Medical Conditions A - Z

Colour blindness

Qn: My brother is colour blind while me (I am a female) and my sisters all have normal vision. Based on this, it appears that my mum has a recessive gene for colour blindness. Is there any ways for me to find out if I am a carrier of this gene? Also, is there any cure for colour blindness?

Ans: Colour blindness may be a hereditary condition or caused by disease of the optic nerve or retina. Inherited colour blindness is most common, affects both eyes, and does not worsen over time. This type is found in about eight per cent of males and 0.4 per cent of females.

Photoreceptors called cones allow us to appreciate colour. These are concentrated in the very center of the retina and contain three photosensitive pigments: red, green and blue. Those with defective colour vision have a deficiency or absence in one or more of these pigments.

The symptoms of colour blindness are dependent on several factors, such as whether the problem is congenital, acquired, partial, or complete. The most common is red green colour blindness.

Colour vision deficiency is most commonly detected with special coloured charts called the Ishihara Test Plates. On each plate is a number composed of coloured dots. While holding the chart under good lighting, the patient is asked to identify the number. Once the colour defect is identified, more detailed colour vision tests may be performed.

There is no treatment or cure for colour blindness. Those with mild colour deficiencies learn to associate colours with certain objects and are usually able to identify colour as everyone else. However, they are unable to appreciate colour in the same way as those with normal colour vision.

Based on your information, your mother is a carrier of the inherited gene and your father is normal. By referring to the chart below, you have 50 per cent chance of carrying the defective gene, as shown in the charts:

A woman who carries the colour blindness gene will pass it on to 50 per cent of her children, as she passes on only one of her X chromosomes to each child (the other 50 per cent get the good gene).

Of the children who inherit the faulty gene, the boys will all have the condition as they inherit only one X chromosome (the faulty one from their mother - they get a Y chromosome from their father so they do not have a healthy X chromosome). But daughters will inherit a healthy X from their father so they will only be carriers of the condition. The exception is daughters in a family where the mother carries the condition and the father has the condition. Here, there is a 50 per cent chance that girls will inherit two faulty genes, one from each parent, and develop the problem.