About 8% of males and 0.5% of females are classified as having defective colour vision whether it is one color, or a color combination . This is moslty genetic in nature, but can also occur because of some eye, nerve, or brain damage. Males are at a greater risk of inheriting the condition due to it being passed on via the X chromasome. Due to females having two X chromasomes , if they inherit a normal X chromosome in addition to the one which carries the mutation, they will not display the mutation.
Color blindness may be partial (affecting only some colors), or complete (affecting all colors). Complete color blindness is very rare. The term Color blindness itself is misleading if taken literally, because colorblind people can see colors, but cannot make out the difference between some couples of complementary colors. Red/Green color vision deficiency is by far the most common form, about 99%, and causes problems in distinguishing the difference reds and greens.
The Ishihara Test
The most commonly used test to detect color vision deficiencies was developed by the Japanese ophthalmologist Shinobu Ishihara. While working at the Military Medical School he was asked to devise a test to screen military recruits for abnormalities of color vision. His assistant was a colorblind physician who helped him test the plates. A collection of 38 plates filled with colored dots build the base of this test. The dots are colored in different shades and a number is hidden inside with shades of another color.
The existence of color vision deficiencie is usually established after a few plates, the testing of the full 24 plates gives a more accurate diagnosis of the level of severity an individual suffers from the color vision defect.
In the above plates you should see the numbers: 74, 6, 12, 42, 2