What causes color blindness, and why are there so many more color-blind men than women? It all starts at the chromosome

Test plate No.9 in the Ishihara Color Perception Test | Source: Wikipedia, public domain.



Most people who look at this photo see the number 74 composed of colored circles, but a rather large minority of the population - about six percent of men and less than one percent of women - see the number 21 instead. These people are color blind, and have difficulty distinguishing between shades of green and red. What does this stem from, and why are there so many more color-blind men than women? As usual, the answers are in the genes.

The cells responsible for our color vision are the light-absorbing photoreceptor cells in the eye retina, termed cones. Found alongside them in the retina are rods, photoreceptor cells that allow us to see in dim light but that cannot tell colors apart. Most people have three types of cones in their retina, each of which responds most strongly to light of a certain color, that is, to a specific wavelength. One responds mostly to the shorter visible light waves within the blue light range, another to shades of green light, while the third one responds strongest to red light. Some animals have more cone types - most birds have four, and the mantis shrimp has no less than 12 types of cones in its eyes.

Activity of the cells in response to different wavelengths is determined by photopsins, the photoreceptor proteins they contain, that react to different ranges of light frequency. A photoreceptor cell that responds to red light contains photopsins that are sensitive to red light, a cell that responds to green will contain photopsins with green light sensitivity, and so on. These proteins are produced according to instructions within our DNA, and most of us have three genes that are responsible for the production of the three photopsins. When any of these genes happens to carry a mutation - a change causing the gene to be either inactive or less active, the result is color blindness. When one of the photoreceptor cells responsible for color vision is not functioning properly, a person will have difficulties distinguishing between shades of color in the respective region of the spectrum. 

A matter of X

The most common cause of color blindness is a mutation in the gene that encodes the protein responsible for the response to medium wavelengths, (i.e. mostly shades of green). Such a mutation confers difficulty in distinguishing between red and green color, and nearly always affects men. The explanation for this lies in the gene’s location. 

Our genome is packed into 23 long DNA molecules, termed chromosomes. Apart from the gametes (the sex cells), each cell carries two copies of the genome - two sets of 23 chromosomes. But these are in no way two perfect copies. One of these sets of 23 chromosomes we receive from our mother, and the other set of 23 chromosomes from our father. Each of these will  have slightly different versions of the same genes, but the biggest difference lies in the sex chromosomes - X and Y. Women almost always have two X chromosomes, while men usually receive a Y chromosome from their father and an X chromosome from their mother. Both the gene encoding green photopsin and the gene encoding red photopsin are located on the X chromosome, of which men carry only a single copy.

If a woman has two X chromosomes, and one of them carries a mutation in the green photopsin gene - the functioning gene on the other chromosome compensates for it, and in most cases she will not have a problem with color vision. She will be a carrier of the mutation, but it will not affect her significantly. 

In contrast, a man has only a single X chromosome, and when this chromosome carries a similar mutation there is no other copy of the gene to compensate for the mutation, resulting in color blindness. The Y chromosome does not carry a photopsin gene. Since men get their X chromosome from their mother, a color-blind man received the mutation from his mother, who likely inherited it from her father. Statistically, a son born to such a woman has a 50% chance to inherit her mutation-carrying X chromosome and therefore to be color-blind. For a woman to be color blind, she would have to have both a color-blind father and a mother who carries the same mutation, which is significantly more rare.   

This form of inheritance is common to many traits and syndromes, as well as diseases in which the underlying gene is located on the X chromosome. A famous example for such X-linked inheritance is hemophilia, a disease characterized by a blood clotting deficiency, which is significantly more common in men.