Ol' blue eyes
Several genes determine eye colour. These genes affect the amount of pigment in the iris of your eyes but some have stronger effects than others. Two brown-eyed parents could still have blue-eyed children. In the parents, the effect of the blue-eye gene is masked by the brown-eye gene, but they may still pass on a blue-eye gene to their children. A child who inherits no brown-eye genes but gets two blue-eye genes will have blue eyes.