It is a moderately severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary. There are 3 forms of Drichromacy:
Protanomaly: is a severe type of color vision deficiency caused by the complete absence of red retinal photoreceptors. It is a form of dichromatism in which red appears dark.
Deuteranomaly: is a color vision deficiency in which the green retinal photoreceptors are absent, moderately affecting red-green discrimnation.
Tritanomaly: is a very rare color vision disturbance in which there are only two cone pigments present and a total absence of blue retinal receptors...