It is a genetic flaw inherited mainly from our parents which causes a defect in the retina-the light sensitive inner lining of the eyes. People suffering from this illness can see other colours except black and white though not as much as those who can see with normal vision.
In the human eye,the retina contains normally three types of cone-shaped sensitive cells. Each type is tuned to the wavelength of a different primary colour of light_red,blue or green. Light of different wavelength triggers the corresponding cones,which signals the brain and enable one to perceive colours.

In people with colour-vision deficency, however,the sensitivity of the cones to one or more colours is weak or shifted in wavelength,and this inturn alters their response to colour.
Many people suffering from this have difficulty differentiating between colours such as yellow,red,orange and brown. For example,it will be hard for them to distinquish a blue-eyed blonde from a green-eyed redhead or see green mold on brown bread or cheese. If a sufferer's red sensitive cones are very weak,then a red rose appears black. Few of them cannot see blue.

Children and colour deficiency: Defects in colour vision are usually inherited and starts right after birth,thus children with this condition often learn to compensate. In other words,they learn to perceive contrast and brighteness and associate these variations with these colour names even if they cannot differentiate between certain hues.

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Because most schools often use colour coded-teaching tools beginning from early grades,parents and teachers alike may mistakenly think that a child has a learning disability when in fact that child is colour-deficient,since many young people remain unaware of their disability throughout childhood. For good reason therefore,vision test is recommended in early childhood.

Did you know? Colour vision deficiency occurs mostly in male genders. This is because inherited colour deficiency are carried on the X chromosomes. Women have two X chromosomes,while men have one X and one Y. Therefore,if a woman inherits a visual defect in an X chromosome,the normal gene in the other chromosome is likely to override it and her vision will not be impaired. But when a man inherits a defect in his X chromosome,he certainly will have no other X chromosome to fall back on.

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Tests to discover the degree of colour-vision deficiency that a person has often employ patterns of dots in various hues and shades. The widely used Ishihara test consists of up to 38 different patterns. If testing reveals a defect,an eye doctor may recommend further tests to determine whether it was inherited or come from some other cause.
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