Eye Color Genetics

Parents expecting a new baby usually wonder what their baby will look like. One common question is what color their baby's eyes will be. But although eye color is determined by genetics, it takes a year for a child's permanent eye color to develop.

A baby with large, blue eyes
 ePhotoAlto / Ale Ventura Collection / Brand X Pictures / Getty Images

It's normal for parents to wonder why their bouncing blue-eyed baby is sporting hazel peepers as a toddler. It's because a baby's eye color will change during the first year of life, as the eye takes on its permanent color.

Understanding how eyes get their color and the role genetics play can take some of the mystery out of this phenomenon. And while eye color is mostly just a physical characteristic, in some cases, it can be a sign that the baby has a health issue.

Scientists once believed eye color was determined by a single gene, but advances in genetic research and genomic mapping have revealed that more than a dozen genes influence eye color.

This article will discuss how genetics decides 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 or gray eyes. Black, Hispanic, and Asian babies commonly have brown or black eyes.

When a baby is born, 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.

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.

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.

Probability of Eye Color
Parent 1 Parent 2 Blue Green Brown
Blue Blue 99% 1% 0%
Blue Green 50% 50% 0%
Blue Brown 50% 0% 50%
Green Green 25% 75% 0%
Green Brown 12% 38% 50%
Brown Brown 19% 7% 75%

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. Babies whose eyes are different colors—known as heterochromia—may have Waardenburg syndrome.

This 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.

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.

Studies suggest fewer than one out of every 60,000 men has ocular albinism.

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

A Word From Verywell

While understanding the genetics of eye color can help you understand how likely a baby will have a certain eye color, there are no certainties. If you have any questions about your child's eye color or overall eye health, bring your concerns to their pediatrician.

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