Red-green colour blindness is a common inherited disorder of colour vision in which a person has difficulty distinguishing between certain shades of red and green. It is one of the most important examples studied in Human Genetics because its inheritance is closely associated with the X chromosome. The condition does not usually affect the sharpness of vision, but it changes the ability to perceive particular colours accurately. Normal colour vision depends mainly on three types of cone cells present in the retina of the eye. These cones are sensitive to different ranges of light wavelengths and are commonly described as red-, green-, and blue-sensitive cones. In red-green colour blindness, the photopigments responsible for detecting red or green wavelengths may be absent, defective, or have altered sensitivity. As a result, colours that appear clearly different to a person with normal vision may appear similar to an affected individual. Red-green colour blindness is generally classified into two major forms: red colour