Your genes determine whether you have blue eyes. If you inherit genes for blue eyes from your parents, your eyes will have less of a pigment called melanin in them.
Approximately 27% of people in the United States and between 8% and 10% of the world's population have blue eyes. The highest density of blue-eyed people overall is around the Baltic Sea in northern Europe where the mutation is thought to have originated, including Denmark where 65% of the population have blue eyes and Poland where nearly 53% have blue eyes.
This article discusses where blue eyes come from, the advantages of blue eyes, and how having blue eyes may increase your risk for certain health conditions.
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Why Some Eyes Are Blue
The colored part of your eye, called the iris, has multiple layers. The top layer, called the epithelium, is where melanin is lodged. If you have plenty of melanin in the top layer, your eyes will be brown. If you lack melanin in the top layer, your eyes will be blue.
Having blue eyes doesn't mean that your irises are blue. In fact, in the absence of melanin, your irises are entirely colorless.
The blue color is caused by the absorption and reflection of different wavelengths of light. The lack of melanin causes fibers in the iris to absorb longer wavelengths of light (like red and yellow) and reflect shorter wavelengths of light (like blue), making your eyes seem blue.
At birth, a baby's eye may only be temporarily blue. It can sometimes take up to six months for the gene responsible for producing melanin to activate. During this time, the baby's eyes may be blue and later shift to brown.
If the gene doesn't activate (or only partially activates), the eyes may remain blue.
What Is the Most Common Eye Color?
Genetics of Blue Eyes
Genetics determine what your eye color will be. For blue eyes, the pattern of inheritance—or the genes you inherit from your mother and father—is polygenic, meaning multiple genes are involved.
Traditionally, schoolchildren have been taught that blue eyes are a recessive trait, meaning that both parents have blue eyes and both contribute a non-dominant blue eye gene. What scientists now understand is that blue eyes are a polygenic trait and that having two parents with blue eyes can sometimes lead to a brown-eyed baby.
With polygenic inheritance, the genes for blue eyes are not recessive but instead have "incomplete dominance" with some gene variations being more dominant and others being less dominant. Depending on the genes contributed by your parents, your eye color may be expressed on a spectrum of blues to browns.
For instance, if both parents contribute less dominant genes for blue eyes, you are more likely to have brown eyes. If both parents contribute more dominant genes for blue eyes, you are more likely to have blue eyes. Anything in between may cause lighter or darker shades of blue or even amber- or hazel-colored eyes.
Blue Eyes as Evolutionary Adaptation
Blue eyes are thought to be an evolutionary trait that only occurred 6,000 to 10,000 years ago in response to dark winters in northern Europe. By reflecting blue light (the bandwidth that predominates at night), blue eyes may have served as adaptive protection from cataracts and other eye diseases caused by excessive blue light exposure.
How Your Genes Determine Eye Color
Benefits of Blue Eyes
A growing body of research has found that eye color may play a role in not only your eye health but also your psychological health and well-being.
Cataracts
Many studies have shown that having blue eyes reduces your risk of getting cataracts (an aging-related disorder characterized by the clouding of the eye's lens). However, the benefits are inconsistent and appear to be different depending on where you live.
For instance, studies in Australia concluded that having blue eyes reduces your risk of cataracts by 2.5-fold, while a study in Barbados reported a decreased risk of nearly four-fold if you have blue eyes.
On the flip side, a study from Iran reported that blue eyes were associated with a two-fold greater risk of cataracts compared to people with brown eyes. Another study from Iceland reported no difference in the risk between blue or brown eyes.
It is possible that your location nearer or closer to the equator (along with seasonal variations in light exposure) might account for these differences.
Seasonal Affective Disorder
While limited, there is some research showing that blue eyes may protect against seasonal affective disorder (SAD).
Several studies have shown that brown-eyed people are more likely to suffer from this type of depression, which develops in response to fewer daylight hours during the winter month. The disruption of the normal circadian (sleep-wake) cycle largely accounts for the onset of seasonal depression.
People living in the far northern hemisphere are especially vulnerable.
Among the possible explanations, researchers have found that the pupils of blue eyes don't contract (narrow) as much when exposed to blue light that predominates after dark. What this means is that more light can enter the eye during the dark winter months, lessening the dramatic shifts in the sleep-wake cycle.
What Having One Blue Eye and Brown Eye Means
Risks of Blue Eyes
Despite the potential benefits of blue eyes, studies have also shown that they can also put you at higher risk of certain health problems compared to people with brown eyes.
Eye Cancer
Eye health providers remind almost everyone with blue eyes to wear sunglasses to reduce their risk of potential eye cancers like eye melanoma. In the same way, you can get melanoma on your skin, you can also get melanoma in your eye.
Eye melanoma is known to be more common for those with fair skin and light-colored eyes. While ocular melanomas may happen at any age, the risk goes up as you get older.
Does Eye Color Affect Your Cancer Risk?
Macular Degeneration
The macula, which is at the retina's center, is susceptible to damage as you age. This damage will cause your vision to become blurrier and more distorted—a condition called macular degeneration.
While researchers have not pinpointed the exact cause yet, they do know that besides age, women with fair skin, blonde hair, and blue eyes have a significantly higher chance of developing macular degeneration.
Are Blue-Eyed People More Likely to Get Macular Degeneration?
Hearing Loss
Scientists are looking into the possibility of those who have blue eyes being at higher risk for sensorineural hearing loss.
This form of hearing loss comes from damage to the inner ear or to the nerve connecting the inner ear to the brain. Since the inner ear uses melanin—and blue eyes come from a lack of melanin—some researchers hypothesize there may be a link between eye color and acquired hearing loss.
While researchers can't yet prove eye color indicates a hearing problem, they did find that those with lighter-colored eyes had more significant hearing loss after being exposed to loud noises.
Summary
Blue eyes are an inherited trait. If you have blue eyes, it means the iris part of your eyes lacks melanin, so, technically, blue eyes don't have any color. They look blue because of how light is reflected.
Having blue eyes has its advantages. They lower your risk of developing cataracts, for instance. However, they might increase your risk of certain health problems like eye cancer. Protecting your eyes and getting regular checkups is important no matter the color of your irises.
How to Protect Your Eyes
