Incomplete and Complete Achromatopsia :: Biology Essays Research Papers

Incomplete and Complete Achromatopsia The human eye requires both rods and cones for normal vision. Over 100 million rods are located in the periphery of the human eye, and about 6 million cones compose the fovea. Rods, the more sensitive of the two to light, are not able to differentiate wavelengths, thus cannot detect color, and perceive shades of grey, black, and white. Cones, on the other hand, are of three types, containing particular pigments. They are categorized as red, blue, and green depending on to which wavelength they are most sensitive. These cones are what render color vision to humans. Red-green color blindness is not uncommon in the general population. The unequal crossover in the X-chromosome which causes this disorder is much more easily achieved than the mechanisms which cause other types of color blindness, due to the proximity of the two pigment genes. Five to eight percent of men are affected with this genetic condition, and due to a lacking pigment, have trouble distinguishing between red, green and brown. (1) Blue color blindness, also known as incomplete achromatopsia or blue-cone monochromatism, is an X-linked recessive disorder in which only the blue cones and the rods are functioning properly. A previously proposed theory states that signals from rods travel in the same pathways which carry signals from the blue-cones, making color vision in a blue-cone monochromat impossible. However, current research on blue-cone monochromats shows that signals from some rods and cones may be traveling by separate pathways to where wavelength discrimination takes place, making color vision possible in this type of monochromat, when both rods and blue cones are working simultaneously under twilight conditions. (6,7) Total color blindness, or complete achromatopsia, is an autosomal recessive disorder. (2) This is defined as little or no function of the cone cells, (3) and is somewhat frequent among Moroccan, Iraqi, and Iranian Jews. (7) There is no treatment for this color deficiency, only ways to alleviate its expression. (4) The consequences on the lives of complete achromats are many, and those of children should especially be considered. Many educational materials use different colors to teach. If vowels are red and consonants are black, the two are virtually indiscernible to an achromat (They may be discerned under certain light conditions, a phenomenon known as spectral differentiation). They may also not have yet learned to discern the different shades of grey, and may need help deciphering traffic signals.