Color blindness and genetics

DEFINITION: Color blindness is a condition in people whose eyes lack one or more of the three color receptors present in most human eyes. It is an important condition to understand because many people experience it to some degree. It is also a window into the inner workings of the eye and a marvelous example of the workings of Mendelian genetics.

Risk Factors

Approximately 1 percent of males and 0.01 percent of females are protanopes (lack L cones); 1.5 percent of males and 0.01 percent of females are deuteranopes (lack M cones); but only 0.008 percent of males and females are tritanopes (lack S cones).

Etiology and Genetics

Light-sensitive structures in the retina called cones are the basis for color vision. A person with normal vision can distinguish seven pure hues (colors) in the rainbow: violet, blue, cyan, green, yellow, orange, and red. People with typical color vision are said to have trichromacy, meaning they have three types of cones: L, M, and S, named for particular sensitivities to light of long, medium, and short wavelengths. The human visual system detects color by comparing the relative rates at which the L, M, and S cones react to light. For example, yellow light causes the M and L cones to signal at about the same rate, and the person “sees” yellow. Strangely, the right amounts of green and red stimulate these cones in the same fashion, and the person will again see the color yellow even though there is no yellow light present. Since people have only three types of color receptors, it takes the proper mix of intensities of only three primary colors to cause a person to “see” all the colors of the rainbow. A tiny droplet of water on the screen of a television or computer monitor will act like a magnifying lens and reveal that the myriad colors that are displayed are formed from tiny dots of only blue, green, and red.

People with dichromatic color vision have only two of the three types of cones. People with tritanomaly cannot distinguish between blue (especially greenish shades) and yellow. The genetic code for the S pigment lies on chromosome 7. The fact that the S pigment gene lies on an autosome explains why yellow-blue color blindness is manifested equally in males and females. The inheritance pattern is that of an autosomal dominant trait: Only one arm of the two arms of chromosome 7 has the defective allele in the affected parent, and since there is a 50 percent chance a child will receive the defective arm, 50 percent of the children will inherit the defect. In fact, the trait is often incompletely expressed, so that the majority of affected individuals retain some reduced S-cone function.

Deuteranomaly is more common than protanomaly. People with this color vision impairment need three primary colors to match the hues of the rainbow, but they match them with different intensities than other individuals with trichromacy because the peak sensitivities of their cones occur at wavelengths slightly different from normal. Their color confusion is similar to that of dichromacy, but less severe. About 1 percent of males and 0.03 percent of females have anomalous L cones, while 4.5 percent of males and 0.4 percent of females have anomalous M cones. The fact that far more males than females have some degree of red-green color blindness implies that the genetic information for the pigments in L and M cones lies on the X chromosome.

The gene structures for M-cone and L-cone pigments are 96 percent the same, so it is likely that one began as a mutation of the other. Small mutations in either gene can slightly shift the color of peak absorption in the cones and produce an anomalous trichromat. Generally, these mutations make M and L cones more alike. The similarity between the genes and the fact that they are adjacent to each other on the X chromosome can lead to a variety of copying errors during meiosis. People with typical color vision have one L-cone gene and one to three M-cone genes. The complete omission of either type of gene will result in severe red-green color blindness: protanopia or deuteranopia. Hybrid genes that are a combination of L-cone and M-cone genes lead to less severe types of red-green color blindness, especially if there is also a normal copy of the gene present.

Red-green color blindness follows an X-linked recessive inheritance pattern. Suppose a man has a defective X gene (and therefore has a color vision impairment) and a woman has typical color vision. Their male children have typical vision because they inherited their X genes from their mother, but their female children will be carriers because they inherited one X gene from their father. If the daughters married men with typical vision, 50 percent of the grandsons would inherit the defective gene from their mothers and have color vision deficiency, and 50 percent of the grandsons would have no vision impairment. Likewise, 50 percent of the granddaughters would have typical vision, and 50 percent would inherit the defective gene from their mothers and become carriers.

Symptoms

Dichromats can match all of the colors they see in the rainbow by mixing only two primary colors of light, but they see fewer (and different) hues in the rainbow than a person with typical vision. Protanopes and deuteranopes cannot distinguish between red and green. More exactly, protanopes tend to confuse reds, grays, and bluish blue-greens, while deuteranopes tend to confuse purples, grays, and greenish blue-greens.

Screening and Diagnosis

The test most often used to diagnose red-green color blindness is called the Ishihara color test. It consists of a series of pictures of colored spots in which a figure (such as a number or symbol) is embedded using a slightly different color. Those with color vision can easily distinguish the figure within the image, but those with color deficiencies cannot.

Treatment and Therapy

While color vision deficiencies cannot be cured, devices such as tinted filters or contact lenses can help an individual distinguish between different colors. Their practical use is somewhat limited, however. Computer software and cybernetic devices can also aid those born with this condition. Gene-therapy research may offer insight into treatments.

Prevention and Outcomes

Color vision deficiencies cannot be prevented, but those born with it may manage so well that they may be unaware of the condition if not tested for it.

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