Colour vision deficiency is carried on an X chromosome and is a recessive genetic disorder. So, if there is an accompanying normal X chromosome the colour vision deficiency won’t manifest itself. Females have two X chromosomes so colour vision deficiency is rare in females. The figure quoted in most reliable sources is 1 in 250, which to me isn’t that rare. The other thing I sometimes see in reliable sources is that the Ishihara colour vision test isn’t sensitive enough to detect colour vision deficiency in females. For that to occur each X chromosomes has to have the code for colour vision deficiency. Males only have one X chromosome so colour vision deficiency is much more common in males. One in 12 males has a colour vision deficiency.
It is easily tested in practice using the Ishihara test. Some attention needs to be given to the lighting used to illuminate the test plates. Tungsten lighting, although being phased out in many countries, is still common and may make the symbols which test red colour vision easier to see. It is better to conduct colour vision testing using fluorescent light.
All boys should be tested as soon as they are able to perform the digit (numbers test) or the tracking test. Colour vision forms a major part of early education. Think about all the colouring, maps and chemical reactions in school lessons. Colour vision deficiency is not treatable but it can be taken into account in the learning environment. Parents cannot rely on detecting colour vision deficiency through colour identification errors made by a child as colour identification strategies can be developed from an early age.
Some careers require perfect colour vision and are precluded by even mild colour vision deficiency. It is far better for a family to know of a child’s colour vision deficiency from an early age as possible in order to guide the child away from careers with a strict colour vision requirement.
A case that comes to my mind from my own clinical practice is of a 16-year-old boy who wanted to enter the Navy in a role that required perfect colour vision. He had wanted to do this from a very young age. He had had his eyes examined around eight times previously in another practice from an early age. I assessed his colour vision and he had severe deficiency. He was with his mother and when I told them, both cried. His mother asked why this had not been detected previously. It had not been detected because the optometrists who had tested the patient previously had not taken the time to test for it.