Multiple genes determine eye color
"Why are eyes different colors?" asked a young patron at the Arlington Heights Memorial Library.
The colored part of the eye is the iris, but whether it is brown, gray, hazel, or blue, eye color is determined by genes.
"The inheritance of eye color is related to multiple genes," said Dr. Janice Lasky Zeid, an ophthalmologist at Ann & Robert H. Lurie Children's Hospital of Chicago, in describing the mechanism that creates color in the iris.
"A particular region on chromosome 15 plays a large role in eye color, as well as other genes that have a smaller role."
Through genes, a pigment called melanin is produced that contributes to eye, hair and skin color.
Zeid added that iris pigmentation is not complete until after birth.
The National Center for Biotechnology Information researched the prevalence of iris color in newborns and revealed that 63 percent are brown, 20.8 percent are blue, and the rest are indeterminate. In a very few cases, babies are born with two different colored eyes. A future study will reveal the percentage of babies who experience eye color changes within their first few months.
The iris controls the amount of light that enters the pupil, the black dot in the center of the eye where light penetrates and is directed to the retina and, ultimately, the brain.
Dr. Steve Wexler, optometrist at Eyecare Inside Waukegan Walmart, explained that the pupil appears black because its purpose is to absorb light.
"If light illuminated the inside of the eye, the pupil would not be black. Just like when it's dark outside and no lights are on in a house, the picture window looks black," Wexler said.
As light is directed into the pupil, the extraordinary visual system kicks into gear and sends signals throughout the body to produce a clear understanding of what's being seen.
The iris uses muscles to contract or enlarge the pupil. On a very sunny day, the muscles push on the pupil to make it smaller, creating a tiny dot. They widen the dot when there's a darker ambience.
Emotions also can cause the iris muscles to react. For instance, when people are very frightened, the iris muscles will respond by enlarging the pupil. Surprisingly, pupil size can increase if there's a high difficulty level in an activity or task.
Less than 1 percent of the population is born with two different colored eyes, what eye doctors call heterochromia. When eye color in one or both eyes changes over time, a doctor's care is required.
Genetics researchers have discovered blue eyes are a fairly recent phenomenon. Originally, all humans had brown eyes. But about 7,000 years ago, a time when people had already developed shared community life and were planting small farms, genes produced blue eyes.
An ancient skeleton found in a cave in northern Spain provided enough DNA for researchers to identify that this man had dark hair, dark skin and eye color lighter than brown.
Eye exams should be given as early as age 1, Wexler said. They assess both visual acuity and overall health since eyes affect vision and can reveal the presence of some health concerns.
Check it out
The Arlington Heights Memorial Library suggests these titles on eyes:
• "All About Pink Eye" by Megan Borgert-Spaniol
• "All In The Family" by Dona Rice
• "Eye Color: Brown, Blue, Green, And Other Hues" by Jennifer Boothroyd
• "Eye: How It Works" by David Macaulay
• "Genetics" by Christine Taylor-Butler
• "Genetics: Investigating The Function Of Genes And The Science Of Heredity" by Trevor Day
• "Inheritance And Variation Of Traits" by Don Rauf
• "Inside The Eyes" by Karin Halvorson