Heidelberg, Germany - Four-year-old Leo can clearly distinguish black, white, blue and yellow but often has trouble with other colours.

His nursery nurses recommended an examination by an ophthalmologist, who told Leo to try to recognize shapes on charts made up of coloured dots. Leo could not, and the doctor's diagnosis was red-green colour blindness.

About 8 per cent of males and 0.4 per cent of females are born with red-green colour blindness, noted Professor Hermann Krastel, an ophthalmologist who works in Heidelberg. Because of a small genetic defect, photoreceptors in the retina of the eye send deficient information on colour through the optic nerve to the brain.

Red-green colour blindness has been known for a long time. It is also called Daltonism, after English chemist and physicist John Dalton. Afflicted with the disorder himself, he was one of the first to describe colour-vision deficiency. Dalton, who died in 1844, perceived orange, yellow and green as various shades of yellow.

Scientists were able to prove, 150 years after his death, that Dalton suffered from colour blindness, remarked Bernd Wissinger, who does research on the genetic causes of colour blindness and is director of the molecular genetics laboratory at Tuebingen University's eye clinic. Dalton's defective gene was located in DNA extracted from his preserved eye tissue.

"We don't see in grey, we just see differently," explained Fritz Buser, an optician in the Swiss city of Olten who is red-green colour blind. Buser trains vision disability specialists and teachers of the visually impaired in Germany.

"When I gather berries with my son, he finds more raspberries and I recognize the blackberries better," he said.

Opthalmologist Krastel noted that some colour-blind people distinguished between shades of ochre, beige and brown better than people with normal sight.

Buser said his colour problems had little impact on his daily life. "The biggest restriction is in the choice of occupation," he said. Colour blind people are not suited to work as airplane pilots, ship captains, and train drivers, all of whom need to recognize colour signals with certainty.

Colour blindness is neither curable nor correctable. But sufferers looking for a "crutch" can easily find internet offers of special lenses with colour filters supposedly able to correct red-green colour blindness.

According to Krastel, the lenses do indeed change colour vision and may enable a person with red-green colour blindness to pass colour chart examinations. However, they impair colour vision in ways not tested by the charts.

"The problem is simply shifted," Krastel said.

Experts advise the colour blind to accept their disorder. Children, regardless of whether they have shown symptoms, should be tested for colour blindness when they start school, at the latest, to avoid learning difficulties.

"Colour-coded materials such as maps are constantly used in schools," Krastel pointed out.

If a child has been diagnosed with colour blindness, the teachers should be informed. The child, too, should be made aware of the problem. One way to get around it is by marking coloured pencils.