Colour Blindness
Colours do not appear the same to everyone. People with a colour
vision deficiency are called colour blind. There are many different
types of colour blindness. The most common is the red-green colour
deficiency, which is very common among boys. Colour blindness is
nearly always congenital. Approximately 7% of all men are colour
blind and less than 1% of all women.
This is how it works
When we look at an object light enters the eye. The light is
registered by sensory cells in the eye that send signals to the
brain. This is also why we cannot see in the dark when there is no
light to be reflected.
Rods and Cones
There are two types of photoreceptor cells in the retina. They
are called rods and cones. With the rods we cannot see any colours
at all, but they function even when there is little light. That is
why it is hard to determine what colour for example a car is at
dusk. With the cones we can see colours. The cones are mostly
located in the part of the retina called the macula. The light
which is reflected from the object we are looking at, focusing on,
falls on the macula where many of the cones are located.
Light of different colours has different wavelengths. There are
three types of cones: those that best absorb light of wavelengths
445-450 nm (violet), those that best absorb light of wavelengths
525-535 nm (green) and those that best absorb light of wavelengths
555-570 (yellow). The different cones can "see" different colours
because they contain different pigments, molecules that change when
they are hit by light. Colour blindness can be due to a lack of one
of the three types of cones, or be due to differences in the
pigments in the cones.
Perceive Colours Differently
People with a red-green colour deficiency cannot tell the
difference between red and green, and may have problems with some
other colours as well. In other words most "colour blind" people
can see colours, but they perceive them in a different way. It is
very rare not to be able to see any colours at all. Complete colour
blindness (acromalia) is caused by a total absence of cones. People
afflicted by this cannot be around bright lights and do not have
very good vision. It is difficult for them to read for example. You
can also have insensitive colour vision. Then you need more time to
distinguish colours and it is unusually difficult for you to
distinguish colours in dim light.
Why are more men than women colour
blind?
This is because colour blindness is hereditary and the gene that
leads to colour blindness is located on the X-chromosome. The gene
also has to be recessive and this is the case with colour
blindness. A recessive gene "loses" if it comes into conflict with
another gene for the same trait. There are two main types of colour
blindness, one for the green field and one for the red. Both are
caused by recessive genes located on the X-chromosome.
The X and Y-chromosomes determine the sex of a person. Men have
one of each while women have two X-chromosomes (and no
Y-chromosome). If a gene that causes a disease is recessive then
the disease will not reveal itself if there is also a normal gene
in the body. The normal gene "wins". That is a possibility in women
since they have two X-chromosomes. If one of them carries the gene
for colour blindness and the other one carries the "normal" gene,
the normal gene will dominate and she will not be colour
blind.
Men only have one X-chromosome, so if it has the gene for colour
blindness the man will be colour blind. For a man to become colour
blind it only takes one gene, but for a woman it takes two genes
for colour blindness, one on each X-chromosome. To get two genes
for colour blindness at once (one from the father and one from the
mother) is much more unusual.
Women who are carriers of one gene for colour blindness, and who
therefore are not colour blind, can still hand the gene on to their
children (in 50% of the cases). Other hereditary diseases which are
much more common in men than in women work in similar ways, for
example classic hemophilia.