Clinical Characteristics
Ocular Features
Human color vision is trichromatic and requires the normal function of three classes of cones responding to wavelengths of approximately 420nm (blue cones), 530 nm (green cones), and 560 nm (red cones). Dichromatic color vision discussed here is based on responses of red and green cones whose pigments are generated from contiguous gene regions on the X chromosome encoding OPN1MW (green pigment), and OPN1LW (red pigment).
The degree of color deficiency is variable and some males are so mildly affected that they are unaware of any defect until tested. The human eye is capable of seeing about a million colors which is made possible in part by the wide range of comparative signal outputs from the three classes of cones. In addition, the ratio of red and green cones varies among individuals and these factors collectively influence how each individual interprets the spectrum of wavelengths that enter the eye. The phenotype of red-green color blindness is highly variable.
Four subclasses of red-green color vision defects are recognized:
Protanopia – only blue and green cones are functional (1 percent of Caucasian males)
Deuteranopia – only blue and red cones are functional (1 percent of Caucasian males)
Protanomaly – blue and some green cones are normal plus some anomalous green-like cones (1 percent of Caucasian males)
Deuteranomaly – normal blue and some red cones are normal plus some anomalous red-like cones (5 percent of Caucasian males)
Blue color blindness (tritanopia; 190900) is the result of mutations in the OPN1SW gene on chromosome 7. ERG flicker responses can be used to define the type and nature of the cone defects.
Systemic Features
There are no systemic abnormalities.
Genetics
Inheritance
Red-green color perception is based on gene products called opsins which, combined with their chromophores, respond to photons of specific wavelengths. The OPN1LW and OPN1MW genes reside in a cluster with a head-to-tail configuration on the X chromosome at Xq28. Red-green color vision defects are therefore inherited in an X-linked recessive pattern. There is a single gene for the red cone opsin but there are multiple ones for the green pigment. Only the red gene and the immediately adjacent green pigment gene are expressed. All are under the control of a master switch called the locus control region, LCR.
These DNA segments undergo relatively frequent unequal crossovers which can disrupt the color sensitivity of the gene products so that red-green colorblindness in some form is the most common type of anomalous color vision. It is found in approximately 8% of males and perhaps 0.5% of females.
Pedigree
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'.
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.