Browns, blues, greens: what defines the color of our eyes?
"You have beautiful eyes you know!", "What seduced me were his eyes." For thousands of years, eye color has been charming, frightening, questioning. Why do we have the colored iris and what defines these differences in colors and shades?
The color of the iris of the eye is defined by a complicated game of genetic roulette but also by the amount of melanin present in the iris. Melanin is this pigment also responsible for the color of your skin, your hair and therefore your eyes.
Today, we know that a collection of 16 genes plays a role in this genetics.
It is the variation in the amount of melanin in the iris that gives it its color. If you have a lot of melanin in your iris, you will have brown (or other dark color) eyes. If you have less, you will have a lighter eye color.
The iris has two tissue layers which both have a tint. Most people have a brown tint in the back layer, but lighter colored eyes have less tint in the front layer. Blue eyes have almost no tint in the anterior layer. People with hazel, gray and green eyes also have shade variations in the front layer of the iris.
Additionally, perceived eye color depends on how light passing through the iris is absorbed, scattered, and reflected (the same effect that makes the sky appear blue). Blue eyes aren't really blue...
It's hard to tell what color your child's eyes are going to be. Many Caucasian babies are born with blue eyes because melanin is not yet fully present in their eyes. The child grows and the melanin will develop more or less depending on the person.
Some fertility institutes offer ratings of a baby's eye color based on the parent's eye color. However, it is also necessary to rely on the eye colors of previous generations and even there, nothing is certain.
Keep in mind that it can take 6-12 months for a baby's true eye color to appear. Even after this time, the color may still change. Most individuals reach stable color by age 6.
However, some people (10-15% of Caucasians) may experience color changes throughout adolescence and into adulthood. If you notice anything unusual about your eye color or if it changes in adulthood, see an optometrist to determine the cause.
In the same way, it is important to consult an optometrist for your children from 6 months and until the first years of his life to control the changes in the level of the eyes (beyond their color) and to ensure that their development is going well.
Some people have a variation called heterochromia. They then have more than one eye color. It can be a small or large part of the iris, sometimes even having one eye of a different color from the other. This is very often caused by the tissue variations in the parts of the iris. Heterochromia is usually harmless when present from birth or early development, but it can also indicate a condition, hence the importance of optometric consultations.
In more specific cases, heterochromia can develop later in life due to disease, injury, or the use of certain medications. This is acquired heterochromia.
There are three types of heterochromia:
- Complete heterochromia: the two eyes are completely different colors. This is the least common form.
- Central heterochromia: color variation that begins with one color near the pupil and changes to a different color towards the edge of the iris. It usually affects both eyes.
- Sectoral or partial heterochromia: color variation that takes on more of a "slice" or "wedge" pattern on each affected eye. It is the most varied type of heterochromia. The secondary color may be just a "thin slice of color" in one eye and occupy 2/3 of the iris of the other eye, in one or both eyes.
Heterochromia is sometimes confused with anisocoria. This condition gives the appearance of different colored eyes but the variation is only in the size of the pupil. This makes one eye appear darker than the other. This was the case with David Bowie.
In most cases, heterochromia is genetic and does not require treatment. On the other hand, if it develops because of a specific condition, your doctor will be able to advise you on the appropriate treatment.
What is the most common eye color? What is the rarest?
Much depends on where you live and where your ancestors came from. For example, brown eyes are the dominant color in many African and Asian countries. The colors will lighten in Europe where the eyes are more often blue in the Nordic countries.
Brown (and its variations) remains the most common color in the world. Green is the rarest natural eye color: 2% of the world's population compared to 70 to 79% for brown eyes.