About 8% of all men and 0.5% of all women are colorblind. The most common form is red-green color blindness which affects much more men than women, as it is encoded on the x-chromosome (sex-linked) and usually inherited from a mother to her son.
But Color blindness is not ‘color blindness’! There are still many people who think colorblind people can’t really see any colors. But the term is misleading. More than 99% of all colorblind people can see colors. A better wording would be color vision deficiency, which describes this visual disorder much more precisely.
So what actually is color vision deficiency also known as color blindness?
Simply put, if you are suffering from a color vision deficiency you are perceiving a narrower color spectrum compared to somebody with normal color vision.
This short definition raises a few more questions which need to be answered to understand the term color-blind more completely:
- Why am I suffering from color blindness at all?
- What means narrower color spectrum compared to normal color vision?
- Are there different types of color vision deficiency?
- How do I know if I’m colorblind?
- Is there some possibility to cure color vision deficiency?
- Can I just live with it or do I have to be afraid of it?
In this article I will among other things answer the first two of those questions. The others will be looked at in the follow up articles of this series about Color Blind Essentials. But first of all I would like to take you back to the 18th century.
History of color vision deficiency
The first scientific paper about color blindness was written by John Dalton in 1793 entitled “Extraordinary facts relating to the vision of colours“. Dalton himself was red-green colorblind and as a scientist he took interest in this topic. He claimed, that a colored liquid inside the eyeball is the source for a different color perception. This was proved wrong only after his death, when his eyes were examined and no such liquid was found.
After that Thomas Young and Hermann von Helmholtz were the first who described the trichromatic color vision. And once a theory for human color vision was ready, the basics of color vision deficiency weren’t far away.
The cause of color blindness
Color perception in the human eye is build up by three different types of cones. Each type is sensitive to a certain wavelength of light (red, green, and blue) and every perceived color is therefore a mixture of stimuli of those three cone types.
Now, if you one of those peaks of sensitivity is shifted towards another one or if one is missing at all, you perceive a narrower color spectrum—in other words you are colorblind. As a peak can be shifted everything between a little bit and the whole way, any type of severity is possible. The closer the peaks are the more severe is your color vision deficiency: slightly, moderately, strongly, or absolutely colorblind.
“What do you mean by «narrower color spectrum»?”
Let’s say somebody with normal color vision can identify and distinguish 150 hues. If you are colorblind this number starts to drop as you have fewer possibilities to create color mixtures from your color receptors. In case of absolute color blindness—missing one type of cone at all—you might be able to distinguish only as many as 20 different hues!
The type of affected cones also has a big impact on your color vision deficiency. As there are three different types of color receptors, there are also three different main forms: red (protan), green (deutan), and blue (tritan) disorders. As red and green deficiencies result in quite comparable color vision problems, they are put together and known under the term red-green color blindness. You will find more information on the different types of color blindness in the following two articles of this Color Blind Essentials series.
Much less common possibilities for color blindness are also glaucoma, aging, alcohol missuse, or a hard injury on your head. Those factors often cause some milder form of blue-yellow color blindness (tritanomaly). Also other facts like signal transmission can cause problems in color perception, but this is not fully understood yet.
Why am I suffering from color blindness?
You know now the cause of color vision disorders, but we still have not evaluated why we can be colorblind at all.
We learned that in most cases color blindness is a genetic disease which is inherited from the parents to their children. This means, if one or both of your parents is suffering from some type of color vision deficiency, there is a certain chance that you or your children will have the same vision handicap. The chance is strongly related to the type of color blindness.
Before I show you a sample inheritance pattern, we will have a closer look at our chromosomes. Unfortunately it is not as simple as it could be, because there are different chromosomes involved in color vision. And on top of that even on the same chromosome several different genetic code pieces are participating. The essence you should know is, that red-green color blindness is a sex linked recessive trait and blue-yellow color blindness is a autosomal dominant trait.
- sex linked: encoded on the sex chromosome X; men only have one of them (XY) compared to women (XX).
- autosomal: encoded not on the sex chromosome, equal for men and women.
- dominant: if it is encoded on one chromosome, you really suffer from it.
- recessive: if you have another healthy chromosome, it won’t show up.
If you combine this all together, we have more colorblind men than women. — Why is that?
Color blindness inheritance pattern
Red-Green Color Blindness Inheritance Pattern
The above genetic encodings lead us directly to the inheritance pattern. This will also show us on a glance, why there are more men suffering from color blindness than women.
The diagram on the right shows the inheritance pattern of red-green color blindness, which is by far the most common type of color vision deficiency. As you can see, this is a disorder which is passed on from a grandfather to his grandson, whereas the mother is only a carrier of it. A carrier is not affected because the trait is recessive. This causes much more men to be red-green colorblind, and even more women to be carriers of this color vision deficiency. You can also learn from this diagram, that a woman can only be red-green colorblind if both of her parents are at least carrying the disease encoded in their genes.
Am I the only colorblind person?
No, definitely not. Color blindness is a very common disease which is found all over the world. Different scientific studies show, that roughly 8% of all men and 0.5% of all women are colorblind. This numbers are supported by different studies and are about the same all around the world. The high difference between men and women is resulting from the facts we just learned, that the most common form red-green color blindness is a recessive sex-linked trait.
Knowing this numbers you can also compute some very interesting probabilities in color vision deficiency:
- Approximately every 500st handshake is between two colorblind people.
- It is almost sure (probability: 94%) that at least one out of a football team is colorblind.
- If you pick 100 persons randomly, there is a tiny chance (< 1.5%) that none of them is colorblind.
In the next article of the series Color Blind Essentials, we will have a closer look at the different types of color blindness. The common and also the very uncommon ones.