February 28, 2021

Why do some people have an eye for each color? | Science


The color of the eyes is a genetic trait and, therefore, depends on the color of your parents' eyes. The most frequent is the brown iris followed by blue and the weirdest is a green iris. Actually, the cause of the hue of our eyes is the concentration of a pigment, melanin, on the anterior face of the iris. A more melanin darker eyes and more light eyes have less melanin.

In medicine, the fact that an individual presents an eye of each color is called iris heterochromia. Complete iris heterochromia is a rare condition with an incidence of six people per ten thousand. But what is much more common is the partial or central heterochromia of the iris.

Partial heterochromia is one in which one of the eyes has a sector with a different color, while the central one, which is even more common, consists of having the outer part of the iris of one color and the inner part of another. The latter is the most common and does not usually occur along with pathologies.

This occurs during development. When children are born they usually have very clear eyes and during growth melanin is deposited in the anterior area of ​​the iris or epithelium. Because of this, many babies have the blue eyes at birth and later, when they grow older, their eyes darken. In fact, when there is a complete iris hemorrhochromy, the lightest eye is the one that due to a lack of melanin has not become pigmented as it should. This does not have to be due to any pathology, simply the pigmentation of one eye has been different from the other. In most cases these eyes are perfectly healthy and look exactly as well.

An ophthalmologist checks the eyes of a child.


An ophthalmologist checks the eyes of a child.

The causes of this anomaly are varied. Most of the cases have a close relationship with the Genetic heritage and they are not associated with any disease. But in some cases, iris heterochromia can occur as a clinical sign associated with a disease. For this reason, when a baby or a girl or boy have different colored eyes, they should visit the ophthalmologist to rule out a pathology that has produced it, although they are rare and have a low incidence.

Finally, iris heterochromia can also be acquired. That is, someone can be born with the eyes of the same color and when he is already an adult, one of the two eyes changes his tone. This change may be indicating an ophthalmological disease, ocular trauma or, occasionally, the use of certain drugs that can cause a change in the shade of the iris, such as the treatment with certain drops for glaucoma (prostaglandin analogues). In the case of adults in which one of their eyes changes color should always seek the assessment of a specialist.

Elena Salobrar-García She is a doctor in Vision Sciences and a researcher at the Ramón Castroviejo Ophthalmological Research Institute of the Complutense University of Madrid.

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