CONGENITAL COLOUR VISION DEFICIENCY

 

FEMI AGUNLOYE

Congenital color vision deficiencies are inherited conditions that is present at birth and usually affect both eyes equally unlike acquired colour vision deficiency. It is one of the commonest inherited disorders of vision: its prevalence may be as high as 8% in males and 0.5% in females. The most common form of inherited color deficiency is the red–green deficiency, which is passed onto offspring via the X chromosome. For males to have the disorder, a faulty gene for red–green vision only needs to be present on his one X chromosome, whereas in females it needs to be present on both. Therefore, this type of colour vision deficiency is significantly more common among men than women. The blue–yellow vision deficiency is passed on through a non-sex chromosome and is equally common among men and women. Congenital colour vision deficiency results from genetic mutations that affect the expression of the full complement of normal cone photoreceptors. They are generally classified by severity (anomalous trichromacy, dichromacy, and monochromacy) and may be further classified by the type(s) of cone(s) affected. 

ANOMALOUS TRICHROMACY

In this condition, all three types of cone cells are functional but in one cone type, the perception of light is slightly inaccurate. It is the mildest form of colour vision deficiency. Like those with normal colour vision, the anomalous trichromat requires three primary colours to match any other colour. However, the way in which they mix the primary colours is aberrant, such that they will accept colour matches that a normal will not. Persons with this anomaly vary in their ability to discriminate colours. It is subdivided into; protanomaly which describes a reduced sensitivity to red light; deuteranomaly which refers to a reduced sensitivity to green light and tritanomaly which refers to a reduced sensitivity to blue light.

DICHROMACY

Dichromacy refers to a condition where one type of cone cell is dysfunctional or missing, meaning a certain portion of the light spectrum cannot be seen. Dichromats have a reduced dimension of colour vision and require only two primaries to match any other colour. It is also subdivided into 3 forms; protanopia which refers to an inability to see red light, deuteranopia refers to an inability to see green light and tritanopia, people cannot see blue light.  

MONOCHROMACY 

Also called total color blindness, this is an extremely rare condition, where two or all three of the cone cells are missing and colour and light perception is one dimensional. It is the severest form of congenital colour vision deficiency and here colour discrimination is absent. Aside from the effect monochromacy has on vision, people with this condition may demonstrate nystagmus: an involuntary, side-to-side movement of the eye. They may also have a sensitivity to light and a visual acuity of between 20/50 and 20/400.

Deficiency	Cone(s) affected	Inheritance
Anomalous Trichromacy ·         Protanomaly ·         Deuteranomaly ·         Tritanomaly	Red Green Blue	XLR XLR AD
Dichromacy ·         Protanopia ·         Deuteranopia ·         Tritanopia	Red Green Blue	XLR XLR AD
Monochromacy ·         Green-Cone monochromacy ·         Red-Cone monochromacy ·         Blue-Cone monochromacy   ·         Rod monochromacy	Red and Blue   Green and Blue   Red and Green   Red, Green and Blue	Dual XLR and AD   Dual XLR and AD   XLR   AR

REFERENCES

References Mandal, Ananya. (2019, February 26). Congenital Color Vision Deficiencies. News-Medical. Retrieved on August 20, 2022 from https://www.news-medical.net/health/Congenital-Color-Vision-Deficiencies.aspx.