How does the color of the iris come about?
The color of the iris is determined by the dye melanin. This dye serves as light protection in eyes and skin. Melanin has a brownish color and absorbs incident light.
A different colored pigment is not produced by humans. Originally, therefore, probably all people had brown eyes. Differently colored eyes develop when less melanin is produced in the eye.
Incident light is scattered by tiny particles in the iris, which is now more transparent. This is called the Tyndall effect. The strength of the scattering depends on the wavelength of the light.
Blue light has a particularly short wavelength and is therefore more strongly scattered than red light. Part of the scattered light is reflected. Thus the eye appears blue.
The situation is similar with green eyes. So the color of the eye depends not only on the pigmentation, but also on the microscopic properties of the iris. Since differently colored eyes are evolutionarily still very young, 90% of the people worldwide have brown eyes. Green eyes are even present in only 2% of the world population.
Heterochromia
In heterochromia, the color of the iris of one eye is different from the color of the other eye. A sectorial heterochromia is also possible. In this case only a section of an iris is affected.
The cause is usually a deficient pigmentation of one eye. Since eye color is genetically determined, heterochromia can also be triggered by genetic causes. Often these are harmless variations.
However, in addition to the harmless cases of heterochromia, there are also genetic diseases. These include certain pigmentation disorders. Hereditary Waardenburg syndrome is a congenital heterochromia associated with hearing loss.
However, heterochromia can also occur during the course of life as a symptom of various diseases. An inflammation of the iris or adjacent tissues can lead to depigmentation of the affected eye. Such inflammation of the iris can also spread to the lens.
If this happens, the lens can become cloudy, a condition known as cataract. A newly occurring heterochromia should therefore be examined by an ophthalmologist.
