Eye Color Genetics

Baby Eye Color

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Expectant parents often wonder what their child will look like. Eye color is one of many traits determined by genetics. The DNA passed down from your ancestors influence whether your baby’s eyes are blue, grey, green, hazel, or brown.

When you first look into your newborn’s eyes—bright, shiny, and full of wonder—they may or may not bear a family resemblance. At birth, our eyes are not yet fully developed. During the first year of life, your baby's eye color will change as the eye develops into its permanent hue. 

How Eye Color Develops

The colored part of our eyes is the iris, a muscular structure that surrounds the pupil and controls the amount of light that gets in. What we perceive as eye color is a combination of pigments produced in a layer in the iris known as the stroma.

Pigments determine the color of a person’s skin tone, hair, and eyes. Three pigments make up an individual’s unique eye color:

  • Melanin, the most common, is a yellow-brown pigment also related to skin tone
  • Pheomelanin, a red-orange pigment that is responsible for red hair, is predominant in green and hazel eyes
  • Eumelanin, a black-brown pigment that determines hue saturation, is abundant in dark eyes

The precise hue of an individual's eye color is based on a combination of factors. The variety of pigments produced, how densely they are dispersed, and how they are absorbed in the stroma determine whether an eye looks brown, hazel, green, gray, blue, or a variation of those colors.

Brown eyes have denser concentrations of melanin than light eyes. A complete absence of melanin results in the pale blue eye color often seen in people with albinism. 

Interestingly, there is no blue pigment involved in eye color. Blue eyes get their color the same way sky and water are blue. They scatter light so that more blue light reflects back out.

Brown is the most common eye color in the world, while amber-colored eyes are the most rare.

Prevalence of Eye Colors Worldwide
Eye Color Percent of Population

Brown

55%–79%

Blue   

10%

Hazel 

5%–8%

Green 

 2%
Amber  1%

Newborn Eyes

At birth, melanin is not widely distributed in the iris. As a result, a baby's eye color is not fully developed until around their first birthday. 

Newborn eyes are typically dark and the hue often correlates to skin tone. Caucasians tend to be born with blue or grey eyes, while Black, Hispanic, and Asian babies commonly have brown or black eyes.

Sometime during the first six months of life, pigment epithelial cells start to pump melanin, pheomelanin, and eumelanin into the stroma. By age 1, eye color is usually set.

Genetics of Eye Color

Scientists once believed eye color was determined by a single gene, with brown eyes a dominant trait and blue eyes recessive. Advances in genetic research and genomic mapping revealed more than a dozen genes influence eye color.

Genes provide the blueprint by which proteins—the building blocks of the body—are assembled. This, in turn, determines the structure of cells and how they work. The collection of genes responsible for your genetic traits (referred to as your genotype) directs the characteristics you inherit as an individual (referred to as phenotype).

Each person's eye is unique, the result of multiple variations of genes (polymorphisms) that are associated with the production and distribution of melanin, pheomelanin, and eumelanin secreted by and other pigments. 

The two genes most commonly associated with eye color are OCA2 and HERC2, which are located on human chromosome 15.

Every gene comes in two different versions (alleles). If two alleles are different (heterozygous), the dominant trait is expressed. The unexpressed allele is recessive. If a trait is recessive, like blue eyes, it generally 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

Since multiple genes and gene interactions are involved in eye color—some of which may be unexpressed in the parents—blue-eyed parents can have a baby with blue, green, or brown eyes.

The probability of a baby's eye color can be assessed in a few ways. An early 20th-century tool, the Punnett square, uses a grid chart to track the genetic traits of the parents.

The genetic traits of one parent are entered in the top rows of the grid, while the genetic traits of the other parent are entered in the far left columns. Plotting the contribution each parent makes provides a better-than-average probability of their offspring's eye color.

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 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%

In recent years, scientists have begun to develop methods for predicting eye color using genetic tests that identify specific polymorphisms. Based on which polymorphism is found, scientists can better predict how much melanin, pheomelanin, and eumelanin will be produced as well as the degree of saturation in the iris.

Eye Color and Health

Your baby's eye color may also reveal congenital diseases and other conditions.

Babies born with two different colored eyes (heterochromia) may have a genetic condition known as Waardenburg syndrome, which 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 also be caused by ocular albinism, in which there is a total absence of iris pigmentation. As an X-linked recessive disorder, ocular albinism occurs almost exclusively in men since they have two X chromosomes. (Women, who have an X and a Y chromosome, may be carriers.) Studies suggest that less than one of every 60,000 men is affected.

A baby may also be born missing all or part of their iris, a genetic condition known as aniridia. The disorder is caused by mutations in the PAX6 gene, which plays a critical role in the formation of tissues and organs during embryonic development.

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