When a child is conceived the parent will pass on any one of these genes. this can result in 2 parents with brown eyes having children with green or blue eyes (BG +Bb = bb). depending of which gene was passed on, but if one parent passes the gene for green eyes and the other passes on the gene for blue eyes, then the child will always end up with green eyes as green is dominant over blue.(BG + Bb = Gb or GB)