Eye color genetics

Whether your child is born with brown eyes or blue eyes — or any hue in between — involves a complicated game of genetic roulette. But human eye color genetics aren’t as simple as looking at the parents’ eyes and then predicting a child’s eye color.

At one time, researchers thought that only one gene passed eye color from parents to their children. This led to the belief that a child whose parents shared an eye color (such as brown) couldn’t inherit a different eye color (such as blue).

It turns out that isn’t quite accurate. Scientists now know that a collection of up to 16 genes plays a role in eye color genetics. So, it’s entirely possible for parents with brown eyes to welcome a child who’s eyes are a different color into the world. Although those scenarios are uncommon, they do happen.

Ultimately, the parents’ eye colors can help predict their child’s eye color, but it’s only one factor.

There’s no guarantee when it comes to your offspring’s eye color. While a baby inherits half of their eye color genetics from one parent and half from the other parent, the way that the multiple genes interact also plays a role in determining eye color.

Differences in eye color are also influenced by differing amounts of melanin, the pigment responsible for eye color (plus hair color and skin tone).

For instance, many white non-Hispanic babies are born with blue eyes because they don’t have the full amount of melanin present in their irises at birth. As the child grows older, if they’ve developed slightly more melanin in their irises, they may end up with green or hazel eyes. When the iris stores a lot of melanin, the eyes will be amber (a golden brown), light brown or dark brown.

Even though you don’t know the amount of melanin your baby will have, you can still get a pretty good sense of eye color from the parents’ eye colors. As the American Academy of Pediatrics explains:

• Two blue-eyed parents are likely to have a blue-eyed child, but it’s not guaranteed.

• Two brown-eyed parents are likely to have a brown-eyed child. Again, it’s not guaranteed.

• Two green-eyed parents are likely to have a green-eyed child, although there are exceptions.

• Two hazel-eyed parents are likely to have a hazel-eyed child, although a different eye color could emerge.

• If one of the grandparents has blue eyes, the odds of having a baby with blue eyes increases slightly.

• If one parent has brown eyes and the other has blue eyes, the chances of having a brown-eyed or blue-eyed baby are roughly even.

The Fertility Institutes, which offers fertility services in California, New York, Utah and Mexico, offers the following odds of a baby’s eye color based on the parents’ eye colors. (Due to rounding, percentages don’t always add up to 100%.)

• Both parents with brown eyes: 75% chance of baby with brown eyes, 18.8% chance of baby with green eyes, 6.3% chance of baby with blue eyes.

• Both parents with blue eyes: 99% chance of baby with blue eyes, 1% chance of baby with green eyes, 0% chance of baby with brown eyes.

• Both parents with green eyes: 75% chance of baby with green eyes, 25% of baby with blue eyes, 0% chance of baby with brown eyes.

• One parent with brown eyes and one parent with blue eyes: 50% chance of baby with brown eyes, 50% chance of baby with blue eyes, 0% chance of baby with green eyes.

• One parent with brown eyes and one parent with green eyes: 50% chance of baby with brown eyes, 37.5% chance of baby with green eyes, 12.5% chance of baby with blue eyes.

• One parent with blue eyes and one parent with green eyes: 50% of chance of baby with blue eyes, 50% chance of baby with green eyes, 0% chance of baby with brown eyes.

Keep in mind that it may take six to 12 months for a baby’s true eye color to emerge, so the color you see at birth can certainly change.

SEE RELATED: Is it true all babies are born with blue eyes?

While a baby eye color predictor or calculator you find online — some even include a baby eye color chart — can offer insights into whether your baby will have Mom’s blue eyes or Dad’s brown eyes, the forecasts they generate aren’t completely accurate.

That’s because the predictor or calculator relies solely on the parents’ eye colors, whereas a baby’s complex genetic makeup ultimately determines the color of their eyes. In the end, a baby eye color predictor or calculator may be fun to play with, but it doesn’t take into account the complicated science behind genetics.

Another tool you can use to help predict a baby’s eye color is the Punnett square. This square diagram lets you test different genetic combinations based on the parents’ eye colors — it also comes in handy when trying to figure out other genetic probabilities.

The Fertility Institutes offers an ethically questionable service: The organization claims to be the first and only genetics-based fertility program to offer high-level genetic screening of parents “seeking to have a voice in determining the eye color of planned children.”

“We are only offering eye color selection at this time in conjunction with our general genetic well-being and gender selection procedures. Eye color preference is available only as an ‘add-on’ procedure to our general procedures,” the Fertility Institutes says.

Most parents are more concerned with their children’s health and vision care than with their eye color.

Throughout development, infants and their eyes will experience many changes — beyond changes to their iris color. So, it’s important to visit a pediatric eye doctor who specializes in eye health and vision care for the wee ones.

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