Question: why are some people colour blind? and why are some more colour blind than others?

Elaine Marshall answered on 18 Jun 2013:

Whether or not you are colour blind is largely the result of an inherited mutation in the genes which encode for the photoreceptor proteins. These proteins are found in the cone cells located at the back of your eye. The gene for these proteins arose by mutation naturally in our evolution and most people have 3 though recently a fourth one has been found. Each protein allows us to detect light over a slight different wavelength. Also the reason more boys are colour blind is that photoreceptor genes are coded for on the X chromosome and boys only inherit one copy from their parents.

Depending on how these genes have been mutated will determine which wavelength of visible light you have difficulty perceiving. Total colour deficiency arises when 2 or 3 of the gene products have been lost. A less severe form involves a mutation in just one gene but again the spectrum of severity depends on whether the mutation leads to a total loss of protein or perhaps one that just doesn’t work as well.