A Guide to Color Blindness Genetics
The human eye is one of the most evolved sensory organ among living organisms. In fact, it is our primary sense organ, and studies have shown that the brain shows a preference for the visual signalling components when there are conflicting signals from auditory or any other sensory system. The eye has also functioned as an efficient tool in asessing potential mates, finding food, and distinguishing friend from foe. Because it carries such significance for survival, it also has the ability to distinguish color in all the objects that come within the field of vision. The human eye detects colors or shades with the help of photoreceptors which are located at the back of the retina.
Cells that detect Color
The photoreceptors that detect color are divided into two groups, referred to as “rods” and “cones”. The rods, or rod cells, are responsible for the sensitivity of vision, meaning that they allow the eye to gather more light under low light conditions and enhance our vision. The rods do not assist in distinguishing particular colors, however. This role is filled by cones. Cone cells are distributed all over the retina and contain regions that are sensitive to certain pigments or color wavelengths.They fire when the particular color is detected and communicates with the brain about the chromaticity of an object. Cone cells require a sufficient amount of light to detect color, and hence cannot help much under low light conditions.
Genetic basis of Color Blindness
Color vision deficiency or color blindness is caused when the cone cells are unable to distinguish among the different light wavelengths and therefore misfire, causing the brain to misinterpret certain colors. Mutations in the following genes results in defects in color vision : CNGA3, CNGB3, GNAT2, OPN1LW, OPN1MW, and OPN1SW. More specifically:
OPN1LW mutations impact cells that are designed to pick up the red end of the light spectrum
OPN1MW mutations impact cells that detect the midrange colors of the light spectrum, such as yellow and green.
OPN1SW mutations impact cells that detect the lower range of the light spectrum, such as indigo and violet
Any mutations that affect the above genes causes the color vision deficiency. For example, a mutation affecting the first two genes would cause a defect in detecting the green and red color of the light spectrum. This is one of the most prevalent forms of color blindness.
Genetic Inheritance of Color Blindness
Color blindness has a sex-linked inheritance, which means color blindness is transmitted from parent to offspring. The most commonly found red-green color blindness is an X-linked defect. Males are said to be heterozygous XY and females homozygous XX. The red-green color blindness is carried in the X-chromosome and therefore is transmitted from the mother to children. The male children of a carrier parent (mother) have a 50% probability of being color blind. These odds don’t change even if the father is color vision defective. The male children of a color vision defective father and a normal mother will be completely normal but all females will be carriers. The children of a color vision defective couple (both father and mother) will be color blind. The above pattern is only for red-green color blindness, since the inability to detect blue is caused by a mutation is a gene that does not follow sex-linked inheritance.