There is more than one gene for eye pigment, as shown in the illustration below, but brown is always dominant over blue. Two people, one with two genes for brown eyes, the other with two  genes for blue
eyes (A), will have children who all have brown eyes. However, if two brown-eyed parents (B) carry the heterozygous recessive blue gene they will have one blue-eyed child for every three brown-eyed children.