Classification of Color Vision Variations
Defects in color vision can be divided into two groups. The first, the congenital color variations, is a result of heredity. The defective gene for color vision is passed in the X chromosome of the mother in a "X-linked recessive" fashion. By the rules of Mendelian genetics, a male (XY) with the defective chromosome will express a color variation, but a female (XX) with one defective chromosome will still have normal color vision. For a female to be affected, both X chromosomes would have to be defective, and this is therefore more rare.
The second group, acquired color variations, results from the loss of color vision due to aging, or disease processes such as diabetes or glaucoma. These variations are not inherited; color vision is initially normal but changes over time.
The differences between congenital and acquired color variations are many. Congenital variations are stable over time, have been fully characterized, and also display typical and reproducible patterns on color vision testing. Acquired variations are variable over time, are not easily characterized, and are not typical or reproducible with color vision testing. Congenital variations affect both eyes equally, whereas aquired variations will usually affect one eye more than another.
Congenital Color Variations
Understanding the classification of congenital color variations requires some knowledge of visual physiology and anatomy. The highly specialized cells responsible for both visual acuity and color perception are called cones and are of three types in the central retina. They have traditionally been termed the "red", "blue" and "green" cones, but this is a simplistic view, as each type of cone is sensitive to a wide range of wavelengths. Each type of cone does have a peak of sensitivity to hue, however, and these peaks correspond roughly to the familiar red, green and blue hues. More accurately, these cones are termed the long, middle and short wavelength sensitive (LWS, MWS, SWS) cones.
Alterations in color vision occur when the visual pigment in the LWS, MWS, or SWS cone, called opsin, is changed as a result of the expression of a faulty gene. The addition of anomalous proteins into the opsin structure presumably alters its spectral sensitivity and gives rise to variations in color vision.
Classification of Congenital Color Variations
People with normal color vision require three primary colors to match any color, and are termed normal trichromats. As a result of anomalies in the opsin molecule, a group of people known as anomalous trichromats will still require three primaries to match the unknown color, but the amount of each primary will differ from that given by normal trichromats. Another group of color variants, termed dichromats, requires only two primary colors to match any color. This condition is thought to result from the lack of expression of one of the three opsin molecules specific for the LWS, MWS, or SWS. Visual acuity for dichromats is not affected, because the affected cone is filled with opsin from one of the other two cones.
A fourth group, termed monochromats, are truly "color blind", needing only one primary to match any color. The rod monochromats suffer poor visual acuity due to loss of the cones themselves. In this case, vision is dependent on the rods. The cone monochromats have only one cone type, and have better visual aquity, but are functionally similar to rod monochromats.
Depending upon the particular type of cone affected, trichromats and dichromats are further classified as protan (LWS cone altered), deutan (MWS cone altered), or tritan (SWS cone altered). The tritan defect is extremely rare, and has a different inheritance pattern (autosomal dominant) from the more common deutan and protan defects (X - linked recessive). A fourth class, termed tetartan, has been proposed on theoretical grounds, but no case of a congenital tetartan defect has ever been documented. Tetartan defects have been noted in certain acquired color variations, however.
Deutan and protan defects involve losses in discrimination in areas of red and green hues, and are thus described as being red - green defects. Tritan (and presumably tetartan) defects involve losses in discrimination in areas of blue and yellow hues, and are thus described as being blue - yellow defects. Anomalous trichromats are sub-classed as either protanomalous, deuteranomolous, or tritanomalous trichromats. Dichromats are sub-classed as either protanopic, deuteranopic, or tritanopic dichromats.
Classification of Acquired Color Variations
Acquired defects are more difficult to classify, but it has been known from the early 1900's that these variations can be described in terms similar to congenital defects. The axis of color discrimination loss generally determines the type of defect, and there are three types.
In the type I acquired red - green defect, loss of discrimination occurs along the red - green axis. It is seen typically in patients with retinal diseases which affect retinal layers including the rods and cones. In the type II acquired red - green defect, loss of discrimination also occurs along the red - green axis, and is associated with optic nerve disease. In the type III acquired blue -yellow defect, loss of discrimination occurs along the blue - yellow axis, and is associated with disease of the retina, choroid and optic nerve. It is also commonly seen with aging in nuclear sclerotic cataracts.
Classification, Inheritance, and Incidence of Color Vision Variations
Classification Inheritance Incidence (% males)
Pratanomalous X - linked recessive 1.0
Deuteranomolous X - linked recessive 5.0
Tritanomalous autosomal dominant 0.0001
Protanopic X - linked recessive 1.0
Deuteranopic X - linked recessive 1.0
Tritanopic autosomal dominant 0.001
Rod autosomal recessive 0.0001
Cone X - linked recessive very rare
type I red - green none
type II red - green none
type III blue - yellow none
Among women, the occurrence of congenital color variations is found to be about 0.4 %. The tritan defects are rare, and affect men and women equally. As can be seen from the table above, the deutan defect is the most prevalent congenital defect, at six percent of the european male population. The protan defect is next at two percent of the male population.