I had a hard time doing titrations in Chem Lab, I can tell you that:
- OK, in this lab you count the drops you add until the solution turns pale green... why are you still dumping acid into your solution?
- I'm waiting for it to turn pale green.
- It's already as green as grass!
- Oh. Damn. Grass, you say? Usually I'll notice that shade of orange.
Pause
- ORANGE?!
- Never mind.

Colour blindness is a disorder in which the individuals affected have a partial or total inability to detect certain wavelengths of the visual spectrum. It was first described by famous chemist John Dalton in 1798, who described his own defective colour vision. Consequently, colour blindness is also known as Daltonism. In 1881, Lord Rayleigh developed a more scientific method of testing colour vision. It was known that a mixture of red and green gave yellow, but Rayleigh showed that the particular mixture of a specific red and green that made a standard yellow could be used as a test for colour discrimination. The total inability to see colour is rare and the disorder should more accurately be known as colour vision deficiency, or colour defective vision.

The various forms of colour blindness are hereditary conditions, and so the incidence varies within populations, and also between men and women. For example, within the USA colour blindness affects 8% of white males, 4% of black males, 1% of white females and 0.8% of black females.

In some isolated populations, the incidence is much higher - if there is a period in the population history where there was a lot of inbreeding, genetic disorders may have a higher prevalence. In the former British Colony of Hong Kong, the incidence of colour blindness in men was so high that people applying for jobs that were normally restricted to those with normal colour vision had to be accepted. For instance, the Royal Hong Kong Police Force allowed colour blind constables.

Causes

The cause of the disproportionately-high numbers of males affected is that the most common form - red-green colour blindness - is an 'X-chromosome sex-linked recessive' disorder. The 'recessive' part means that a person must carry two copies of the defective gene to develop the condition. 'X-linked' means that the gene is on the X-chromosome. Women have two copies of the X-chromosome, and so they may have normal colour vision, even if they carry one copy of the defective gene. Men only have one X-chromosome, and so will be colour blind with only one defective copy of the gene. Men inherit the X-chromosome from their mother, so a woman with one copy of the defective gene (and therefore normal colour vision) can still pass colour-blindness to her sons - she is a 'carrier'.

The 'red' and 'green' genes are known to reside at the tip of the long arm of the X chromosome¹. These genes encode different forms of 'opsin', which is a protein found in the cone cells of the retina. Normal opsin binds to the visual pigments in the red, green and blue cone cells. The combination of visual pigment and opsin is sensitive to a certain wavelength of light, which causes the stimulation of the optic nerves - with the end result that you see the light. Therefore, if the red-opsin gene is absent or defective, the functions of the red cones are impaired.

The most common defect involves the green receptors and leaves people unable to distinguish red and green, but still sensitive to red light. The next most common failure is in the red receptors. These people also confuse red and green but are insensitive to red light. Less common are failure of the blue receptors, total failure of all receptors and failure of the rods². These defects are rare and are often associated with other disorders. The gene involved is found on chromosome seven and so the condition is found equally in men and women.

Effects

My partner suffers from colour blindness and sees greens as varying hues of muddy brown (his description) - not that this has stopped him being a successful stylist and clothes designer

Colour blindness varies between individuals in both the insensitivity and the wavelengths they are unable to see. There are several different defects and they are named after the Greek word for the primary colours: 'protos' for red, 'deutros' for green and 'tritos' for blue. Someone with a complete red defect is said to have 'protoanopia' and if he has partial defect they are said to have 'protoanomaly'.

Red-Green Colour Blindness

The effects of red-green colour blindness can be described as follows:

• Protanopia (red deficiency) - blue-green appears grey, red-purple appears grey.

• Protanomalia (partial red deficiency) - blue-green appears indistinctly greyish, red-purple appears indistinctly greyish.

• Deuteranopia (green deficiency) - green appears grey, purple-red appears grey.

• Deuteranomalia (partial green deficiency) - green appears indistinctly greyish, purple-red appears indistinctly greyish.

These are extremes of the different variables. More commonly found are individuals with greater or lesser abilities to distinguish between the various colours, and that the lighting affects the perception of the colours. A particular mutation does not necessarily produce a definite level of impairment, and severity may vary even within a family.

Evolutionary Aspects

Colour blindness is a non-fatal disorder. Female carriers are not affected and male sufferers are not generally prevented from reaching breeding age. Therefore, evolutionary pressure against the gene is very slight. However, in the past, it may have conferred some advantages or disadvantages on humans and our primate ancestors. For example, in palaeolithic times, when humanity lived predominately by hunting and gathering, colour blindness in females would have been a disadvantage because of the inability to select ripe (that is not green) fruit.

When I was in the US Army ROTC, they pushed heavily for me to be in an infantry combat area rather than my desire to be an engineer. The Army was well aware of my 'hidden ability' to detect certain types of camouflage and better-than-average night vision (there are some benefits after all!)

Colour blind people have a tendency to better night vision and an ability to be able to distinguish hues that remain unseen to those who do not have the disorder³. In males, this may result in improved hunting skills in low light levels. The prevalence of colour deficiencies ranges from 1% to 14% in different populations and may be moderately correlated with how much twilight there is in the region. The duration of twilight is short at the equator but progressively longer at higher latitudes.

Colorectal Cancer

Men are statistically more likely to die of colorectal cancer than women, and it is thought that one reason for this is that men are more likely than women to be colour blind. The link is that if you're colour blind, when you look at a piece of used toilet paper it may all look the same colour, even though there's red and brown on there. Hopefully the other symptoms will prompt the colour-blind person to seek medical attention.

Diagnosis

The diagnosis of the disorder is still based on principles outlined by Rayleigh. Several tests have been devised, the most basic is to have many coloured discs on a screen and ask the patient to identify the colours. The commonest test - the Ishihara test - involves the use of special test plates called 'pseudo iso-chromatic plates' or 'confusion plates'. The plates are made up of a series of spots of varying colours and hues so that some number or letter stands out from the background. Those with defective vision are unable to distinguish these figures or will see a different figure due to the different hues. By changing the colour and hues and the background, all basic types of defective colour vision can be identified and classified. Other more specific tests can pinpoint the more subtle defects in colour vision.

Treatment

There is currently no treatment for colour blindness. In the future, gene therapy - inserting a working copy of the defective gene into the cells of the retina - may be an option, although this would have to be carried out quite early in the development of the embryo. Gene therapy is still in its early infancy, and there are many safety and efficacy issues to be resolved before such techniques become common.

Prognosis

As far as living with a colour deficiency, it was much harder as a youth than it is now. I got teased a lot and was the butt of a few jokes, but it wasn't anything worse than what I think every kid goes through in those early school days.

Although there may be some discrimination with regard to certain forms of employment, most colour blind people can adapt and carry on normal lives. For example, traffic light colours (red and green) could potentially cause a problem, but colour blind individuals simply have to show in their driving test that they know the sequence of light changes. Some changes have been made in everyday life for the benefit of colour blind people - the colours of electrical mains wires in the UK used to be red (live), black (neutral) and green (earth). These were changed to brown (live), blue (neutral) and green/yellow (earth) to avoid potentially dangerous confusion of the live and earth wires.

As an adult now in my 30s, I am rarely embarrassed by people that correct me when I call something by the wrong colour, though it can get annoying very quickly to be continually corrected or have to ask for more information when someone uses colour alone to indicate something...
- My car is the green one
- Where? Near the tree? Curb? Pointing which way?

In society, little importance is attached to colour blindness. People with the disorder are either ignored or slightly discriminated against. In fact, the only contact that some people have with colour blindness is when it used as an example of a sex-linked condition in a basic biology lesson.