Color blindness, a color vision deficiency in animals, is the inability to perceive
differences between some of the colors that others can distinguish. It is most
often of genetic nature, but may also occur because of eye, nerve, or brain
damage, or due to exposure to certain chemicals.Colour blindness occurs when
one or more of the cone types are faulty or missing. This makes it difficult to
see the cone colour that is missing or faulty. For example, if the red cone is
missing you won't be able to see colours containing red as clearly.
About eight in 100 men and one in 100 women are affected by colour blindness.
Most people cannot distinguish certain shades of red and green. Very few
people are not able to see any colour at all and are truly colour blind. The gene,
which is responsible for the condition, which is carried on the X chromosome
and this, is the reason why many more men are affected than women are. The
inheritance process is explained in more detail in the section Inherited Color
8% of the male population and 4.5% of the population of the UK as a whole are colour blind
and there are estimated to be over 250 million colour-blind people worldwide.
There are two types of light sensitive cells in your eyes, called rods and cones.
They are both found in the retina, which is the layer at the back of your eye that
processes images. Rods allow you to see in dim light or at night, whereas cones
allow you to see in detail and distinguish colours.
There are three types of cone cells – red, green, and blue. Each cone has a
different level of sensitivity to light. When you look at an object, light enters
your eyes and stimulates the cone cells. Other cells in your retina and brain
then interpret the signals from the cone cells and allow you to see the colour of
an object. The red, green and blue cones all work together to allow you to see
the whole spectrum of colours. For example, when the red and green cones are
stimulated to a certain level you will see the colour yellow.
There are many types of color blindness. The most common are red-green
hereditary (genetic) photoreceptor disorders, but it is also possible to acquire
color blindness through damage to the retina, optic nerve, or higher brain areas.
This is the most severe form of colour blindness. Monochromatism (also called
achromatopsia) is the least common type of colour blindness, where none of
your cone cells function properly or where only one type works, as they should.
This results in no colour vision – all you see is black, white, and shades of grey.
Dichromatism is when one of the cones is missing. There are three types:
protanopia – the red cones are missing
deuteranopia – the green cones are missing
tritanopia – the blue cones are missing
This is the mildest and most common form of colour blindness. Anomalous
trichromatism is when you have all three cones but there is a fault in one of
them, making you less sensitive to certain colours. Depending on which cone is
faulty, this will cause:
protanomalous trichromatism – the red cones are less sensitive
deuteranomalous trichromatism – the green cones are less sensitive
tritanomalous trichromatism – the blue cones are less sensitive
Red and green cone defects are known as red-green colour blindness. People
with anomalous trichromatism vary in their ability to distinguish between
different colours – some are more affected than others are.
Symptoms of colour blindness
If you have colour blindness, the main symptom you will have is a difficulty in
distinguishing certain colours or you may make mistakes when identifying
them. You may see only a slight difference in the different shades of colour, or
if you have severe colour blindness, all you see is black, white, and shades of
In particular, situations this may be made worse, for example:
in low level lighting
if the area of colour is small
if you view a large area of colour at a distance
if you try to distinguish pale colours or dark shades
Causes of colour blindness
Either colour blindness is usually inherited or you develop it at some stage of
your life, for example, because of an illness, ageing, or exposure to
chemicals.The effects of colour vision deficiency can be mild, moderate or
severe so, for example, approximately 40% of colour blind pupils currently
leaving secondary school are unaware that they are colour blind , whilst 60% of
sufferers experience many problems in everyday life.
Colour blindness can be difficult to detect, particularly in children with
inherited colour vision deficiency, as they may be unaware that they have any
problems with their colour vision. A child with a severe condition such as
deuteranopia may seemingly be able to accurately identify colours which they
can’t see (e.g. red) because they have been taught the colour of objects from an
early age and will know for example that grass is green and strawberries are red
even if they have no concept of their true colours.
Inherited colour Vision Deficiency
Red/green colour blindness is a common hereditary condition which means it is
usually passed down from your parents. Colour blindness is usually passed from
mother to son on the 23rd chromosome, which is the sex chromosome.
Chromosomes are structures which contain genes – these contain the
instructions for the development of cells, tissues and organs. If you are colour
blind it means the instructions for the development of your cone cells are wrong
and the cone cells might be missing, or less sensitive to light or it may be that
the pathway from your cone cells to your brain has not developed correctly.
The X chromosome is the sex chromosome: males have an X chromosome and
a Y chromosome and females have two X chromosomes. For a male to be
colour blind the faulty colour blindness gene only has to appear on his X
chromosome. For a female to be colour blind it must be present on both of her
X chromosomes. This is why red/green colour blindness is far more common in
men than women. Blue colour blindness affects both men and women equally,
because it is carried on a non-sex chromosome.