Eye color determination and changes

A child’s eye color is always inherited from its parents, even if the influences from each parent may be uneven. Eye color is a polygenic phenotypic character. Generally speaking, eye colors range from the darkest shades of brown to the lightest tints of blue. There has been a graded system for eye color classification. Regular colors include black, gray, green and some combinations of them.

The principle of eye color

Eye color is determined by the pigmentation in the iris of the eye. Brown eye color is usually considered as “dominant” and blue being “recessive”. But it is proven that this once-held view is wrong. It is impossible to conclude that blue eye color is a simple recessive trait. Actually any parent-child combination of eye color is possible. In other words, eye color involves quite a few variations. It is neither the simple blend of parents’ eye colors, nor the result of certain regular combinations. There are still some eye colors such as gray, hazel and other unusual combinations that are beyond clear scientific explanation.

A genetic explanation of eye color inheritance

One point is obvious that three genes in the body are responsible for eye color inheritance and determine three most common colors: green, brown and blue. And each parent has two pairs of genes on each chromosome that affect the eye color of their children. In general, a genetic explanation of eye color is complex that multiple genes may play a role in determining an eye color. It has been found that some genes are related to eye color, e.g. EYCL1, EYCL2 and EYCL3. Researchers are also focusing on DNA analysis to find out adults’ eye color.

Brown-colored eyes and exceptions

A brown pigment named melanin develops as a baby ages, so that its eye color will darken. It has been revealed that a child’s eye color can be completely different from both of its parents. But children are always born with brown eyes if their parents both have brown eyes. Some exceptions may happen during births. For example, some children have different colors in two eyes. This exceptional eye color condition is usually caused by faulty developmental pigment transport. The underlying reasons for this disease may be local trauma in the womb, benign genetic disorder or inflammation of the iris.

Natural and disease-caused changes in eye color

Eye color does not stay unchanged lifelong. The eye’s iris controls the pupil size, which enlarges under dim light conditions and contracts in a bright lighting environment. These pupil size changes may lead to slight eye color change. Eye color changes can be natural or signs of eye diseases. Normal eye color changes relate to mainly two types of factors: emotion and aging. Extreme emotions such as anger and delight can change both pupil size and iris color. Some of Caucasian people are reported to have eye color changes as they age. Diseases such as Fuch’s heterochromic iridocyclitis, Horner’s syndrome and pigmentary glaucoma can lead to harmful eye color changes.