What it is
The iris is a ring of muscle that widens and narrows the pupil, letting in more light in the dark and less in bright sun. A coloboma is a piece of that ring that never formed while the puppy was developing in the womb. The gap may be a small notch at the edge of the pupil or a hole through the iris, and one eye or both can be affected.
The defect is present from birth and has been linked to coat colour genes, especially the merle coat, where the iris may also be thinner.
Because the pupil cannot close down properly, more light reaches the back of the eye than it should. In bright sun or snow your dog may squint, look away or seek shade. A shaded spot outdoors and walks outside the brightest hours make life more comfortable, and the eyes are worth checking at every vet visit, since other changes in the eye can come with it.
Signs you may notice
You may notice that the pupil has an unusual shape, such as being elongated or notched, or that there is a darker patch or gap in the iris. The dog may squint or avoid bright light, and sometimes there are no signs at all and the defect is only spotted when the eyes are examined closely.
When to see a vet
Have a vet check the eyes if you notice an unusual pupil or a gap in the iris. A red, cloudy or painful eye, or any sign that your dog is not seeing well, is worth a prompt visit.
This is general information, not veterinary advice. For your own dog's health, tests and treatment, see a vet.