Why do cats have different coloured eyes?

Cats can have eyes of different colors due to a number of genetic factors. The primary gene related to cat eye color is the OCA2 gene, which is responsible for producing the protein that helps determine how much melanin is produced.

Cats with blue eyes have less melanin in their irises, and cats with green, yellow, or gold eyes have more melanin. Cats with brown eyes have the highest amount of melanin in their irises. The color is actually a reflection of light off the tapetum lucidum, a layer behind the retina that helps improve night vision, passing through the pigmented iris, rather than a color inherent to the eye itself.

If a cat has two different-colored eyes — a condition known as heterochromia — it's generally the result of the relative presence or absence of melanin in the eyes. There are three types of heterochromia:

1. Complete heterochromia: This is when one iris is a completely different color than the other.

2. Sectoral heterochromia: This is when a segment or sector of the iris is a different color than the rest of the iris in the same eye.

3. Central heterochromia: This is when the central (pupillary) zone of the iris is a different color than the mid-peripheral (ciliary) zone, around the pupil.

In cats, complete heterochromia is common and is most often seen in white cats or those with point markings. It's generally caused by a lack of melanin affecting one eye and not the other. This can be genetic, as it often is in white cats with blue and green or gold eyes, or it can be due to mosaicism, where different parts of the body are genetically distinct.

Please note that sudden changes in a cat's eye color can be indicative of a health problem, such as uveitis, glaucoma, or other eye diseases. If you notice any such changes, please consult a veterinarian.