Eyes that change color, a mysterious phenomenon

Iris coloring occurs primarily due to the presence and distribution of pigments in the iris, as well as the scattering effect of light. This process is determined by several biological and genetic factors. Here are the main steps in the iris coloring process:

1. Genetics: the role of melanin

- Eye color is inherited genetically and depends on several genes, the most influential of which are *OCA2* and *HERC2*, which regulate the amount and distribution of melanin, the dark brown pigment that colors skin, hair, and the eyes.

- There are two main types of melanin in the iris: eumelanin (rather brown) and pheomelanin (rather yellow-red). The proportion and concentration of these pigments influence the color of the iris.

2. Formation of pigment in the iris

- In the fetus, the iris begins without melanin, which gives newborns often gray or blue eyes. During the first months after birth, melanin begins to develop in the iris.

- The quantity and density of melanin will increase or not depending on genetics. For example, a high amount of melanin produces brown eyes, while a lower amount results in blue, green, or gray eyes.

3. The dispersion of light: Tyndall effect

- For light-colored eyes (blue, green, gray), the color is not only due to pigments, but also to an optical phenomenon. When light enters the iris, it is scattered by the layers of collagen present in the stroma (front layer of the iris).

- This scattering effect, called the Tyndall effect, is similar to the process of scattering light in the sky which turns it blue. So, blue eyes appear blue because the stroma reflects more blue light due to the absence or low presence of melanin.

4. Color variations with age and environmental conditions**

- Eye colors may also change over time. For example, eyes may darken slightly with age, as pigment continues to develop in the first few years of life.

- Certain medical conditions or even certain hormonal treatments can influence the coloring of the iris. For example, some glaucoma medications can darken eye color by increasing melanin production in the iris.

5. Rare colors and mutations

- Specific mutations or genetic variations can also result in rarer eye colors, such as purple eyes or red eyes (albinism). In albinism, the almost complete lack of melanin makes the iris transparent, and the underlying blood vessels give a red sheen.

Summary

In short, iris color is determined by genetics, the amount of melanin, and light scattering effects. The combination of these factors determines the natural color palette, ranging from blue to dark brown, with variations depending on pigments and light.