Brown, blue and green, these are the most common colors of our eyes. Besides them, there is a variety of other colors that are a combination of colors mentioned above. Many people are born with nondescript gray eyes that change into blue or brown color during the development. What do we really know about eye color, in fact?
The fact that our eyes have a certain color is given mainly by a concentration of pigment called melanin in the iris. The iris is a part of the eye, situated between cornea and intraocular lens. It is a color visible part of the eye. If the iris contains pigment on both sides, light entering into the eye is reflected in such a way that iris has a brown color. Sometimes, however, there is no pigment or in a very small amounts on a surface of the iris. In this case, light reacts with gray fibers of the iris, which gets blue color.
Although most of the irides have a density of pigment on the back side very similar, some percentage of the population has a lower density of pigment. It is caused by light falling into the eye that is not bounced back from the iris, but passes through the retina where reflections occur. The light is reflected from the network of blood arteries and capillaries on the retina. The result is that light reflected has a red color and so-called red reflex occurs, that normally we can see in photographies as red eyes. If a red reflection causes an interaction with blue or brown color, the result is that final color of the iris is markedly blue or purple. This is apparent in people with a lower density of pigment.
There is a rare genetic disorder - albinism. In this case stricken individuals have no pigment and color of their eyes appears to be pink or red.
Sometimes it may also happen that pigment from the back side gets in the front side to edges of the pupil. The result is a brown ring around pupils that acts in a combination with blue eyes very oddly.
Many of us learned a simple Mendeleev's or so-called "two-genic" genetic model to predict eye color in school. This model is based on the fact that blue color of eyes has a recessive characteristics. This means that children will have particular eye color on the basis of mother's and father's eye color. However, this model no longer appears to be entirely reliable, but still is presented as an explanation for eye color of offsprings.
Gradually, as we heard about genetics and cell chemistry more and more it was proven that melanin is influenced by genes for brown and blue color on the 15th. chromosome and by genes for green and blue color on the 19th. chromosome.
However, new and more accomplished genetic models still do not work perfectly, since it has been found out that there is at least one other gene determinig eye color. It is a gene of a brown type on the 15th chromosome. Scientists also found out that in irides of green, purple and tan color there is a tawny pigment called lipochrome.
Many children, especially those of white-skin, are born with neutral eye color. As eyes are exposed to sunrays, the melanocytes in the iris start to produce melanin pigment and eye color gradually starts to change and resembles to parents' eye color. The density of melanin in the 1st. year of life is about 50% of density compared with an adult. Pigmentation is complete in the 3rd. year of life.
Eye color changes in adults are also very interesting phenomena. As we have already mentioned, eye color is the result of a light rays reflection from outside of the structure of the iris. People with light-color irides often argue that color of their eyes changes depending on color of their clothes. It is no mystery, as in this case color of the iris is conditioned by a combinations of colors reflected from clothes or even make-up and is not always of what you would expect.
The health state can also affect eye color. Many people are faced with the fact that when they are sick or stressed, the color of their eyes is darker or lighter. This is due to distribution and density of melanin or lipofuscin on the iris. The exact mechanism is not fully understood yet.
Change of eye color can also be caused by ingestion or a specific disease such as glaucoma. The mechanism of drug action for changing eye color is ascertained only partially but it has a similar effect as the hormone prostaglandin, which can be commonly found in a human body. It is partially proven that influence of hormones has an impact on pigmentation of the iris, which can also be explained the change of eye color in adults by.
As some diseases can cause the change of color of the iris, it is good to see a doctor when noticing such a change.