Why do different people have different eye colors? Why can someone have brown eyes and light skin, or blue eyes and dark skin? The answers are coded in your DNA.
Blue, Brown, and Green
Genetics is the study of genes. Your DNA contains thousands of genes that tell every cell in your body what to do and when to do it. Though every one of your cells has the same DNA, each gene works differently depending on what kind of cell it's in. Melanin is the name of the pigment that gives your skin, hair, and eyes their color. Therefore, the genes that control melanin production determine those aspects of your appearance. When a cell makes more melanin, it has a darker color. So, a person who has brown hair has more melanin in her hair than a person who has blonde hair. Since genes express themselves differently in different types of cells, the same person may have varying amounts of melanin in her hair, eyes, and skin.
Scientists have spent a lot of time studying how eye color is determined. They've found that eye color is largely decided by two main genes. One gene can make eyes either brown (lots of melanin) or blue (very little melanin). The other makes eyes either green (a moderate amount of melanin) or blue. Both your parents have two copies of each of the genes. You received one copy of each gene from your mother and one copy of each gene from your father. The combination you inherited determined your eye color. You and your siblings may have different eye colors, which means you received a different combination of copies from your parents. Scientists can use probability to determine all the possible eye colors that can appear in one couple's children, as well as the likelihood of each outcome in the sample space.
Roughly 300 million people in the world have blue eyes. In a study published in 2008, a team of geneticists from Denmark managed to track down one specific genetic mutation that leads to blue eyes. The researchers compared the DNA of blue-eyed people from places as diverse as Scandinavia, Turkey, Jordan, and India and were amazed to find that they all shared that exact same mutation—a fact that suggests that all blue-eyed people may be descended from a single person. This common ancestor would have lived near the Black Sea 6,000 to 10,000 years ago.
See for yourself: http://www.sciencedaily.com/releases/2008/01/080130170343.htm
Most babies are actually born with blue eyes, even if they later become brown-eyed adults. Watch the video to find out why. Use the "eye color calculator" at the next link to calculate the probabilities of different eye colors for your hypothetical children. See how the calculator works at the last link.