The Science of Eye Color

Answering the question “what color will my baby’s eyes be?” requires a quick trip back to bio class—specifically, the anatomy of the eye and its many pigments, for some background info.

• Iris – The iris is the colored part of the eye. It’s where you’ll be looking for hints of your partner’s chestnut brown eyes or maybe even your great uncle’s signature green gaze.
• Melanin – Melanin is a pigment that’s found in skin, hair, and eyes. When your baby is exposed to sunlight, the UV rays of the sun trigger specialized cells called melanocytes to produce melanin, which causes eyes, hair, and skin to take on color. Hazel, light, and dark brown eyes contain high amounts of melanin.
• Pheomelanin – Pheomelanin is a particular type of melanin. This pigment is red-orange in hue and gives hazel and green eyes their unique colors. The amount of pheomelanin produced in the body is also responsible for answering if and when do babies get freckles?
• Eumelanin – Another type of melanin, eumelanin, is black-brown and creates brown shades in eyes.

Green, hazel, and brown eyes are accounted for in this list of pigments. But what about blue eyes? Is there a blue pigment—blumelanin, maybe?

Blue eyes actually have very little to no pigment. The reason blue eyes are blue is the same reason the sky is blue—the Tyndall Effect.

When we open our eyes, light enters the iris—which is the structure of cells and muscles in the eye. The pigment in green, hazel, or brown eyes absorbs the light waves. But in eyes with little to no pigment—aka “blue eyes”— those light waves scatter, shortening the wavelengths and transforming the color into the blue end of the light spectrum. The result? The eyes look blue!

Babies and Eye Color

There is some scientific etymology to the term “baby blues.” When babies are born with blue eyes, those hues don’t always stay. Because melanin production is only stimulated by sunlight, as your baby is exposed to the sun, the baby blues may transform into gorgeous greens, hazels, or browns, usually by age 1.

So as your blue eyed child enjoys early morning cuddle time, afternoon strolls to the park, or sunset suppers in Grandma and Grandpa’s backyard, more sunlight shines into her body—and her melanocytes get to work. Your baby’s eyes will find their final color anywhere between 6 months to three years.

(If you’re wondering, “will my baby’s eyes stay blue?” check out our in-depth guide to blue eyes and babies.)

How Genetics Play into Eye Color

The spectrum of human eye pigment is extraordinary and certainly outdoes a typical box of crayons. But the genetics behind how much of which pigment your baby has is a bit more complex than pulling a color out of a box.

To understand how to determine baby eye color, it may help to break down the genetic components of what goes into the unique color of your child’s eyes.

• Genes – Genes are the instruction manual for your baby’s DNA. They tell the body what kind of hair color to make, how tall your baby’s body will grow, and what your baby’s eye color will be —among many, many other things that make up the human body. In fact, The Human Genome Project reports that humans have between 200,000 and 250,000 genes.
• Alleles – An allele is a distinct manifestation of a particular gene. Each parent contributes an allele to a child for each gene.
• Genotype – Through alleles, your child has two copies of each gene, one for each parent—thanks, mom and dad! The combination of alleles for a specific gene is known as the genotype for that trait.
• Phenotype – This is what actually manifests as your child’s physical traits. It’s what genotype information gets put into production by the body.

Putting Away the Punnett Squares

In middle school biology class, you might have learned about dominant and recessive traits through Punnett Squares. Punnett Squares were based on the hypothesis that phenotypes were determined by the dominance of certain traits over others—the recessive traits.

You may remember the detached versus attached earlobe experiment. Detached earlobes are considered a “dominant” trait, while attached earlobes are considered a “recessive” trait.

For example, if one parent contributes their dominant, detached earlobe allele to a child and the other parent contributes the recessive attached earlobe allele, the child’s phenotype will show the dominant trait—in this case, detached earlobes. But the child’s genotype will have one of each trait—one dominant and one recessive.

If both parents contributed recessive traits—or two attached earlobe alleles—the child’s phenotype and genotype would show attached earlobes.

So can you answer “what color eyes will my baby have?” with a Punnett Square?

Nope!

For one thing, this only works for autosomal genes—or genetic traits that are determined by only two alleles. Just like with the question of “Where does the hair gene come from?”, the answer to where your baby gets their eye color is not simply Mom or Dad.

According to scientists, eye color is determined by at least 16 genes.

How Eye Color Is Determined

There’s a whole kaleidoscope of genes that go into your child’s eye color. But these genes aren’t exactly the Blue Eye Gene or the Brown Eye Gene.

The genes that play into eye color are involved in many different facets, including the production, transport, or storage of different kinds of pigment.

Here are just two genes to give you an example of how that works in your child’s DNA:

• OCA2 – The OCA2 gene makes a certain protein called the P protein. This protein helps the eye’s melanosomes—the cells that produce melanin—develop and mature to make eumelanin or pheomelanin. That means that the more P protein your child has, the more eumelanin or pheomelanin he will have, resulting in hazel or brown eyes.
• HERC2 – The HERC2 gene controls whether the OCA2 gene is activated. So, for example, a child could have the OCA2 gene, but the HERC2 gene may decide to deactivate the OCA2 gene—this would result in much less melanin (of any type) and lighter eyes.

So essentially, eye color genes control the production and amount of pigment that goes into your child’s peepers. It’s a lot like mixing 16 different colors of paint to get that perfect shade. The different amounts of each color will determine the shade, and those amounts are decided by different genes.

Your baby’s genetics determine how many of certain pigment-producing cells your child has and which of those cells are actively adding “tint” to the eye color mix. But where does your child’s DNA get the instructions for his unique mix of iris paint? Mom and Dad of course!

Eye color is not one of those physical traits inherited from Father or Mother alone. Parents each contribute one allele for each of the (at least) 16 genes that determine eye color.

Which is why Punnett Squares or newborn eye color charts don’t quite help if you want to find out the color of your baby’s eyes.

Understanding the color of your baby’s eyes goes beyond a simple genetic lottery, instead involving complex processes guided by at least 16 genes. Though Punnett Squares and newborn eye color charts may not offer the final answer, appreciating the intricacies of genetics and eye color brings a sense of wonder as we anticipate the final hue that emerges in our baby’s eyes, adding another unique element to their personality.

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