Eye Color Genetics

Eye color is a complex genetic trait determined by pairings of genes passed on from each parent. Scientists once believed that eye color was determined by a single gene, but advances in genetic research have revealed that eye color is influenced by more than 50 genes.

A baby's eye color is controlled by the amount and type of a pigment called melanin that is stored in the iris. While it's mostly just a physical trait, eye color can sometimes signal that the baby has a health issue.

This article discusses eye color genetics. It explains how genes trigger different combinations of pigments to determine what color your baby’s eyes will be.

How Eye Color Develops

The colored part of the eye is called the iris. What we see as eye color is really just a combination of pigments (colors) produced in a layer of the iris known as the stroma. There are three such pigments:

• Melanin is a yellow-brown pigment that also determines skin tone.
• Pheomelanin is a red-orange pigment responsible for red hair. It is mostly found in people with green and hazel eyes.
• Eumelanin is a black-brown pigment abundant in dark eyes. It determines how intense the color will be.

The combination of pigments, as well as how widely they're spread out and absorbed by the stroma, determine whether an eye looks brown, hazel, green, gray, blue, or a variation of those colors.

For example, brown eyes have a higher amount of melanin than green or hazel eyes. Blue eyes have very little pigment. They appear blue for the same reason the sky and water appear blue—by scattering light so that more blue light reflects back out.

When you don't have any melanin at all, you end up with the pale blue eyes of people with albinism. 

A newborn's eyes typically are dark, and the color is often related to their skin tone. White babies tend to be born with blue eyes or sometimes gray eyes. Black, Hispanic, and Asian babies commonly have brown or black eyes.

If your baby has blue eyes, they may not stay that way. As a newborn, the pigment is not widely spread throughout the iris. During the first six months of life, more of the pigments are produced. By age 1, you usually have your permanent eye color.

Genetics and Eye Color

Eye color is determined by multiple variations of genes that are in charge of the production and distribution of melanin, pheomelanin, and eumelanin.

The main genes influencing eye color are called OCA2 and HERC2. Both are located on human chromosome 15.

Each gene has two different versions (alleles). You inherit one from the mother and one from the father. If the two alleles of a specific gene are different (heterozygous), the trait that is dominant is expressed (shown). The trait that is hidden is called recessive.

If a trait is recessive, like blue eyes, it usually only appears when the alleles are the same (homozygous). Brown eye color is a dominant trait and blue eye color is a recessive trait. Green eye color is a mix of both. Green is recessive to brown but dominant to blue.

Other genes that help determine eye, skin, and hair color include the genes ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1. These genes are thought to interact with OCA2 and HERC2 to decide eye color.

Predicting Eye Color

Without knowing exactly which genes a baby will have, it's impossible to predict with total certainty what color their eyes will be. But there are ways to make fairly accurate predictions.

One of these is by using a simple grid chart called the Punnett square. You enter the genetic traits of one parent in the top rows of the grid. The other parent's genetic traits are entered in the far-left columns. Plotting the contribution each parent makes provides a better-than-average probability of what their child's eye color will be.

Still, the Punnett square chart has its flaws, as it is not an accurate way to predict more complex problems of inheritance. For example, according to the Punnett square, two blue-eyed parents cannot have a brown-eyed child. However, if the blue-eyed parents carry DNA for brown eyes from a parent of their own, then it is still possible—albeit unlikely—that their child will inherit brown eyes.

Determining each parent’s alleles can get a little complicated depending on the eye color. As a dominant trait, brown eyes can come from six different genetic combinations. They can also hide recessive (hidden) traits of green or blue eye color. To find any recessive traits, it's helpful to know the grandparents' eye colors.

For example, a blue-eyed parent whose entire family has blue eyes and a brown-eyed parent whose mother and father were brown- and blue-eyed has a 50/50 chance of having a blue-eyed or brown-eyed child.

Scientists have begun to develop methods for predicting eye color. They use genetic tests that identify specific polymorphisms (versions of a gene) that can indicate how much melanin, pheomelanin, and eumelanin will be produced.

Eye Color and Health

A baby's eye color may also reveal congenital diseases (diseases you're born with) and other conditions.

Waardenburg Syndrome

Heterochromia, in which a person has two different colored eyes, can be a sign that a person has Waardenburg syndrome. Heterochromia often presents as one blue eye and one brown eye.

Waardenburg syndrome is a genetic condition that can cause hearing loss in one or both ears. People with Waardenburg syndrome may also be born with very pale eyes or one eye that is two colors.

On the contrary, having multiple colors within the same eye—known as central heterochromia (or tie-dye eyes)—is not associated with health problems.

Ocular Albinism

Very pale blue eyes may be caused by ocular albinism. This is when there is absolutely no pigment in the iris.

As an X-linked recessive disorder, ocular albinism occurs almost exclusively in men. This is because men have one X and one Y sex chromosome. The gene for the condition is on the X chromosome. So, in men, the gene for the condition will be expressed even though it's recessive.

Women, on the other hand, have two X sex chromosomes, so they may be carriers. They may have one gene for ocular albinism that is hidden by another normal gene. So they may not have the condition themselves but be able to pass on the gene for it.

Aniridia

A baby also may be born missing all or part of their iris, a genetic condition known as aniridia.

Aniridia is bilateral, meaning that it affects both eyes at the same time. The condition causes the pupil to appear much larger and unevenly shaped. Eyes affected by aniridia have difficulty adjusting to light, resulting in blurry vision and light sensitivity.

Aniridia is caused by mutations in the PAX6 gene. This gene plays an important role in forming tissues and organs during an embryo's development.

Summary

Your baby's eye color is determined by genetics. Eye color is a combination of pigments produced in the stroma. Brown eyes have more melanin than green or hazel eyes. Blue eyes have very little pigment.

The mix of genes inherited from each parent determines which pigments are produced and the baby's eye color. These genes can also lead to certain conditions.