What Determines Eye Color?

How is Eye Color Determined?

For years everyone thought that eye color was a simple genetic process of brown eyes are dominant and blue eyes are recessive. Each parent has two pairs of genes on each chromosome. Blued eyed parents had to have blue eyed children. Brown eyed parents would have a seventy five percent chance of browned eyed children and twenty five percent chance of a blue eyed children. We now know that it is not that simple.

Factors that Determine Eye Color

Eye color in humans is due to three factors and is a variation of pigmentation of the iris from light brown to nearly black.

Iris Pigment Epithelium

The amount of melanin pigment in the iris pigment epithelium located on the back side of the iris contributes to the iris color..

Iris Stroma

The amount of melanin pigment in the iris stroma located on the front of the iris contributes to the iris color.

Density of the Stroma

The cellular density of the stroma also contributes to the eye color. Blue eyes have thinner irises and brown eyes have thicker irises.

The appearance of blue, green, and hazel eyes results from less brown pigment and the scattering of light in the stromal layer of the iris.

Genetics of Eye Color

Eye color is not just a simple dominant and recessive genetic result. The genetics of eye color is determined by multiple genes. More than fifteen genes have been found to be associated with determining eye color. The OCA2 gene contributes the most in eye colors genetics. The genetic controller next to the OCA2 gene controls its action and therefore the amount of pigment produced in the eye.Eye color can vary from the most common color of brown to the least common green. Many scientists think that a mutation caused the lighter colors and that originally everyone had brown eyes.

Eye Color Change in Babies and Children

Most babies who are Caucasian have blue or light colored eyes when they are born. As the child develops, the melanocytes found in the iris begin to produce melanin pigment. As the child approaches age one, the iris begins to change and continues up to age three or four before the final eye color will be known in many cases.

Diseases that Effect Eye Color

Heterochromia

Complete Heterochromia

Complete Heterochromia is an iris condition where the iris of one eye is totally different from the other eye.

Partial Heterochromia

Partial Heterochromia is an iris condition where a portion of the iris in one eye is different from the others portions of the eye.

This is caused by the increase or decrease in the amount of pigment in the iris.

Other Possible Causes for Changes in Eye Color

Injury to the eye with a loss of pigment

The use of prostaglandin drugs in the eye for treatment of glaucoma

Horner’s’ Syndrome

Waardenburg syndrome

Wilson’s disease

Ocular Albinism

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