Mutations in the CNGA3, CNGB3, GNAT2, OPN1LW, OPN1MW, and OPN1SW genes cause color vision deficiency.
The retina, a light-sensitive tissue at the back of the eye, contains two types of light receptor cells called rods and cones. These cells transmit visual signals from the eye to the brain. Rods are responsible for vision in low light. Cones provide vision in bright light, including color vision. Three types of cones each contain a special pigment (a photopigment) that is most sensitive to a particular wavelength of light. The brain combines input from all three types of cones to produce normal color vision.
Specific genes provide instructions for making the three photopigments. The OPN1LW gene makes a pigment that is more sensitive to light at the red end of the visible spectrum, and cones with this pigment are sometimes called long-wavelength-sensitive or L cones. The OPN1MW gene makes a pigment that is more sensitive to light in the middle of the visible spectrum (yellow/green light), and cones with this pigment are often called middle-wavelength-sensitive or M cones. The OPN1SW gene makes a pigment that is more sensitive to light at the blue/violet end of the visible spectrum, and cones with this pigment are usually called short-wavelength-sensitive or S cones.
Genetic changes involving the OPN1LW and OPN1MW genes cause red-green color vision defects. These changes lead to an absence of L or M cones or the production of cones with abnormal visual properties that affect red-green color vision. Blue-yellow color vision defects result from mutations in the OPN1SW gene. These mutations inactivate the short-wave-sensitive pigment, which probably leads to the premature destruction of S cones or the production of defective cones. A loss of S cones impairs perception of the color blue and makes it difficult or impossible to detect differences between shades of blue and green.
Changes in the CNGA3, CNGB3, and GNAT2 genes are responsible for achromatopsia. Each of these genes provides instructions for making a protein that is involved in the normal function of cones in the retina. Mutations in any of these genes prevent all three types of cones from reacting appropriately to light. As a result, most people with mutations in one of these genes must depend on rods alone for vision. They typically have no color vision and often have other visual problems as well. Some people with mutations in CNGA3 have incomplete achromatopsia, which may allow some cone function and limited color vision.
A particular form of incomplete achromatopsia, called blue cone monochromacy, occurs when genetic changes prevent both L and M cones from functioning normally. People with this condition have only S cones. Because the brain must compare input from at least two types of cones to detect color, people who have only functional S cones have very poor color vision.
Some problems with color vision are not caused by gene mutations. These nonhereditary conditions, which are described as acquired color vision deficiencies, occur in people with other eye disorders. Specifically, acquired color vision deficiences can result from diseases involving the retina, the nerve that carries visual information from the eye to the brain (the optic nerve), or areas of the brain involved in processing visual information.
Read more about the CNGA3, CNGB3, GNAT2, OPN1LW, OPN1MW, and OPN1SW genes.