Dichromacy

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In dichromacy pairs of colours, that seem very different to a normal viewer, appear to be the same colour (or different shades of same colour). This is caused by full loss of function of a class of retinal cones. The incidence is 2.4% in males and about 0.03% in females. Most mammals are dichromates and it can have advantages as it allows better detection of chromatic contrasts that are rapidly modulated over time[1]. Humans have been reported with a single pigment gene on their X chromosome[2] so the cause is not necessarily a dysfunctional opsin gene. The subtypes are:

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Protanopia

Protanopia is a form of dichromacy where the severely defective color vision based on the use of only 2 types of photoreceptors, blue plus green. L cone cells are absent. Incidence is 1.3% in males and 0.02% in females as it is linked to absence of function of the OPN1LW gene at Xq28 that would normally express long-wave-sensitive opsin 1. Full dysfunction of an opsin leads to degeneration of the receptor cell that would normally express it.

Deuteranopia

Deuteranopia is a form of dichromacy where the severely defective color vision based on the use of only 2 types of photoreceptors, blue plus red. M cone cells are absent. Incidence is 1.2% in males and 0.01% in females as it is linked to absence of function of the OPN1MW gene at Xq28 that would normally express opsin 1 medium-wave-sensitive. Full dysfunction of an opsin leads to degeneration of the receptor cell that would normally express it.

Tritanopia

Tritanopia is a rare form of dichromacy where the severely defective color vision based on the use of only 2 types of photoreceptors, green plus red. The S cone is absent. The incidence is about 1 in 13,000 in the UK[3]but some population studies suggest rates up to 0.03%. It is linked to absence of function of the OPN1SW gene at 7q31.3-q32 that would normally express opsin 1 short-wave-sensitive. Full dysfunction of an opsin leads to degeneration of the receptor cell that would normally express it.


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