Explanation of Eye Color Genetics

Reviewed by Terri Forehand RN
Close up of an eye

There are a number of genetic factors that influence whether you inherit your great grandmother's baby blues or your father's warm brown eyes. Scientists are still discovering genes that influence eye color, and the simple dominant and recessive rules you may have learned in high school biology aren't completely accurate. All it takes is a look at your family portrait to see that eye color is a trait with endless variation. What's more, eye color isn't a fixed characteristic; it can change throughout your lifetime.

It's About Melanin

According to Hudson Alpha Institute for Biotechnology, the color of your iris is dictated by a substance called melanin. Your eye has cells called melanocytes that have special pigment-storage compartments, or melanosomes. How much melanin is stored in these melanosomes is what determines the color of your eye. The possible range in colors is infinite, but in general, the more melanin in the eye, the darker it is.

Tone Eye Color Melanocytes
Light-colored eye
Light

Blue, gray or violet

Low concentration
Medium green eye
Medium
Hazel or green Moderate concentration
Dark brown eye closeup
Dark
Brown or black High concentration

More Than a Single Gene

Your eye color is a characteristic you inherited from both your parents. Each parent contributed half of your chromosomes, and the interactions of the genes on these chromosomes are what determines whether you have eyes that are blue, brown, or another shade.

For many years, scientists thought that eye color was controlled by a single gene that determined exactly how much melanin you had in your iris; however, greater knowledge of human genetics is changing the way people think about this trait. In fact, according to review of the literature published in the Journal of Human Genetics, scientists now believe that at least 16 different genes contribute to iris pigmentation, or eye color. Among these 16 genes, there are a few main players that account for most eye color variation.

OCA2

A 2007 study published in The American Journal of Human Genetics found that a gene called OCA2 was responsible for 74% of human eye color. This gene is located on the 15th chromosome, and it controls how much melanin is produced in the cells of the iris.

HERC2

Another gene, immediately next to OCA2 on the 15th chromosome, is HERC2. HERC2 affects the way that OCA2 is expressed, and the blue version of this gene is present in almost everyone with blue eyes. What's more, a 2008 study published in Human Genetics found that it's very likely all people with this HERC2 presentation share a common ancestor.

Gey

A third gene, called gey, influences OCA2 in combination with HERC2. Gey comes in two possible colors: blue and green. Green is dominant over blue. If you have the blue version of HERC2 and the green version of gey, you'll have green eyes. The following table shows how this interaction works:

Eye Color HERC2 Color Gey Color
Brown Brown Green or blue
Blue Blue Blue
Green Blue Green

Eye Color Changes

Everyone who has held a newborn baby knows that many babies are born with blue eyes. Babies of Asian or African decent often have light brown eyes when they are born, but at birth, the cells in the eyes of caucasion babies have not begun producing enough melanin to provide any coloration. In any race, over the first few months and years of life, the eyes can change.

What many people don't know is that changes in eye color can continue throughout childhood and adulthood. A study published in JAMA Opthomology found that as many as 10% to 15% of people may experience this continually evolving eye color. The study postulates that this trait is also inherited, which means that if your mother's eyes changed color over her lifetime, yours may as well.

Predicting Eye Color

Wondering what color your children's or grandchildren's eyes will be? Unfortunately, even having a clear idea of the genetic contributions of both parents won't give you a foolproof prediction. According to a 2009 study in Current Biology, it's relatively easy to predict basic colors like blue and brown, but it's still extremely difficult to tell shades of green, hazel, and other intermediate colors.

If you want to take an educated guess, you can try the eye color prediction test from the Tech Museum of Innovation. It uses information you provide about your parents and siblings to give you options for your children's eye color. It's still limited to brown, blue, and green, but it can give you an idea of the possibilities.

Implications for Genealogists

You may remember the eye color of your grandparents or even your great grandparents, but it can be difficult to image what color eyes many of your ancestors may have had. In some cases, you can use a combination of genetic information and historical records to get a sense of how your ancestors looked. Military records, such as World War I draft registration cards, sometimes include the eye color of the individual. If you learn that your great grandfather had blue eyes, then you know that he had the HERC2 gene for blue and shared a common ancestor with almost all other blue-eyed people.

Complex and Fascinating

The genetics of eye color are complex and fascinating. Although you can't know the exact shade of your ancestors' or descendants' eyes, you can use your understanding of this interesting field to make educated guesses about these individuals.

Explanation of Eye Color Genetics