Some people and cats have eyes of two different colors. This is called heterochromia. The color of the eye comes from a pigment called melanin. If one eye has less melanin, it can look blue.

In people, this is often harmless. In white cats, the gene that makes the fur white can also change the eyes and ears. So white cats with blue eyes are more often deaf.

Heterochromia is a condition in which the two eyes have different colors. Eye color depends mainly on the amount of melanin in the iris. Blue eyes do not actually contain blue pigment. When there is little melanin, the tissue in the iris scatters blue light, in a way similar to how the sky looks blue.

In humans, heterochromia that is present from birth is usually harmless, but when it appears in adulthood, it can be caused by an injury, inflammation, or certain eye drops. In white cats, a change in a gene called KIT makes the fur white. Because pigment cells may then fail to reach the eyes and inner ears, blue-eyed white cats are more likely to be deaf.

Heterochromia, the condition of having irises of different colors, arises when the two irises contain different amounts of melanin. According to a 2012 review, blue eyes contain no blue pigment at all: when melanin is scarce, fibers in the iris scatter short-wavelength light, much as the atmosphere makes the sky appear blue.

In humans, congenital heterochromia is usually benign, although it can occasionally signal conditions such as Horner syndrome, in which nerves running to the eye are damaged. In early twentieth-century experiments, removing a nerve cluster in a cat's neck changed iris color, so nerves seem to help maintain pigment. In white cats, heterochromia stems from the gene that whitens the fur. A 2014 study traced dominant white coats to an ancient virus-derived sequence inserted into the KIT gene.

Pigment cells that fail to reach the iris leave it blue, and their absence in the inner ear can lead to deafness. A 2015 review of 256 white cats found deafness in 65 to 85 percent of those with two blue eyes, but only 17 to 22 percent of those without blue eyes. The exact mutation behind the deafness has not been pinned down. Blue-eyed Siamese cats show no such link.