There is an old adage that the eyes are the windows to the soul, but a more accurate saying would be that eyes are the windows to a fascinating world of intricate genetics. Human eyes come in a wide range of colors, and each of them has its own special genetic code.
Eye Color and Genetics
A person’s eye color is based on the number and color of melanin (small coloring proteins), a trait influenced by as many as sixteen different genes, with the most prominent of the two stemming from chromosome 15 of the usual 46 that make up human DNA. There are three gene pairs (HERC2 and OCA2 in chromosome 15, and gey, all of which are located on chromosome pair 19) that influence eye color. Each of the alleles contains instructions for the positioning of melanin, a pigment in both human skin and the iris of an eye, which will determine how light or dark the eye is to be. Depending on the combination of these genes, an individual’s eye color may range anywhere from gray to brown.
Changes in Eye Color
It’s difficult to tell if an infant born with blue eyes will have the same color throughout the duration of their life, because most infants develop eye pigments with age. Eye color in both sexes will lighten or darken during childhood and puberty, and may continue to change with extraneous events such as pregnancy or physical trauma. Light-eyed people (such as those with blue eyes) are more likely to notice a darkening of hue as they age.
Less noticeable changes in eye color, such as variations in hue, may occur as a result of the pupil and iris expanding or contracting in different light and during different emotional stages. Dramatic changes in eye color may indicate an ocular disease, in which case a trip to the eye doctor is always recommended.
Range of Eye Colors in Humans
Amber: The difference between amber eyes and brown eyes is that amber eyes display a more orange color than brown ones do, due to an increased placement of yellow pigment (lipochrome) around the iris. This is a less common color in humans, and is more often seen in animals such as wolves.
Blue: Blue eyes are the result of low levels of melanin within the iris, and are a common eye color, especially for those of Northern European descent. Babies tend to be born with blue eyes since the melanin hasn’t been fully distributed in their irises yet. Blue eyes contain little or no brown pigment, and therefore interact with blue light more than with red or low-frequency light – the result is an effect similar to the way we see the sky’s color as blue, during which the interaction of light with the iris causes the blue light to refract and scatter.
Brown: Brown eyes are the most common variation among humans, and may be the exclusive eye color in many parts of the world, especially Asia, Africa, and Eastern and Southern Europe. This coloration, which ranges from dark brown to a lighter “honey” brown, is the result of a high concentration of melanin.
Gray: Like blue eyes, gray eyes are most commonly found in those of Northern European descent and also appear in the Middle East. Gray eyes are similar to blue in that they too have low levels of melanin, but with less refraction of blue light. Some may even include brown or yellow flecks of pigmented areas.
Green: Ranked as the most attractive eye color, green eyes are caused by the combination of light brown pigments with the scattering of light that causes blue eyes. They tend to be most frequently found in Europe, and may range anywhere in saturation from deep green to a light green/brown shade.
Hazel: Hazel eyes may appear to range in color from brown to green, but, similar to green eyes, are the result of a moderate amount of melanin combined with the refraction of blue light. Eyes of this hue often seem to have a dominant shade, such as brown, amber, or green, or may be perceived as a multicolored iris. Hazel eyes are commonly found in the Middle East, Australia, and those of European descent.
Red: Red is a rare eye color for humans, and is a natural effect of albinism, as the level of melanin in the irises is so low as to be virtually nonexistent. The low level of pigmentation allows for blood vessels to show through in certain light conditions, which creates the perception of red eyes. Red and violet eyes are particularly sensitive to light, since there is a lack of pigment protection against light waves.
Violet: Like red eyes, violet eyes are a result of albinism, though deep blue eyes may be incorrectly labeled as violet. Violet eyes occur as a result of a low melanin level, like red eyes, coupled with the refraction of light found in blue eyes. Violet eyes may be light-sensitive, due to the lack of melanin protection that eyes of darker shades have.
While no singular eye color is cause for alarm, due attention should be paid to changes in eye color, especially in regard to the whites of the eye. In individuals with a high level of melanin pigment in their eyes and skin, yellowing of the whites of the eyes is due to a natural level of pigment. For individuals who experience a sudden yellowing of the whites of the eye, this may be a symptom of jaundice or liver disease, and should act as a cue to visit a doctor.
Lighter iris colors, such as blue, gray, and green, may experience a higher percentage of macular degeneration with age. Macular degeneration is a loss of vision towards the center of the eye, and is a common cause of blindness or visual impairment in senior citizens. Lighter iris colors are also at a higher risk for ocular melanoma, or cancer of the eye.
A person born with eyes of different colors since birth has congenital heterochromia; this is not a medically alarming condition, but rather the result of a change in an iris’s early development. Heterochromia may manifest as two eyes of completely different colors, or an eye with a discolored section. If heterochromia develops over time, it is more likely to be due to changes in melanin and may be a symptom of a disease within the eye.
Eyes that appear to be completely black, or without an iris, are an example of aniridia. Aniridia is a mutation or congenital disorder that affects vision and unfortunately is associated with an increased level of ocular complications.
Eye color is the complex result of melanin proteins and genetic source code, and while there are trends as to where certain colors may be geologically found, the end result is due to a game of genetic chance. In the end, eye color is simply another component of what makes each individual unique, and is an excellent and beautiful example of the complexity of human DNA.