Daltonism (color blindness)
Colors are not perceived the same by all people. Daltonists are those who have a color perception deficiency. There are many types of daltonism. The most common is the inability to distinguish red and green, especially at boys. In almost all cases daltonism is congenital and manifests itself in about 7% of men and in less than 1% of women.
When we look at an object, light enters the eyeball. Light is recorded by ocular sensory cells that send signals to the brain. Therefore we can not see in the dark, because no light is reflected.
Rod and cone cells
In the retina there are two types of photoreceptor cells. They are rod and cone cells. The rods can not see all colors, but they work even when light is poor. Therefore it is difficult to realize, for example, what color has a car that we see in the twilight. Cones allow us to distinguish colors. They are located mostly in the macula lutea, an area of the retina. The light reflected from the object looked at intensely reach the macula lutea, where cone cells are concentrated the most. Light of different colors have different wavelengths. There are three types of cones: those that best absorb light with wavelengths between 420 and 440 nm (violet), the second type responds most to light of medium-wavelength 534 � 545 nm, peaking at green, and the third, responding mostly to light of long wavelengths, 564 � 580 nm, peaking in the yellow region. Different cells can “see” different colors, because they contain different pigments, molecules that change in contact with light. Daltonism may be due to the lack of one of the three types of cone cells and to the pigment differences between cones.
Different color perception
Persons with color perception deficiency (discromatopsy) of the red-green axis may have problems with perception of other colors. In other words, most “color blindness” people see colors, but they perceive them in a different way. Cases where one can not distinguish any color are rare. Absolute color blindness (total discromatopsy) is due to the total absence of cone cells. Persons affected by this disease do not support bright light and have a weak vision, and their reading capacity is affected. Another problem is the lack of color sensitivity. In this case, you need more time for distinguishing colors and it is very difficult to distinguish colors in low light.
Rainbow seen by people with different color perception deficiencies
|Normal color perception||Blue discromatopsy|
|Red discromatopsy||Absolute color blindness (total discromatopsy)|
Why does daltonism affect men more frequently than women?
This is because daltonism is hereditary, and the responsible gene is located on chromosome X. The gene is recessive, as well as daltonism. The recessive nature of the gene is lost when it conflicts with another gene for the same feature. There are two main types of discromatopsy: in the spectrum of green and red, respectively. Both are caused by recessive genes located on chromosome X.
X and Y chromosomes determine the sex of the individual. Men have one of each, and women have two X chromosomes (and no Y chromosome). If a gene that causes a disease is recessive, then the disease will not occur if the body has a normal gene, too. The normal gene “wins the fight”. This is possible for women, as they possess two X chromosomes. If one of them is wearing the daltonism gene and the other one is wearing the “normal” gene, then the normal gene has dominant character and daltonism does not appear.
Men have one X chromosome and if one is wearing the daltonism gene, then the man will be daltonist. For a man to be daltonist, it takes only one gene, while a woman requires two genes, one for each chromosome X. The odds of inheriting two daltonism genes (one from the father and one from the mother) are much lower.
Women who are carriers of a daltonism gene, therefore not daltonists, can still transmit the gene to their children (in 50% of cases). The mechanism is similar to the other hereditary diseases that are more common in men than in women, such as classical haemophilia.