Heterochromia

How it works

Eye colour comes from melanin, the same pigment that colours skin and hair. It sits in the front layer of the iris. More melanin gives brown, less gives blue, and the amounts in between give green and hazel¹. 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.

When heterochromia is present from birth, it usually comes from a small difference in how pigment cells settled in each eye during development. 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.

Fewer than 1% of people have any form of heterochromia¹. It is far more common in cats, dogs, and horses, where breeders select for it.

History and culture

The actor Kate Bosworth has sectoral heterochromia, with a hazel wedge in one blue eye². 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. Both irises were the same blue. That condition is called anisocoria, a difference in pupil size³.