Heterochromia Types and Causes

Heterochromia is  the name for a condition where two or one irises have different colors. This means that instead of one eye being a complete blue, the iris part, the colored circle enfolded around the black pupil and surrounded by the white sclera, the colored area will be half brown in the bottom half and half blue on the upper half, or visa versa. Heterochromia is how the eye coloring is structured and altered. The eye may have spots of green flecked throughout a solid brown eye. The word “hetero” translates to ‘different,’ while “chromia” translates to ‘color.’ Heterochromia can be congent(inherited at birth or soon after birth) or acquired later in life due to either an accident, disease, or some traumatic experience. There are three separately known types of Heterochromia: Complete heterochromia, partial heterochromia, and sectoral heterochromia. 

Complete heterochromia shows up rarely in humans, but is seen most common in animals. Complete heterochromia is when one iris tissue is a solid blue or some other color, while the other eye is a solid brown or some other color. Both eyes are a full complete color, but the coloring is different from the other eye. Usually the color difference is very dramatic and shocking. 

Partial heterochromia is characterized by the same iris having two different colors. The outer circle of the eye is what the true eye color is that will be passed on. Sometimes this type of Heterochromia has more defined flecks of color patterning the inner ring of the eye, this is known as having ‘cat eyes.’ 

Sectoral heterochromia occurs mostly due to a disease and is seen the most rarely. Sectoral heterochromia is recognized by the irregular spots receding throughout the iris as well as the uneven circle around the pupil. This can be in one or both eyes and usually needs treatment before this worsens vision or other vein or vessel pathways  in the eye. 

The birth and formation of heterochromia begins in most cases by a small mutation. This happens in early development, when the animal or human is still forming into a embryo. One cells picks up a mutation in it’s DNA. Maybe some chemical damaged it or the cell simply made a mistake when it copied its DNA. Whatever the cause, now all the cells that come from that cell have a different DNA code.

If, for example, the mistake happened when the fertilized egg first divides, then half of the human or animal’s DNA will have different DNA. Now both eyes have different eye colorings. Both complete and partial heterochromia are inheritable traits.

Genetic heterochromia is an autosomal dominant condition. This means the genetic abnormality must be dominant in just one parent to pass the condition to a child. Each child of a parent with the dominant trait for heterochromia has a 50% chance of developing the condition. Children with the heterochromia gene have a much higher chance of passing the characteristic down to their offspring. Children who do not have the heterochromia trait have no chance of ever passing heterochromia on to their offspring.  Inherited heterochromia very rarely has any strings attached that causes the animal or human any harm or future vision loss. This doesn’t shorten the subject’s lifespan or make them more prone to any illnesses or diseases. 

Sectoral heterochromia is different though. Sectoral, in most cases cannot be passed down to offspring. Sectoral can be linked with a variety of issues and is not pinpointed on any one disease. Many factors can cause sectoral heterochromia and the consequences for these factors are unique to each one. Sometimes the underlying cause of the condition can be treated; but the different eye colors will always be distinctly different colors.