Genetic Traits Eye Color

Many people wonder why their eyes are a certain color. Why are brown eyes so common, and why are green eyes relatively rare? Why does a child of brown-eyed parents sometimes have blue eyes? Why do fraternal twins sometimes have different-colored eyes?

The answer to all these questions lies with genetics. Scientists have understood for many years that eye color is an inherited trait. However, recent research has provided more insight into what specifically causes one person’s eyes to be blue, and another person’s to be brown.

About Eye Color

Eye color is a trait that is influenced by the genetic material that a person inherits from both parents. People used to believe that brown eyes were a dominant trait, and that it was impossible for two blue-eyed parents to have a brown-eyed child. However, scientists now know that the genetics of eye color is more complex than experts previously thought.

Brown is the most common eye color in the world, followed by blue. Green eyes are less common. Some other eye colors occur quite rarely, including red or pink eyes (common in albino individuals), and violet eyes. One study found that the frequency of blue eyes among Caucasians in the United States is decreasing. Almost 60 percent of people born between 1899 and 1905 had blue eyes, compared to about 34 percent of people born between 1936 and 1951.

Biology of Eye Color

Eye color is determined by the amount of melanin found in the stroma of the eyes iris. Most babies have blue eyes when they are born. A childs eye color may change or darken as they get older.

Differences in eye color are caused by single nucleotide polymorphisms, or SNPs. This means that changing just one tiny portion of a persons genetic sequence can affect the color of their eyes. One study found that these single letter changes account for about 75 percent of all differences in eye color. This is because the SNPs likely control how much pigment is in a persons eyes. A person who has dark eyes has a lot of pigment, while someone with light-colored eyes has less.

Genetics of Eye Color

Scientists have recently made progress in understanding the genetics of eye color. While there is no single gene for eye color, the main gene that determines eye color is called OCA2. This gene is significantly involved in determining whether a person has blue or brown eyes. A mutation in the OCA2 gene also causes albinism. However, scientists are still studying what factors cause people to have some less common colors, such as green eyes and hazel eyes.

Frequently Asked Questions about Eye Color

Q: Can my eye color change?

A: Yes, it is possible for your eye color to change over time. Children are usually born with blue eyes, and their eye color will change as they get older. Older adults may find that their eyes get lighter or darker as they age. If your eyes change color dramatically, or if one eye is suddenly a different color than the other, you should see a doctor.

Q: What does my eye color mean?

A: In most cases, your eye color has no specific meaning. However, some eye colors, such as red, may be a sign of another condition, such as albinism. Light-colored eyes may put a person at greater risk for age-related macular degeneration.

Q: Does my eye color affect my vision?

A: Scientists are still studying whether eye color affects a persons vision. Preliminary evidence suggests that eye color may have some minor affects on a persons vision. For example, people with dark eyes may have quicker reaction times than those with light-colored eyes.

Recent News on Eye Color and Genetics

In March of 2009, researchers in the Netherlands announced that by analyzing a subjects DNA, they were able to predict with 90 percent accuracy whether a person had blue or brown eyes. Other eye colors could be predicted with 75 percent accuracy. Progress in this field is advancing, and one can only assume that more findings will be announced as they surface.


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