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How rare is heterochromia?

Young woman with heterochromia has one gray eye and one brown eye

Heterochromia is definitely rare — fewer than 200,000 Americans have the condition, according to the National Institutes of Health. That’s only about six out of every 10,000 people.

This form of heterochromia is called complete heterochromia, meaning someone has two completely different colored eyes.

SEE RELATED: How eye color develops, and why it changes

Why is complete heterochromia so rare?

The family gene pool is rarely responsible for heterochromia. There is a slight chance it can get passed down from parent to child, but it’s unlikely.

This might be the biggest reason heterochromia is so rare — different colored eyes is a random genetic mutation that simply doesn’t happen very often. The mutation often happens before you’re born, and it’s usually harmless.

Certain forms of heterochromia can occur as the result of a secondary condition, injury or medication, but these aren’t classified as congenital heterochromia — present at or shortly after birth.

Sectoral heterochromia and central heterochromia rarity

There are a few different types of heterochromia. Sometimes splashes and streaks of color show up in one or both eyes, but the dominant color stays the same.

When a different color displays on a random section(s) of the iris, it’s called partial or sectoral heterochromia. This can show up as a single streak or a much larger portion.

Central heterochromia usually involves two colors in both eyes, with one color forming a ring around the pupil. A common expression of this condition is predominantly blue eyes with a smaller ring of brown or hazel that seems to radiate from the center.

It’s currently unknown how rare central and sectoral heterochromia are, but we do know that they aren’t quite as rare as complete heterochromia. In fact, you may not even notice sectoral or central heterochromia in some people.

SEE RELATED: Why are green eyes so rare?

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