Human beings have 23 pairs of chromosomes. Out of these 23 pairs 22 are autosomal chromosomes which are equal in both sexes and encode body functions. Only one pair consists of two sex-chromosomes which are different for men and women. The 22 pairs of equal chromosomes are numbered from 1 through to 22. The sex-chromosomes are labeled with X and Y, whereas women carry the combination XX and men the combination XY. This all sums up in a total of 46 chromosomes which make the human genome.

Diagnosis

Example of an Ishihara color test plate.

The numeral "74" should be clearly visible to viewers with normal color vision. Viewers with dichromacy or anomalous trichromacy may read it as "21", and viewers with achromatopsia may not see numbers.

An Ishihara test image as seen by subjects with normal color vision and by those with a variety of color deficiencies.

The Ishihara color test, which consists of a series of pictures of colored spots, is the test most often used to diagnose red–green color deficiencies. A figure (usually one or more Arabic digits) is embedded in the picture as a number of spots in a slightly different color, and can be seen with normal color vision, but not with a particular color defect. The full set of tests has a variety of figure/background color combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described above, is also used in diagnosing anomalous trichromacy.

Because the Ishihara color test contains only numerals, it may not be useful in diagnosing young children, who have not yet learned to use numerals. In the interest of identifying these problems early on in life, alternative color vision tests were developed using only symbols (square, circle, car).

Most clinical tests are designed to be fast, simple, and effective at identifying broad categories of color blindness. In academic studies of color blindness, on the other hand, there is more interest in developing flexible tests to collect thorough datasets, identify copunctal points, and measure just noticeable differences.

Problems:

Colour blind often confuse red and green items. For example, they may find it difficult to distinguish a Braeburn apple from a Granny Smith and in some cases, the red and green of a traffic light without other clues (for example, shape or location). The vision of dichromats may also be compared to images produced by a color printer that has run out of the ink in one of its three color cartridges (for protanopes and deuteranopes, the magenta cartridge, and for tritanopes, the yellow cartridge). Dichromats tend to learn to use texture and shape clues and so are often able to penetrate camouflage that has been designed to deceive individuals with color-normal vision.