Geneticists have not completely solved the riddle of eye color, but they do know that it is determined by genes inherited from parents. At least two genes are involved in eye color.
The gene OCA2 is thought to affect the amount of pigment melanin in the stroma of the iris. A large amount of melanin makes eyes brown, while little or no melanin makes eyes blue. Geneticists, including those at the Stanford School of Medicine, think that all other eye colors probably come from other genes, but this has not been proven scientifically.
Irises that don't match in color are likely caused by genetics, such as each eye having a different set of eye color genes, or disease or injury before birth.