Complete heterochromia is very rare in humans and it results in having two completely different coloured eyes for example one blue eye and one brown eye. Complete is more common in animals like dogs and cats and nearly always results in them having one blue eye. Partial or sectoral results in having part of an eye a different colour than the rest. In Partial the outside of the iris is the true colour of the iris. A form of partial or sectoral is central heterochromia where the center ring of the iris is a different colour than the mid peripheral of the iris. Central is more common than complete in humans. Below are some causes off having different coloured eyes..
Congenital heterochromia causes
This is usually inherited as an autosomal dominant trait. An autosomal dominant trait is a gene on one of the non sex chromosones that is always expressed even if there is only one copy of it present. A single copy of the mutation is enough to cause a difference in eye colouration. This is in contrast to a recessive disorder where two copies of the mutation are needed to cause a disease or condition. Other causes of a difference in eye colouration include injury, inflammation, eye drops or tumours. These types of causes are called aquired heterochromia.
Aquired heterochromia causes
Aquired heterochromia means aquiring heterochromia rather than inheriting it. This can be done through injury, inflammation, eye drops or tumours. David bowie appears to have heterochromia however his eye was damaged in a fight and remains dilated which gives the appearance of him having different coloured eyes. Eye drops that have been used by glaucoma patients often leads to them becoming heterochromic. Heterochromia can also be aquired through tumours and a variety of different diseases.