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My grandfather and both my
brothers have colour blindness. An eye doctor told me once that it can be hereditary through the maternal side of the family and I am curious if my five-year-old could possibly have it too?
A. A person who is colour blind has a hard time telling the difference between different colours of the spectrum. The difficulties can be mild or can be severe. A person with colour blindness is certainly not blind! The colours they see are in a much more limited range.
In order to see colour normally there are specialized cells in the back of the retina. These special receptor cells are called cones. There are three types of cones: red, blue and green. You require these receptor cones to work properly to see the wide spectrum of colours. You can imagine that if you have an abnormality of the cone cells, colour will be perceived differently. The most common kind of colour blindness is called red/green. About 8 percent of white males are affected but less than 0.5 percent of females. On the other hand, blue colour blindness is very rare. In this case you cannot see blue or yellow.
As you mentioned in your question, there is a difference in boys and girls with respect to colour blindness and that is because of how it is inherited. If you are a girl you have two X chromosomes inherited from each parent. Boys have an X and a Y chromosome – the Y coming from the father and the X coming from the mother. The X chromosome is one of the locations for the gene relating to colour blindness.
If you are a male who inherited the colour blindness gene from your mother, you will express that gene and be colour blind. Females however would require 2 X locations carrying the colour blindness gene. It is very unusual for a girl to inherit the same colour blindness gene from both parents – what are the odds that a colour blind male marries a female carrier of the colour blindness gene?
If the mother carries the colour blindness gene, her son has a 50 percent chance of inheriting that X gene from his mother. If he did inherit it, he would be colour blind. If he did not inherit it, he would have normal vision and not have the ability to pass it on. A daughter born from the same mother has a 50 percent chance of inheriting the gene, but as her other X chromosome from her father is normal she would not express the colour blindness. However if she carried the gene and passed that gene onto her son at a later time, that son would have colour blindness.
This phenomenon is called a sex-linked mutation. It explains why boys manifest the disease but girls are carriers and do not express the disease.
Published in March 2011.
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