It's all in the chromosomes

Nine out of 100 males are colourblind, says consultant opthalmologist Dr Tan Niap Ming. She said there were many types of colour deficiency, but the most common was the "red-g reen" colour blindness.

"Many people with the condition cannot tell the difference between green and tones of red.

"But a big majority of them can differenti at e solid colours, even if it is between green and red; and they usually face difficulty in identifying desaturated hues of colours." Dr Tan said to help such people drive on public roads, some countries had added a tinge of blue to the traffic light 's green bulbs, so they have a greater contrast with red bulbs.

"They can also read the red light's position on traffic lights." The traffic light system, where red is the highest bulb and green the lowest, is universally standardised.

"Abnormal colour perception, or colour deficiency, is the termprefer red by medical practitioners as the word 'blindness' implies it is a disability or handicap," Dr Tan said.

Even though the condition could not be cured and visual aid to correct the abnormalities had not proven to be effective, she said people with abnormal colour perception could function just as well in their daily commitments as others, such as driving.

"People with abnormal colour perception should be allowed to drive as their condition is really not a big hindrance." However, she maintained that other professions which required perfect colour and eye vision, such as airline pilots or bus drivers, should continue to keep the ruling as the risks were much g reater.

Dr Tan urged parents to check their children to see whether they had the condition.

She said it was important to do so because only then could they provide clearer career guidance.

Dr Tan, who is also an aviation opthalmologist, said she had to tell many young, aspiring pilots they could not pursue their dream career because they had colour deficiency. Most of them did not know it.

"They had wanted to become pilots all their lives. Imagine having to ditch 10 to 15 years of preparations just because they did not know they had abnormal colour perception." She said parents should start testing their children from the ages of 5 to 7.

"Grownups above the age of 40 should test their colour perception as well, as the ability to detect different hues of colours tapers off at that age due to the natural yellowing of the lens."

She said the reason the condition was apparent in males was that males only had one X chromosome, whereas females had two.

The abnormality is caused by a mutation in the red-green receptors in one's eye, and the genes of which are located in the X chromosomes.

"Since men only have one X chromosome, the condition will show if it is inherited from their parents (one of whom or both carry the mutated chromosome)," said Dr Tan.

"Women are less susceptible to display colour deficiency because they have two sets of X chromosomes." She said if one X chromosome carried the hereditary mutated gene, there was a second healthy X chromosome that could override the deficiency.

"But some women, although not colour-blind themselves, can be a carrier of that gene. Her male children may inherit and display the condition." Dr Tan said other than genetic inheritance, colour blindness could also be caused by nerve damage or degeneration of the eye receptors.