A coloboma is an inherited birth defect in which your dog's eye is either incompletely formed, has a hole in one of its structures or is missing a structure normally present in the eye. A coloboma may occur in the iris, lens, retina or optic nerve. Large colobomas and colobomas which occur in the optic nerve can lead to blindness. 

Colobomas of the optic nerve are known to accompany a condition called Collie Eye Anomaly.

You may be able to see a lesion or a discoloration on your dog's eye. However, in some cases the coloboma may be located in a part of your dog's eye that cannot be easily observed. Other symptoms may include squinting in bright light (as in the case of iris colobomas) reduced vision, and, in severe cases such as with large colobomas or colobomas of the optic nerve, blindness.

A coloboma is an inherited birth defect.