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Everything you need to know about Heterochromia and Eye Coloring

Have you ever seen a person or an animal with eyes of a different color? It’s not a common trait (it’s even less common for humans), but it exists. It’s called heterochromia. It means the production, delivery, and concentration of a pigment (melanin) is affected.
There are three types, and one of them is extremely rare in our species but can be found in white cats that possess the white spotting gene. The same can be said about Siberian Huskies: but why is their breed affected by heterochromia so much?
But Is it dangerous? Is there a treatment? Does it only affect the eyes or can other body parts be affected as well? Why does it happen? Sometimes it’s syndromic. Sometimes it’s acquired because of an injury or a disease. There are cases that are purely genetic: you’re just born with the condition.
Keep reading and learn more about heterochromia.

Coloration

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Heterochromia is a difference in coloration. This can affect the iris (the circular, thin structure of the eye), but it can also affect the hair or skin, or sometimes even the three.

Melanin

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How is heterochromia determined? It’s all about the production, delivery, and concentration of melanin, a group of natural pigments that can be found in living organisms. Since melanin is a pigment, this means it can change the color of lights that are reflected or transmitted.

Causes

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Heterochromia can be caused by genetic mosaicism. This means there are two or more populations of cells with different genotypes where there should be only one. Organisms composed of these cells are called ‘chimera’. Heterochromia can also appear after an injury or as a consequence of a disease.

Types

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There are three types of heterochromia of the eye (also known as heterochromia iridium). You can have one iris colored differently than the other (complete heterochromia), part of the iris is a different color than the rest (segmental heterochromia) or different shades and colors radiating from the pupil (central heterochromia).

Segmental Heterochromia

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Segmental heterochromia is not often found in the human species. According to studies, the estimation is that only 1% of humans have this type of heterochromia. Segmental or sectoral heterochromia is recognizable because the same iris has two different colors.

Eyedrops

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According to some research, acquired heterochromia can be caused by using certain eye drops. This is why you should always check with your doctor about what options are best for you, so you can make informed decisions.

Animals

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This condition is also common in other animals, especially certain breeds of cats and dogs. It has also been seen in koalas and horses. Horses with pinto coloring (large patches or white combined with smaller patches of any other color) are more prone to heterochromia.

Blue Eye

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You may have noticed that those who have heterochromia almost always have one blue eye. This happens because melanin (the pigment we were talking about before) is absent, and so the blue coloring occurs within a white spot (since there isn’t any pigmentation there).

Dog

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Dogs have to meet certain standards to be considered purebred. Special associations decide what these standards are, and they’re known for being very inflexible about it. Dogs with heterochromia often don’t meet the criteria, thus they don’t qualify as purebred simply because of this trait.

Siberian Huskies

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The Siberian husky is one of the dog breeds more prone to have this condition. Many Siberian Huskies have heterochromia. Purebred Siberian Huskies have blue eyes, but particolored individuals are accepted by American Kennel Club.

Sight

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Heterochromia is not an eye disease (although it can be caused by diseases and syndromes, more on that later!) This means that it does not affect an individual’s vision in itself. People and animals with heterochromia don’t suffer any alterations in their sight.

The Eye With The Lighter Colored Is The One Affected

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As we’ve explained before, heterochromia happens because there are alterations in the production, delivery, and concentration of a pigment called melanin. In cases where this condition is present, melanin is absent. The eye with the lighter color is considered the one that has been affected by this deficiency.

Complete heterochromia

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Complete heterochromia is more common in dogs, cats, and other animals. There are cases of complete heterochromia in humans, but those aren’t the most common. Siberian huskies usually have complete heterochromia, as do some cat breeds.

Iris

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How much do you know about the iris? It’s a part of the eye that is responsible for controlling the size and diameter of the pupil and the amount of light that reaches the retina (a layer that is sensitive to light). Heterochromia affects the coloring of the iris.

Two Layers

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The iris has a fibrovascular and pigment layer of tissue called stroma that connects the muscles responsible for pupil contraction and dilation. Beneath the stroma, you will find a second layer with pigmented epithelial cells.

Two Major Regions

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There is an inner region of the iris, known as the pupillary zone. Its edges form the boundary of the pupil. Then there is an outer region, known as the ciliary zone. This is the rest of the iris and it extends to the ciliary body, where the iris originates.

Cats

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As you may have noticed, cats that have this condition often have white coats. Why is that? It’s because these animals possess the white spotting gene. The lack of melanin is the cause of the white fur (as we’ve mentioned before, heterochromia sometimes affects skin or fur pigmentation as well).

Congenital Syndromes

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Heterochromia in itself does not affect the vision of an animal or a person, but there are many syndromes that cause heterochromia. This is why it’s always important to check with your doctor if you notice any changes in your eye color.

Benign

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However, in most cases heterochromia is benign and it only is a variation in color due to a lack of melanin, so it shouldn’t be something to worry over. But, as you know, better safe than sorry! Never miss a doctor’s appointment!

Tuberculosis

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This infectious disease is caused by a bacteria and is terribly contagious in some cases. It most commonly affects the lungs, but sometimes it also affects other parts of the body. It’s listed as one of the medical conditions that can cause acquired heterochromia.

Cats Of Any Colors

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Even if cats with heterochromia are often white because the condition also affects the pigmentation of their fur, the truth is that all cats can have any of the three types, no matter the color. The heterochromia doesn’t necessarily have to affect their fur.

Central Heterochromia

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As we’ve stated before, central heterochromia happens when there are different colors radiating from the iris due to different pigmentation of the two regions. The inner and mid-peripheral zones are the most affected, thus the color of the outer zone is considered the ‘true color’.

The Process Not Understood

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However, it’s worth mentioning that scientists still don’t fully understand how the processes that determine eye color work. They do know that the genes we inherit from our parents determine our eye color and that there are other factors (such as the environment) that can come into play.

Siberian Huskies From Inbreeding

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Perhaps we’ve left you wondering why Siberian Huskies are so prone to heterochromia. The answer is simply: this is a breed that came to be as a result of inbreeding. The standard of the breed is blue eyes, but there’s also a genetic disposition that causes these dogs to have heterochromia.

Coat Pattern

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However, heterochromia in Siberian huskies doesn’t affect their fur pattern or fur color. It only affects these dogs’ eyes. You will notice that all Siberian huskies with this condition have perfect pigmentation in their white and dark fur.

Unkown reasons

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As we’ve told you before, Siberian huskies have heterochromia because of a genetic disposition that resulted from the inbreeding. They know it’s not linked to the coat, but they don’t know much more. Experts are still researching more about this gene.

Cute

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What we do know is that, whether they have heterochromia or not, Siberian Huskies are absolutely cute. They can have normal lives and excellent vision in spite of the lack of melanin in the irises. Don’t let the fact that they’re not considered purebred discourage you: adopt one!

Go To The Doctor

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If you notice that your kid’s eyes are different colors or that the color begins changing, take them to a doctor for a check-up. Remember that heterochromia in itself is benign, but lots of syndromes and diseases manifest themselves through heterochromia.

Only Six In 1000 People

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Heterochromia is more common in cats, dogs, and other animals than it is in humans. Did you know that only 6 out of 1000 humans have this condition? If you look at the numbers, you can see why it’s considered extremely rare! In some cases, you can’t even notice it!

Glaucoma

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Glaucoma is an eye disease that damages the optic nerve. A lot of cases result in permanent loss of vision. There are treatments, but the sooner glaucoma is detected, the better. If you have blurred vision, severe pain and redness in the eyes or mid-dilated pupil, check with your doctor.

Treatment

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Another reason why a person has heterochromia can be inflammation caused by iritis or uveitis. This happens when the pigmented layer between the inner retina and the outer layer (uvea) gets swollen. This requires immediate professional treatment and medical care.

Pigment Dispersion

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The pigment is the material that gives your irises color. Sometimes the pigment rubs off the back of the iris and floats around until it reaches other parts of the eye. This can be a cause for heterochromia, too, and it’s known as pigment dispersion syndrome.

White Spotting Gene

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But what is the white spotting gene, exactly? This is a gene that affects the coloration pattern. In fact, it’s not a single gene: there are several genes involved. The white spotting gene is dominant, and it also affects the length and texture of the fur.

Shades Of Blue

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All kittens are born with blue eyes. As they grow older, the “true” color begins to appear. Sometimes it remains blue, whereas on other occasions it turns to amber or green. Kittens with heterochromia have blue eyes when they’re born, but the shades are different, the lack of pigmentation already manifesting.

Deafness

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But it doesn’t mean that heterochromia causes the hearing loss. In fact, the age of an animal can be the reason why their hearing cells begin to die. But, just in case, if your cat has heterochromia pay attention to any signs that could indicate they’re not hearing properly.

Van Cats

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Van cats are found in the Lake Van Region (Eastern Turkey). Also known as the ‘swimming cat’, their coats are white like chalk, they’re large in size, and they often have one blue eye and one amber eye (cats that don’t possess the condition have either blue or amber eyes).

Dalmatians

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A lot of dalmatians are born with heterochromia. This means that they’re not considered purebred. Dalmatians also have hearing problems, so you should always remember to schedule frequent vet appointments for your pet and ask for their sight and hearing to be tested.

Horner’s Syndrome

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Horner’s syndrome is a combination of signs and symptoms that appear in an organism when the sympathetic trunk (a group of nerves that form the sympathetic nervous system) is damaged. This can cause heterochromia as well as other eye conditions.

Exotic

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A lot of people consider animals with heterochromia ‘special’ or ‘exotic’ for several different reasons. Some say it’s because they’re magic, mythical creatures and it can be seen in their eyes. Others simply like the fact that their pet has an eye of each color.

Red

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Have you noticed how some people or animals appear with red eyes on flash photos? Cats with heterochromia don’t show this red-eye effect in the non-blue eye because melanin is present there and it will form a layer over the eye tissue, selectively removing the color red that would have appeared otherwise.

Treasure

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In some cultures, cats with heterochromia are considered special animals. These creatures are even protected and preserved by special programs, like the one that began in Turkey in 1817 with government support. This breeding program still exists over two hundred years later.

Calico Cats

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Calico cats are also prone to this condition. This is a domestic animal with a white coat covered in patches or one of two different colors (black and orange are the most usual, but there are exceptions). It’s not a breed, it’s a color pattern on the cat’s fur.

Retinal Dysplasia

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This is a disease that affects the retina of animals. Retinal dysplasia is not a very common condition in humans. Folds can be found in the retinal tissue when examined. This condition can be genetic or acquainted (some viral infections, drugs and vitamin deficiencies cause it).

Turkish Folklore

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Odd-eyed cats (that’s how felines with heterochromia are usually called) are very popular in Turkish folklore since Turkish Van and Turkish Angora cats are prone to this condition. As we’ve mentioned before, at the beginning of the nineteenth century the Turkish government even began a program to protect them.

Temperament

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Cats with heterochromia are still that: cats. The condition doesn’t change the behavior of the individual that possesses it. So don’t worry, your kitten may have eyes of a different color but it’ll still be happy to drop off the table whatever it is you’ve just put on it.

They Thought They Were Blind

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There was a time when it was believed that the blue eye was blind in cases of individuals with heterochromia. It is been proven that both eyes work well and that heterochromia in itself does not affect eyesight. Take your pet to the vet if you suspect they’re having eye trouble!

Mystical Creatures

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Some people believe that dogs with heterochromia are special in more ways than having eyes colored differently. They think these dogs are special, mystical creatures with the ability to see both heaven and earth simultaneously. This is one of the reasons why many people want a pet dog with heterochromia.

Optic Nerve

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Optic nerve hypoplasia is a medical condition that happens when one or both optic nerves are underdeveloped. Regarding optic nerve anomalies, this is the most common condition and it is believed hormone deficiency is what causes it.

Corneal Dystrophy

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Corneal dystrophy is a condition caused by an accumulation of external material in the cornea (the front part of the eye, that is transparent and covers the anterior chamber, pupil and iris). It requires proper treatment from its early stages so vision is not affected.

David Bowie

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Singer and songwriter David Bowie had particularly interesting eyes. However, he did not have heterochromia. Bowie had anisocoria (his pupils were unequal sizes), which means that even if it looked like his eyes were different colored, they were not. His condition was not genetic but rather acquired after an incident.