How rare is heterochromia in humans?

According to Dr. Andrew A. Dahl, the chance of having congenital heterochromia iridis is 6 out of 1000. Heterochromia iridis is an uncommon condition in which the two eyes are different colors. Heterochromia can also be acquired, often as a result of disease or other abnormalities, explains Dahl.

Congenital heterochromia iridis is either present at birth or it begins a short time later. Congenital heterochromia is inherited as an autosomal dominant trait and is unassociated with any other ocular or systemic abnormality. Dahl explains that eye of lighter color usually shows some loss of iris and is regarded as the affected eye. The lighter iris may be differently colored throughout or only in part.