Causes of Colour Blindness

Colour blindness is usually a genetic (hereditary) condition (you are born with it). Red/green and blue/yellow colour blindness types are usually passed down from your parents. The gene which is responsible for red/green colour blindness is carried on the X chromosome and this is the reason why many more men are affected than women. The inheritance process is explained in more detail at Inherited Colour Vision Deficiency

Inherited blue/yellow colour blindness is very rare because it is inherited differently to red/green types. The majority of our website concentrates on red/green types but we definitely do support people with blue/yellow (tritan) deficiencies! If you are affected, rest assured that all of the advice we provide will support people with all forms of CVD!

8% of the male population and 4.5% of the population of the UK as a whole have a form of red/green colour blindness. There are estimated to be over 300 million colour blind people worldwide. The vast majority of people with a colour vision deficiency have inherited their condition from their mother, who is normally a ‘carrier’ but not colour blind herself. Some people also acquire the condition as a result of long-standing illness such as diabetes, multiple sclerosis, some liver diseases and many eye diseases. Read more about acquired conditions.

The effects of colour vision deficiency can be mild, moderate or severe depending upon the defect. If you have inherited colour blindness your condition will stay the same throughout your life – it won’t get any better or worse.

The retina of our eyes have two types of light-sensitive nerve cells called rods and cones. They are found in the retina which is the layer at the back of your eye which processes images. Rod cells work in low light conditions to help night vision, but cone cells work in daylight and are responsible for colour discrimination.

There are three types of cone cells and each type has a different sensitivity to light wavelengths. One type of cone cell perceives blue light, another perceives green light and the third perceives red light. When we look at an object, light enters our eyes and stimulates the cone cells. Our brain then interprets the signals from the cones cells so that we can see the colour of the object. The red, green and blue cone cells send electrical signals to our brain which then interprets the electrical information allowing us to see the whole spectrum of colours. For example, when the red and blue cones are simulated we will see the colour purple.

The exact physical causes of colour blindness are still being researched, but colour blindness is usually caused by changes to the genetic code sequencing which result in  ‘faulty’ electrical signals being sent to the brain.

People with normal colour vision have all three types of cone cells/electrical pathways working correctly, but colour blindness occurs when one or more of the cone cell types have abnormal sequencing. For example, if the red cone is ‘faulty’ you won’t be able to see colours containing red accurately. Most people with colour blindness can’t distinguish certain shades of red and green and colours which contain red and green. In people with severe forms of red, green or blue CVD one set of cone cell types does not exist at all.

If you have no cone cells which can distinguish red light you will have protanopia and people with severe forms of colour blindness are dichromats. Di- meaning two i.e. only two of the three cone cells are present in your eyes. For more information see Types of Colour Blindness.