Why this eye color?

The eye color is a genetic trait that is determined by the amount and distribution of melanin in the iris. It is a complex process involving several genes towards the final result. Similarly, there are three elements that contribute to iris give your colored : the melanin in the iris epithelium, the melanin in the front of the iris and the density of the iris stroma. In addition to melanin, other pigment acting on the process is the lipochrome. Also, the two types of melanin are involved in the process of eumelanin, the dark brown appearance, and pheomelanin, looking brown-red amarillante. In all eye colors, except for abnormal conditions, the iris pigment epithelium is always eumelanin, performing in large quantity. In this way, the eye color variation is produced in principle by the pigment the front of the iris and how much light is absorbed by the stroma according to their density.

In the world there is relative uniformity in the color of eyes, the brown being common. However, the exception is Europe, where it generated a variety of colors: hazel, green, blue and gray. At first it was thought that this peculiarity was a side effect of natural selection, associated with lighter skin to ensure sufficient vitamin D at latitudes considerably distant from the Earth Ecuador. However, the pigmentation of the skin is affected only slightly by the genes for eye color, there being no relationship in many cases. was also identified as causes hybridization with Neanderthals. However, the mitochondrial genome recovered from the latter shows a genetic discontinuity compared to modern Europeans, so that if we claim that there was some gene flow between them, this was not provided, considering the percentage of Europeans with light eyes. Finally, some geneticists like Luigi Cavalli-Sforza, say the answer lies in sexual selection. When one sex outnumbers the other, individuals in the first group to compete for a partner, through strategies to attract attention, such as the use of bright colors or flashy.

Brown eyes contain large amounts of melanin in the front of the iris, which serve to absorb lots of light, especially the wavelengths shorter than this. One of the most common phenotypes for eye color in humans. Studies in the Graduate Institute of Medical Biochemistry and Genetics Danish Centre for Genome Research found that these are genetically related to the polymorphism D15S165, accompanied markers D15S156 and D15S144, all located on chromosome 15 (15q11-21).

The amber eyes are a shade which can range from yellowish to reddish golden coppery tint. Amber is an intermediate color between brown and hazel. The pigment responsible for this coloration is lipochrome, also called dusky pigment, which is also present in green eyes. Genetically there is no explanation for this color.

The hazel eye is a tone intermediate between brown and green. It may also contain areas where the brown, amber or green are presented clearly. From the genetic point of view it is not clear explanation for this. However, a geneticist at Stanford University, Barry Starr, has suggested a probable model, based on a hypothetical gene that act on M EYCL1 gene, making the latter produce more melanin. The dominant allele would generate this effect, while the recessive allele would be a neutral factor. So EYCL1 altered by the action of M gene, and together with EYCL3, would equal hazel eyes. However, this is just a likely model.

The green eyes are the product of moderate amounts of melanin. Green is an intermediate color between brown and blue or gray. The genes involved in this effect are EYCL1 and EYCL3. Thus, the dominant allele EYCL1 conjugated with any allele for EYCL3, except for the dominant allele of the latter, obtained as a result the green color of eyes.

Blue eyes or blue contain low amounts of melanin in the front of the iris. Are produced, and including the previous feature, because the stroma, transparent tissue composed partly of collagen, gets in the way of light when it is reflected from the iris epithelium outwards. Thus, the light bends when it hits with collagen, being reflected blue. is the combination of genetically recessive alleles of genes and EYCL3 EYCL1. first appeared 10,000 to 6,000 years northwest of the Black Sea, a product of genetic mutation of a single individual. present are relatively common in parts of Europe and outside it in some areas with populations of European descent. The blue-eyed world population is 150 million people.

Still unknown factors that lead to acquiring the iris gray appearance. However, there are two theories. The first of which indicates that a stroma, composed of a large amount of protein or by a large size of these, is interposed in the path of the light when it is reflected from the iris epithelium outward, bending the light of such way that is reflected gray. The second theory is related to the amount and location of melanin, and creates two subcategories: dark gray and light gray. The dark gray would be caused by a thin layer of melanin in the front of the iris, which cause the reflection of blue be overshadowed. For its part, the gray would be caused by a tiny amount of melanin in the front of the iris, even smaller than for blue eyes. However, none of these theories have been explained genetically.