Different studies show that 6 to 7 percent of all men are suffering from some kind of color blindness. And as we know from human genetics, color vision deficiency is—in most cases—encoded on the sex chromosome. This is a single Y chromosome for men and two X chromosomes for women. So why are not more women colorblind, if only one of those X chromosomes is working?
We also learned from researchers, that only about 0.4% of alle women are colorblind. Why is this number not closer to 3 or even 4 percent? Does nature know, which X chromosome has to be used for the color receptors inside the eye?
Actually it is quite simple to understand and to tell you the truth, the low number of 0.4% is not really the whole story.
Females are Mosaics
The biological process called X-inactivation, which takes place in a very early stage for each female embryo, is the source for this low number of women who suffer from color blindness. So let me show you what happens in that stage and afterwards, to understand it in more detail:
- At first every cell of a woman has two X chromosomes.
Let’s assume that one of them has some sort of red-green color blindness encoded in it.
- In a very early stage of the embryo, X-inactivation takes place.
Each cell inactivates one of its X chromosomes. This means about 50% will have the defective gene and the rest is not affected.
- Every cell passes the inactivation on to its successors.
Because of this, the ratio stays about the same during the whole life.
Now, what does this mean? — In the end we have actually three different possibilities concerning red-green color blindness in women:
- Both X chromosomes are not affected: The woman has normal color vision.
- Both X chromosomes are defective: The woman is red-green colorblind.
- One X chromosome is affected: Due to X-inactivation this woman has about 50% of defective genes in her eye as well as 50% genes which are working perfectly right.
The last case of course is the most interesting one. Usually only one part of your family would be affected by red-green color blindness. In this case, as a woman, you are called a carrier of the defect, as only one X chromosomes is carrying this genetic irregularity. But if we have a closer look at it, you actually also have those defective color receptors inside the retina!
As we have several millions photoreceptors (cones) which are responsible for our color vision, the defective signals are in a way oversteered by the correctly working ones. So in “real life”, you as a carrier woman have normal color vision just with only about half of your for example green receptors working correctly.
Researchers could show in some cases that such women have slightly more problems in perceiving certain colors under special light conditions, for example dim light. But if I think about it, those women actually not only have three different types of receptors (red, green, and blue) but one more (red, green, defective-green, and blue)! Doesn’t this mean, that they might have an even better color vision then most of us? Might some of them have tetrachomatic vision?
If you want to learn more about X-inactivation in color blindness, read the article by by Shuai Chen from Stanford University on exactly this topic.