The iris belongs to the optical system of the eye, it regulates the amount of light that passes through the pupil. It is the colored part of the eye.

The iris is an element of the bulb of the eye, it belongs to its vascular tunic (middle layer). It is located in front of the eye, between the cornea and the lens, in the continuity of the choroid. It is pierced in its center by the pupil which allows light to enter the eye. It acts on the diameter of the pupil by the action of circular smooth muscles (sphincter muscle) and rays (dilator muscle).

Pupil control

The iris varies the opening of the pupil by contracting or dilating the sphincter and dilator muscles. Like a diaphragm in a camera, it thus controls the amount of light entering the eye. When the eye observes a nearby object or the light is bright, the sphincter muscle contracts: the pupil tightens. Conversely, when the eye observes a distant object or when the light is weak, the dilator muscle contracts: the pupil dilates, its diameter increases and it lets more light pass.

Eye colors

The color of the iris depends on the concentration of melanin, a brown pigment, which is also found in the skin or hair. The higher the concentration, the darker the eyes. Blue, green or hazel eyes have intermediate concentrations.

Aniridie : results in an absence of iris. It is a genetic defect that appears at birth or during childhood. Rare pathology, it affects 1 / 40 births per year. The amount of light entering the eye is not controlled: too much, it can damage other structures of the eye. Aniridia can be complicated by cataracts or glaucoma, for example.

Ocular albinism : genetic disease characterized by an absence or reduction of melanin in the iris and retina. In this case, the iris appears blue or gray with a red reflective pupil because of the blood vessels visible in transparency. This depigmentation is due to the absence or deficiency of tyrosinase, an enzyme involved in the production of melanin pigments. The symptoms observed are generally:

• nystagmus: jerky movements of the eyes
• photophobia: intolerance of the eyes to light which can cause eye pain
• a decrease in visual acuity: myopia, hyperopia or astigmatism can affect people with albinism.

This depigmentation can also affect the skin and hair, we speak of oculocutaneous albinism. This disease results in very fair skin and very pale white or blond hair.

Heterochromia : commonly called “wall eyes”, it is not a disease but only a physical characteristic which results in a partial or total difference in color of the iris. It can affect the irises of both eyes and appears at birth or can result from a disease such as cataracts or glaucoma.

Heterochromia can affect dogs and cats. Among celebrities, David Bowie has often been described as having dark eyes. But the brown color in his left eye was due to permanent mydriasis, the result of a blow he received in his teenage years. Mydriasis is the natural dilation of the pupil in the dark in order to bring as much light as possible into the eye. For Bowie, the muscles in his iris were damaged by the blow causing his pupil to permanently dilate and change the color of his eye.

There are no treatments for these diseases. Exposure to the sun of people with albinism can cause skin damage and their risk of skin cancer is high. The World Health Organization (WHO) (6) therefore advises never to expose yourself to direct sunlight, from early childhood. Wearing a hat and sunglasses is recommended since the depigmented iris no longer plays its role as a barrier against the sun’s ultraviolet rays.

Iridologie : literally “study of the iris”. This practice consists of reading and interpreting the iris to see the state of our body and perform a health check. This contested approach has never been scientifically validated by research.

Biometrics and iris identification

Each iris has a unique structure. The probability of finding two identical irises is 1/1072, in other words impossible. Even identical twins have different irises. This characteristic is exploited by biometric companies which are developing techniques for identifying people by recognizing their irises. This method is now used worldwide by customs authorities, in banks or in prisons (8).

Why do babies have blue eyes?

At birth, the melanin pigments are buried deep in the iris (9). Its deep layer, which is blue-gray in color, is then visible in transparency.

This is why some babies have blue eyes. Over the weeks, melanin can rise to the surface of the iris and change the color of the eyes. A deposit on the surface of the melanin will cause brown eyes whereas if it does not rise, the eyes will remain blue. But the phenomenon does not affect all babies: most African and Asian babies already have dark eyes when they are born.

Blue eyes, a genetic evolution

Originally, all men had brown eyes. A spontaneous genetic mutation affected at least one main eye color gene, and blue eyes appeared. According to a 10 study (2008), this mutation appeared 6000 to 10 years ago and originated from a single ancestor. This mutation would then have spread to all populations.

Other explanations are also possible, however: this mutation could have happened several times independently, without a single origin, or other mutations could also cause blue eyes.