The colored part of your eyes is called the iris. While some people have blue eyes, others have green or brown. Some are more blue-green or hazel. Everyone’s eye color is a bit different. Here’s what you need to know.
How Eye Color Is Determined
In the past, people believed you could predict the eye color of children by looking at the eye colors of the parents and grandparents. Based on the belief that brown eyes are a dominant trait and blue is recessive, you could get a good idea of what color the child’s eyes would be.
Today, we know that eye color isn’t as easy to guess as looking at the parents’ eyes. While genetics does play a role, eye color is not the work of a single gene. Instead, several genes contribute to determining eye color.
Your eye color is the result of the amount and distribution of melanin (a natural pigment) in your irises. Brown eyes have more melanin in them than blue, and there are various shades in between. Darker eyes tend to be more dominant, but as different genes factor in, this doesn’t mean darker colors always win out.
So, while two brown-eyed parents are more likely to have a child with brown eyes, the result isn’t a guarantee. Nor will it always be the case that a child of one brown-eyed and one blue-eyed parent will have brown eyes.
Approximately half of all people in the U.S. have brown eyes. The color is also more prevalent in areas of the world with warmer climates. People with blue eyes have no melanin in the stroma, the front layer of the iris. The lack of pigment in the eyes causes light to scatter when it hits them, making the irises appear blue.
Green eyes are the rarest. Only about 2% of the world’s population has green eyes. The color comes from both melanin and the effect of light scattering when it hits the eye.
People born with albinism often have little to no melanin in their bodies. As such, they typically have light blue eyes. In rare cases, they may have clear irises, which can make their eyes look pink or red.
Can Eye Color Change?
Eye color can change in infancy. Many babies are born with blue eyes that eventually become a different color as melanin develops in the stroma. Their eye color generally becomes permanent around their first birthday.
In general, it’s rare for eyes to change color. They may appear to change when your pupils dilate or shrink, but this occurs because the pigments in the irises come together or spread apart. In some cases, eye color can darken slightly during puberty or pregnancy, or as you reach your later years.
Health Problems That Can Affect Eye Color
In some cases, health problems can affect or change the color of your eyes.
Trauma. An injury or trauma to the eye can result in iris damage. Any tissue loss that occurs can alter the appearance of the eye’s color.
Neurofibromatosis. Neurofibromatosis is a condition that affects the nervous system. It can cause small tumors to grow on nerve cells throughout the body and may lead to small nodules on the irises. These generally harmless growths are called Lisch nodules. While they don’t affect your vision, they can alter the color of your eyes.
Uveitis. Uveitis is a term used to describe a group of inflammatory diseases that cause swelling in the eyes. It can affect your vision and even lead to loss of sight. You may also notice changes in the color of the affected eye.
Fuchs' heterochromic iridocyclitis. Also called FHI, this condition is a form of chronic uveitis. It can lead to atrophy of the iris, cataracts, and eye inflammation. The condition can also cause pigment loss, which can change the color of one eye, leading to heterochromia, or two differently colored eyes.
Horner’s syndrome. Horner’s syndrome is a rare condition that may occur as a result of a stroke or spinal cord injury that damages facial nerves. Signs include pupil constriction (miosis), eyelid drooping, and a lack of sweating on one side of the face. It may also cause depigmentation in the iris, causing the color of the eye to change.
Cataracts. A cataract is the clouding of the eye’s lens, which is behind the pupil. While cataracts don't affect the iris directly, they can change the appearance of the affected eye’s color, making it appear cloudy or milky. Cataracts are more common in older adults and can affect your vision. They can be treated with surgery. While the exact reason isn’t known, those with darker eyes are at a greater risk of developing cataracts.