The topic of eyes, and more specifically eye color, has been as fascinating as it has been mysterious. For this reason, there are endless researches and writings on the topic. Most of them revolve around how eye color is connected to personal traits, DNA, genes, and many others. On this, we shall look at the concept behind different eye colors among people of the same origin.
The eye color (more accurately, the iris color) is usually as a result of the pigmentation of a structure in the eye, which surrounds the pupil, called the iris. When babies are born, most of them have their iris color ranging from almost colorless, to a very light blue. This is because they do not have melanin in the eyes.
As the child develops, melanocytes (these are cells that are found in the hair, skin, and the iris) start producing melanin. In the next one-three years, the iris changes to its permanent color.
The color that a child will have has been continually linked to the color of the eyes of the parents. This is not all true. It was based on findings by researches that showed that iris color was a simple Mendelian trait, following a predictable inheritance pattern and was triggered by a single gene. Further studies showed that iris color is a polygenic trait. Although the parents do have an influence on the iris color that the child will have, it is not directly. The genetic materials that are contributed by each parent could mix in many different ways. Two blue-eyed parents, although rarely, could give rise to a green-eyed child.
Terming iris color as a polygenic trait means that a number of genes influences it. The genes that are involved in the coloring include TYR, TYRP 1, IRF4, ASIF, SLC24A5, SLC24A4, SLC45A2, and TPCN2. Mendelian traits, as previous researches had concluded, meant that it was determined by a single gene. Of the endless number of genes that determine the iris color, the most dominant are found on the human chromosome 15. They are the OCA2 and the HERC2.
The OCA2 is responsible for transporting a precursor of melanin, tyrosine. Mutations in this gene may cause problems related to vision such as increased light sensitivity and reduced brightness. This condition is called oculocutaneous albinism. HERC2 is more closely associated with the blue iris color in the Asian and European population.
Eye color is also dependent on the lighting conditions as well as the structural color. Having blue or green iris color does not mean that you have the pigment green or blue in your iris. This is where the scattering of color by the eyes come in. Using a similar phenomenon to that of the Rayleigh scattering (which causes the blueness of the sky), the eyes scatter light to cause a different appearance of the iris. This is called the Tyndall scattering.
The green color of the eyes contains a yellowish pigment called lipochrome. It is caused by a combination of two things. A blue shade that is created by the Rayleigh scattering of light, and the light brown or amber pigmentation of the stroma in the iris. This light pigmentation has a relatively low melanin concentration.
Blue eyes, also have a relatively low melanin concentration in the stroma when compared to brown, or darker eyes. Under the stroma, is a dark epithelium. Light with longer wavelengths is absorbed by the epithelium while that with shorter wavelengths undergoes Tyndall scattering. The result is a “Tyndall blue” color that tends to vary with lighting conditions.
How common is green eye color?
Green color, as abovementioned, is only found on people with yellowish colored stroma. The yellow, plus the blue from Rayleigh scattering, equals green. Generally, green iris color is the rarest in the world. Only 2% of the world population is estimated to have green eyes.
However, it is most common in Europe and some parts of Asia. In some European countries, it is the most prevalent of all colors. For example in Iceland, more people have green and blue eyes, compared to those having all the other colors. One study found that green eyes are more predominant in females than in males. 17 percent of the women in the study had green eyes, as compared to only 8% of the men.