EyeMark Jan 2017 1A beautiful eye colour can be a striking feature, but what about two beautiful eye colours?? Six out of every thousand people in the world have eyes that are different colours from each other, although in most of these cases the difference is so subtle, its hardly noticeable. This striking phenomenon is called Heterochromia Iridum, which literally means different coloured irises (the coloured area of the eye), and presents in various ways. With some people and animals only a segment of the iris is a different colour, or there may be a different coloured ring around the pupil, while others may have two completely different coloured eyes.

Heterochromia is more common in dogs than in humans, and is commonly observed in sheep dogs, dalmations and huskies. Horses and certain breeds of cats have been found to have it, too.

EyeMark Jan 2017 2What makes a blue eye different than a brown eye, or a green eye different than a blue eye? It all comes down to pigment. Eye colour is determined by your genes and the combination of alleles you inherit from your parents. Alleles are segments within a gene, which are received from both parents. These alleles code for different levels of a pigment called melanin to be produced in the iris. Since each person receives genetic information from both parents, a child can have a completely different eye colour than both of the parents.

EyeMark Jan 2017 3Melanin is a pigment that is found in the skin and also the eyes. More melanin means darker eyes and less melanin means lighter eyes. There are two distinct layers to the iris. All normal eyes have melanin in the back layer. The melanin concentration in the front layer determines eye colour. Brown eyes have the most melanin, green eyes have less and blue eyes have no melanin in the front layer of the iris.

In most cases, heterochromia is present at birth, but it can be acquired later in life. Although a common feature of a number of inherited genetic disorders, heterochromia is not necessarily a sign of any underlying health condition, is usually harmless, and does not interfere with visual acuity. Irregular iris colouring can be caused by an injury to the eye, an infection or mild inflammation that affects only one eye, or the presence of a foreign object in the eye which can lead to inflammation. Heterochromia can also be caused by glaucoma. The sudden onset of heterochromia could be the sign of an underlying medical condition, and a complete eye examination should be conducted to rule out any serious causes.

EyeMark Jan 2017 4Is having heterochromia a blessing or a curse? Though unquestionably beautiful, the look is striking precisely because of how unusual or out-of-the-ordinary it is, which, of course, often means that those who have heterochromia iridum may be teased or bullied for their mismatched eyes while growing up. However, as adults, they may be admired and considered extremely attractive because of this particular feature. Celebrities such as Mila Kunis, Kate Bosworth and Jane Seymour all have heterochromia, with exotic eyes that command attention. Other celebrities with this condition include Kiefer Sutherland, Henry Cavill, Christopher Walken and Benedict Cumberbatch. Less noticeable for their differences are the eyes of Demi Moore and Jonathan Rhys Myers.

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Although the late music legend David Bowie may be included in this list, strictly speaking he did not have heterochromia. Rather, the pupil of one eye was permanently dilated after an incident in adolescence, resulting in the captivating appearance of his eyes, and giving him his signature look.