Color Blindness, Red-Green, Partial
Patient Information
Characteristics
Background and History
The human eye is capable of detecting about a million colors and does so using the responses from only three types of light receptor cells, called cones, in the retina (the light-sensing tissue in the eye). Each type of receptor responds to either blue, red, or green light but it is the relative intensity of the responses when integrated in the brain that makes such color discrimination possible.
Defects in color perception have been known for centuries. While numerous disorders that impact the retina can interfere with the detection of colors, most also cause other vision problems. Certain drugs such as digitalis can also modify color perception. In the heritable type described here, though, red and green colors specifically are difficult to distinguish while vision is otherwise normal.
Clinical Correlations
Defects in red-green color perception are the most common type of color blindness in humans. These are often divided into several types depending on the degree and nature of the color defect. A defect in seeing red colors is called protanopia, while difficulty in seeing green colors is called deuteranopia. Many individuals have a more mild form of the red and green color deficiencies and these are called protanomaly and deuteranomaly with the latter being the most common, affecting about 5% of males. Together, these affect about 8% of males and 0.5% of females.
Most individuals with color vision anomalies function without difficulty. Some, in fact, are unaware of difficulties in color perception. For others, occupational choices are impacted because of work-related requirements.
There are no other health problems associated with red-green color vision deficits.
Genetics
Inheritance
The genes responsible for red-green color perception are located on the X chromosome. Hence, males are primarily affected although a few females have color blindness. Males with red-green color blindness do not usually have children with the same problem although they do pass the mutation-containing X chromosome to all their daughters who have another normal X chromosome and therefore are not color blind. These daughters, however, can expect that half their sons will have the same color blindness as their mother’s father.
Pedigree
X-linked recessive, father affected
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
X-linked recessive, carrier mother
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
Diagnosis and Prognosis
The diagnosis can usually be made by an ophthalmologist by testing with simple color charts. There are no associated health problems elsewhere in the body and individuals can look forward to a healthy, normal lifespan. There is no treatment for color blindness but tinted lenses may be helpful for selected tasks.
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