Heterochromia can result from a genetic mutation, but it doesn’t always. Most cases of congenital heterochromia (the kind you’re born with) trace back to random variations in how pigment-producing cells develop during embryonic growth, not a single identifiable mutation. In some cases, though, a true gene mutation is responsible, particularly when heterochromia appears alongside other symptoms like hearing loss or changes in skin pigmentation. The condition affects roughly 0.063% of the population, or about 1 in 1,600 people.
How Eye Color Works at a Cellular Level
Eye color depends on melanin, the same pigment that colors skin and hair. The iris has two relevant layers: a pigmented layer at the back and a layer of tissue at the front called the stroma. The amount of melanin in that front layer, along with the number and size of pigment-containing packets inside each cell, determines what color you see.
Blue eyes and brown eyes actually contain similar numbers of pigment-producing cells (melanocytes). The difference is that blue eyes have fewer pigment-containing packets and less melanin overall, while brown eyes have high melanin levels and densely packed pigment particles. Green and hazel eyes fall somewhere in the middle. Heterochromia happens when this melanin distribution ends up uneven between or within the two irises.
The Genetic Landscape Behind It
Up to 150 genes play some role in determining eye color, but two genes on chromosome 15 do most of the heavy lifting. One, called OCA2, produces a protein that helps pigment-containing structures inside cells mature properly. The other, HERC2, acts as a control switch for OCA2, turning its activity up or down. Variations in either gene can shift how much melanin ends up in the iris.
For most people born with heterochromia, the cause isn’t a harmful mutation in the traditional sense. It’s more often the result of uneven gene expression during fetal development. As pigment cells migrate from the developing spinal area to the eyes during embryonic growth, small random differences in which genes are active can leave one eye (or one section of an eye) with more or less melanin than the other. This is a normal quirk of development, not a disease-causing mutation.
Two specific biological phenomena can amplify these differences. In genetic mosaicism, a person’s own cells carry slightly different genetic instructions because a spontaneous change occurred after fertilization, meaning pigment genes express differently in one eye than in the other. In chimerism, which is far rarer, a person carries cells from two genetically distinct cell lines, sometimes from a twin that was absorbed early in pregnancy. Both can produce eyes with visibly different pigmentation.
When a Mutation Is the Cause
In a smaller number of cases, heterochromia does stem from a clear-cut gene mutation, usually one that disrupts how melanocytes form and spread throughout the body. The most well-known example is Waardenburg syndrome, a group of inherited conditions caused by mutations in genes like PAX3, MITF, SOX10, and others. These genes are involved in the formation of melanocytes, and when they’re disrupted, pigmentation develops unevenly across the skin, hair, and eyes.
There are four recognized types of Waardenburg syndrome. Types I and II look similar, with pale blue eyes or two differently colored eyes, though type I also causes widely spaced eyes and type II more commonly involves hearing loss. Type III adds abnormalities of the arms and hands. Type IV combines pigment and hearing changes with Hirschsprung disease, a serious intestinal condition. People with Waardenburg syndrome often have a white forelock of hair, patches of lighter skin, and one blue eye paired with one brown eye, or an iris with segments of two different colors.
Mutations related to albinism can also play a role. In oculocutaneous albinism, which is inherited recessively, mutations affect the enzymes responsible for producing melanin. While albinism typically lightens both eyes uniformly, partial or carrier-state effects can occasionally produce asymmetric pigmentation.
Three Types of Heterochromia
Not all heterochromia looks the same, and the type you have can hint at its origin.
- Complete heterochromia: One eye is an entirely different color from the other, such as one brown eye and one blue eye. This is the most recognizable form.
- Sectoral (partial) heterochromia: One iris has a wedge-shaped section of a different color, like a pie slice. This pattern often results from localized differences in melanocyte activity during development.
- Central heterochromia: The iris has an inner ring around the pupil that’s a different color from the outer ring, sometimes appearing as spikes radiating outward. This is the most common type and is usually a benign developmental variation.
Acquired Heterochromia Has Nothing to Do With Genetics
Eye color can also change after birth for reasons completely unrelated to your DNA. Trauma to the eye, iron deposits from a foreign body (a condition called siderosis), and growths like a diffuse iris nevus or melanoma can all alter pigmentation in one eye. Damage to the nerve pathways that control the pupil, as in Horner syndrome, can cause one iris to lose pigment over time.
One particularly common acquired cause is a glaucoma medication called latanoprost, a prostaglandin-based eye drop. Up to one-third of people who use it for five or more years experience a gradual darkening of the treated eye. The drug stimulates melanin production in the iris, and since it’s typically applied to only one eye or affects each eye differently, the result can be noticeable heterochromia.
A chronic inflammatory condition called Fuchs heterochromic iridocyclitis can also cause one eye to gradually lighten. First described in 1906, it involves low-grade inflammation inside the eye that damages pigment cells over months or years. The exact cause remains unclear more than a century later, though immune reactions within the eye appear to play a role. It’s often discovered incidentally during an eye exam rather than from symptoms the patient notices.
What Heterochromia Means for Your Health
The vast majority of people with heterochromia have the congenital, benign form. It developed before birth, causes no vision problems, and requires no treatment. It’s a cosmetic variation, like a birthmark.
The distinction that matters is whether heterochromia was present from birth or developed later. Congenital heterochromia in an otherwise healthy person is almost always harmless. But if your eye color changes as an adult, or if a child’s heterochromia appears alongside hearing difficulties, unusually pale skin patches, or a white streak of hair, those patterns suggest an underlying condition worth investigating. Acquired color changes in particular can signal inflammation, nerve damage, or even a tumor inside the eye.
In infants, eye color naturally shifts during the first year of life as melanin production ramps up. A baby who appears to have two different-colored eyes at three months may simply be going through normal pigment development. Persistent heterochromia that’s clearly established by age one, especially if accompanied by a smaller pupil on one side or drooping eyelid, warrants an eye examination to rule out conditions like Horner syndrome.