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Biology in Action: The Mystery of Eye Color

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Overview:  Genetic Variation and Eye Color
Eye color is determined by a number of genes that control the amount of pigmentation in the iris.  There are no blue or green pigments that determine eye color.  The amount of pigmentation controls the reflection of light from the iris, and the amount of light scatter governs the apparent color of the eye.  Eye color depends on lighting conditions, as well as changes in background.

What Eye Colors Are Possible?
The genetics of eye color are extremely complex, with nearly 20 different genes contributing to what color can be expressed.  Eye color can vary from the lightest blue to the darkest brown.  It is not as simple as a combination of dominant or recessive genes.  Recently, a study of high-resolution pictures showed even more variations in eye color than blue, green, or brown.  Genetic mapping showed that even in a relatively small population, human eye color exists in many more combinations than was once thought.

Where Do Most Eye Colors Exist?
Some variation in eye color exists in all populations of the world.  Brown eyes are the most common eye color, and have the most pigmentation.  Blue eyes have the least pigmentation, but are also seen in all parts of the world, although they are more common in Europe and people of European ancestry than people of Asian ancestry.  Infants of European ancestry tend to have blue eyes when they are born, but if they have enough melanin present, they will turn dark as the child grows older.

Can Eye Colors Change?
Eye colors can change for a number of reasons, such as hormonal changes and injury, as well as the darkening of eyes as children grow older.  Certain changes of eye color, such as rings in the iris of a totally different color that were not there before, can indicate disease.  Some eye colors are very rare, and are actually a variation of more typical eye colors.

Blue Eyes can Be Traced to One Ancestor
Recent studies suggest that in prehistoric times, everyone had brown eyes.  The color variation could be explained by the amount of melanin controlling the color.  However, people with blue eyes all have a common genetic mutation that occurs the same way.  It limits the production of melanin, making eye color lighter.  Mitochondrial DNA ( inherited from the mother) is so similar in blue-eyed people in countries all over the world that scientists think that blue-eyed people may have had the same ancestry and migrated to different regions in the world.  To paraphrase the song, something special made those brown eyes blue.

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