The legendary actress Elizabeth Taylor has lovely, if rather unusual, lavender-colored eyes, while the late-and-great Paul Newman was noted for his “baby blues,” as was the pop icon Frank Sinatra. The distinguished actor Morgan Freeman, on the other hand, has big, dark liquid brown eyes that look almost black in certain lighting. So, what is it exactly that determines the color of someone’s eyes? Then again, to understand eye coloring, one must possess at least some basic knowledge of genetics and of the eye itself.
The Structure of the Eye
Approximately spherical, the eye is a sense organ that enables you to perceive light. It is basically similar to a camera, with the cornea and crystalline lens being comparable to the camera’s lens system, and the retina being comparable to the negative upon which an image is captured and photographed. Moreover, there are three main parts to the eye: the cornea, the retina, and the iris, although only the iris plays a role in determining eye color.
The Role and Function of the Iris
The iris is the colored part of the eye, in which is located the pupil (dark spot in the center of the eye). The iris contains two sets of muscles that dilate or constrict in order to acclimate the pupil to varying amounts of light. By constricting in bright light and dilating in low light, the iris increases one’s ability to see under even the most extreme conditions, for example, during a sudden power outage or when power is abruptly restored, although it usually takes the pupil a few minutes to adjust completely.
How Heredity Contributes to Eye Color
The amount of pigmentation in the eye and the pattern of that pigmentation are both determined by a person’s genetic makeup (DNA), which is provided by his or her parents. Some facts about genetics:
- A human has 46 chromosomes divided into 23 pairs, with one chromosome in each pair inherited from the mother and one from the father.
- The pieces of DNA on each chromosome are called genes, which are the basic units of heredity and determine physical characteristics.
- The alleles within the genes determine the actual appearance of physical characteristics, for example, a person’s hair color, height, nose shape, and eye color.
- For each inherited trait, there are two dominant alleles and two recessive genes.
Regarding how allele affects eye color, since green allele is dominant over blue, and the brown allele is dominant over both green and blue, the following results can be expected:
- If a baby possesses a brown allele, he or she will have brown eyes.
- If a baby has a green allele on chromosome 19, but all others are blue, he or she will have green eyes.
- If all four alleles are blue, a baby will have blue eyes since only two blue-eye genes produce blue eyes.
According to the author of The Genetics of Eye Color, Linda Claire Guttery:
If both parents have a blue and brown gene, their eyes are brown, but if the child inherits the blue gene from each parent then the child will have blue eyes.
If, however, the child inherits only one blue gene, he or she will have brown eyes, not blue. Then again, although genetics determine someone’s eye color, other factors come into play when it comes to the actual formation of that color and its intensity, and these factors are pigmentation and melanin.
The Role of Pigmentation in Determining Eye Color
The iris contains variable amounts of pigmentation, and the amount and type of pigmentation are what chiefly determine the appearance of someone’s eye color, although that color was originally established by genetics. For instance, blue eyes contain black pigment with closely packed brownish granules, and the pigment is restricted to the back of the iris. Dark brown eyes, on the other hand, are those in which the pigment is distributed throughout the entire surface of the iris, and varying amounts and distribution of pigment produce green, gray, or hazel eyes.
How Melanin Production Affects Intensity of Color in the Eye
The more melanin contained in the iris, the darker the eye color appears, which is why some people’s eyes are darker shades and others lighter shades of the same basic color. It also explains why all newborns have blue eyes—because melanin production hasn’t yet begun, but once it does begin, babies’ eyes gradually darken, and their true eye color becomes apparent around the time they reach three.
In summary, the development of eye color is a complicated physical process but possessing at least some knowledge of that process can help people understand why their eyes are brown while other people are blue, green, gray, hazel, or perhaps even lavender like those of the still strikingly beautiful and legendary Ms. Elizabeth Taylor.