What Color Will My Baby's Eyes Be? A Genetic Explanation

by: Rebecca Desfosse
Blue, brown or somewhere in between? Check out some of the genetics that determine your little one's eye color.
What color will my baby's eyes be? A genetic explanation

Are you crossing your fingers hoping for a brown-eyed baby? Or, are you wondering how on earth you ended up with blue-eyes with two parents with brown eyes? Don't worry, you weren't switched at birth and your little one could have brown eyes no matter what color her parents' eyes are. The genetics behind eye color is an extremely complex and intricate game of chance. When it comes to the question, "What color will my baby's eyes be?" the truth is that it's impossible to tell, but it's really fun to guess.

Eye Color and Melanin

what color will my baby's eyes be

Eye colors are completely fascinating. In any given group of people, you can find dozens of different shades of gray, green, hazel, blue and brown. What we call eye color is actually the color of the iris. The amount of a pigment called melanin found in your iris produces your eye color, with blue eyes being produced by the least amount of melanin and brown being produced by the most. The resulting eye color can be any and every shade in between -- all determined by a complex process of genetics.

Here's the quick lowdown: Typically, two blue-eyed parents end up with a blue-eyed child, but not always. Likewise, two brown-eyed parents are likely (but not guaranteed) to have a child with brown eyes. If one parent has brown eyes and the other has blue eyes, their child has about a 50/50 chance of either ending up with brown or blue eyes.

The Theory of Mendelian Genetics

It was originally thought that eye color was determined by a simplistic model of Mendelian genetics, meaning it was determined by a single gene, with brown eyes considered dominant and blue recessive. It has now been determined that eye color is a polygenic trait, meaning it is determined by multiple genes. According to Dr. Jennifer Stagg, a biochemist turned naturopathic physician and author of Unzip Your Genes: 5 Choices To Reveal A Radically Radiant You, "There are a number of genes that are responsible for eye color, but the two primary genes, OCA2 and HERC2 are on chromosome 15." The OCA2 gene produces a transporter of tyrosine, which produces melanin, while HERC2 regulates the OCA2 genes' expression.

Using a Punnett Square

Punnet squares aren't completely useless even with this new information. According to Dr. Stagg, "Old Mendelian Punnett squares are a fun tool to predict eye color, but we now know that charts are too simplified an approach when it comes to genetics." For example, using the Mendelian model, two parents with blue eyes wouldn't be able to have a baby with blue eyes, but we know that it is not impossible. That being said, the Punnett square does accurately predict that it is possible for two parents with brown eyes to have a blue-eyed baby. "Even with the outdated Mendelian model, if both parents each have a dominant and recessive gene for eye color, they could end up having a blue-eyed baby only if both parents pass on a recessive gene," says Dr. Stagg.

Let's explore this example further with a Punnnet square:

To make a Punnett square, you first need to create a table. Put your two possible gene versions on the top using a large B or brown, the dominant trait, and a small b for blue, the recessive trait. Add your partner's gene versions on the side of the table:

  B b

Next, fill in each square with the letters from the top and side to figure out all possible variations:

  B b
b Bb Bb

As you can see, there is a one out of four chance that a child of two parents with brown eyes will have a blue-eyed baby if their parents carry the recessive gene for blue eyes. Fun right?

Eye Color at Birth

As if predicting eye color weren't confusing enough, the color eyes your baby has at birth might not stick around. According to the American Academy of Pediatrics, it takes up to one year for the melanocytes to build up to the right amount of melanin in the eyes. Most babies are born with blue eyes (remember, it's the eye color produced by the least amount of melanin), but that can quickly change to green, hazel or even brown.

If you're a pregnant mama wondering, "What color will my baby's eyes be?" try playing around with Punnett squares for fun, but remember that it's all up to chance -- and the complex mystery that is genetics.

Featured Image source: Contra Curva/Flickr