Heterochromia

How it works

Eye colour comes from melanin, the same pigment that colours skin and hair. The amount of melanin and how it is spread through the layers of the iris set the colour. The way light scatters in the iris also plays a part¹. Heterochromia means the melanin is spread unevenly. There are three forms¹:

• Complete: one iris is a different colour from the other.
• Sectoral: a wedge of one iris is a different colour from the rest.
• Central: a ring around the pupil differs from the outer iris.

Most heterochromia present from birth is inherited or has another genetic cause¹. In some cases the body is a mosaic, with two slightly different sets of cells¹. It also occurs as one sign of a few inherited conditions, such as Waardenburg syndrome, which combines it with hearing loss and a white forelock⁴.

Heterochromia can also appear later in life. Injury, inflammation inside the eye, some glaucoma eye drops, and a few other conditions can darken or lighten one iris¹. A change in one eye’s colour in adulthood is worth showing to a doctor.

How common it is

Heterochromia is uncommon in humans. It affects fewer than 200,000 people in the United States¹. It is unknown how rare sectoral heterochromia is in humans. It is thought to be less common than complete heterochromia¹. The trait is common in some breeds of cats, dogs, cattle, and horses because of inbreeding¹.

Is it a problem?

Usually not. Most heterochromia present from birth is harmless and not linked to any disease¹. It sometimes travels with an inherited condition such as Waardenburg syndrome, which also affects hearing, so a child with unusual eye colour and a white forelock of hair is worth a paediatric check⁴.

A change in one eye’s colour that appears later in life, rather than being there from birth, is different and worth showing to a doctor, since injury, inflammation, or certain eye drops can cause it¹.

History and culture

The actor Kate Bosworth was born with heterochromia. Her right iris is hazel and her left iris is blue². David Bowie is often listed as a famous example, but he did not have it. A fight in his teens left one pupil permanently wide, so that eye looked dark. That condition is called anisocoria, a difference in pupil size³.