Why are my eyes the color they are?
There are two factors that determine your eye color. One factor comes from the basic structure of an eye and the other factor comes from your parents.
Your eye color is created by the amount of melanin in your eye. Melanin is a pigment that is also found in your skin or in freckles. The more melanin in the eyes or the skin, the darker it is. People with blue eyes have no melanin in the front of part of their eye. People with more layers of melanin have green or hazel eyes and people with the most layers of melanin have brown eyes. The amount of melanin in your eye is determined through your genetics.
Genetics are what traits you inherit (the characteristic that get passed down) from your parents and grandparents. We inherit half our genes from each parent. In the case of eye color, the gene is polygenetic. This means that more than one gene affects the color your eyes will be. Some of the genes, however, are stronger than others. The stronger versions of genes are called dominant and the weaker versions are called recessive. Scientists use family trees in studying genes to help them determine whether a gene is dominant or recessive. A family tree neatly shows a family's information and how traits are passed down. They are like a map of a family's traits.
There is a standard way of making a family tree used by scientists. In a family tree, girls are always circles and boys are always squares. Horizontal lines connecting a circle and a square represent a mother and father that had a child. One or more vertical lines under the parents represents the couple had a child or children.
Let's look at this family tree and see if we can tell if brown eyes or blue eyes are dominant.
We can tell several things from this family tree:
1) The grandparents have three children, one son and two daughters.
2) Even though the grandmother has blue eyes, all three of the children have brown eyes.
3) One of the daughters has a child with a man who also has brown eyes, yet one son has blue eyes.
4) When the blue-eyed grandson has a child with a blue-eyed woman, all of the great-grandchildren have blue eyes.
So what's going on here? Can we determine if brown eyes or blues eyes are dominant from this family tree? Yes, but there are few important rules that apply to help us know.
1) If a person has the dominant brown eye trait, then at least one of the parents must also have that trait. That means if a child has brown eyes, at least one parent must have brown eyes. That is because brown eyes always dominate over green and blue.
2) If a person has two parents with a recessive blue eye trait, they will always have blue eyes. That is because there is no dominant brown eye trait to overrule the recessive blue eye trait.
3. Green eyes are recessive to brown and dominant to green.
Did you figure it out yet? Brown eyes are dominant! Using a graphic organizer, like the one below, gives us another way of understanding why that is so. Brown eyes are represented with a B and blue eyes with b. To have brown eyes, one can have either a Bb or BB combination inherited from their parents. But to have blue eyes, one can only have a bb combination. This diagram shows how two brown eyes parents have a 1 in 4 (or 25,) chance of having a blue-eyed child.
Scientists have applied this concept to all the possible combinations, and we now know all the possible outcomes in determining the color a child's eyes. They are illustrated on the chart below:
Of course, there are exceptions to all rules! Some other factors can interfere with the typical outcomes shown in the chart above.