Why are my eyes the color they are?Featured science projectScience project video

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Abstract

You may have noticed that not everyone in your family has the same eye color. Are your eyes the same color as one or both of your parents? Did you ever wonder why your eyes are the color they are? There are three types of eye colors that you can have - brown, green and blue. Of course there are light and dark variations in these colors, but there are really only three basic eye colors.

Objective

In this science project, you will learn the basics of genetics through examining eye color on your family tree. You will create a family tree going back four generations to help determine how your eye color was inherited.

Background

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.

eye color dominance science project

(source: http://genetics.thetech.org/ask/ask363)

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.

Genetics - eye color chart - brown and blue eyes

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:
Eye color combination chart - parent and children eye colors

Of course, there are exceptions to all rules! Some other factors can interfere with the typical outcomes shown in the chart above.

Scientific Terms

melanin , inherit , genes , genetics , polygenetic , dominant , recessive , family tree

Materials

pencil and paper

Procedure

1. Choose one side of your family on which to focus to begin, either your mother?s side or your father?s side. You can add on the other side later if you choose to do so. You will be designing a family tree similar to the one shown in the Background section of this experiment. Remember, connecting a line between a circle and square means a couple is married. For siblings, be sure that you first draw a vertical line above each shape then connect the vertical lines with a horizontal line above the shapes.

2. Turn an 8.5" x 11" sheet of paper the long way (horizontally). Starting at the bottom of the sheet, leaving room for three rows above, draw yourself and any siblings as circles for girls and squares for boys. Leave about ? inch in between each shape. Color in each circle or square brown, green or blue. Hazel is a subset of brown, so color all family members with hazel eyes brown.

3. Draw a short (about a 1/4 inch tall) vertical line above each sibling. Connect siblings, if any, with a horizontal line that is attached to the vertical line. Label this line "Great-Grandchildren". Inside the circle or square representing you, write the word "ME".

4. Above you and any siblings, draw a circle and a square to represent your parents. Leave about ? inch space between each shape. Color in the shape brown, blue or green to represent their eye color. Connect your parents with a horizontal line (about ? inch wide) from the center of each shape. Inside your mother?s circle, write "mom" and inside your father?s square write "dad".

5. If you?ve picked your mother?s side, add her siblings next to her as circles or squares and color their shapes the corresponding color. If you?ve picked your father?s side, do the same as you would for your mother. Again, leave about ? inch spacing between shapes. Connect the siblings with a horizontal line from the center of each shape. Label this line "Grandchildren".

6. Repeat this process by adding your grandparents (chose only your grandmother or grandfather) and their siblings to the right or left of your parents. Label each shape grandma and grandpa. Label this line "Parents".

7. Repeat again by adding the last of four generations, which would be your great-grandparents. Label this line "Grandparents".

8. Using the chart below, analyze why you have brown, blue or green eyes. Write a statement determining why your eye color is brown, blue or green.

eye color science project

Discussion

pencil and paper

Questions & Answers

Is your eye color the same as one or both of your parents?

Why are my eyes the color they are?

Make it Your Own

In this experiment, you drew and examined your family tree on one side of your family going back four generations. Expand this study to both your mother and your father's side. You will need a wider piece of paper to do so. If there are unknown eye colors from your family tree, can you make an education guess on what their eye color could have been?

Use what you learned about eye color and reproduce a similar family tree using other inherited traits such as having freckles, red hair, dimples, ear lobes, or the ability to curl your tongue.

References

No Author (2013). The Tech Museum of Innovation: Dominant vs. Recessive. Retrieved November 6th, 2013 from http://genetics.thetech.org/ask/ask363

#Materials and Equipment:

  • pencil
  • paper

#Experimental Procedure:

1. Choose one side of your family on which to focus to begin, either your mother's side or your father's side. You can add on the other side later if you choose to do so. You will be designing a family tree similar to the one shown in the Background section of this middle school science experiment. Remember, connecting a line between a circle and square means a couple is married. For siblings, be sure that you first draw a vertical line above each shape then connect the vertical lines with a horizontal line above the shapes.

2. Turn an 8.5" x 11" sheet of paper the long way (horizontally). Starting at the bottom of the sheet, leaving room for three rows above, draw yourself and any siblings as circles for girls and squares for boys. Leave about a quarter inch in between each shape. Color in each circle or square brown, green or blue. Hazel is a subset of brown, so color all family members with hazel eyes brown.

3. Draw a short (about a 1/4 inch tall) vertical line above each sibling. Connect siblings, if any, with a horizontal line that is attached to the vertical line. Label this line "Great-Grandchildren". Inside the circle or square representing you, write the word "ME".

4. Above you and any siblings, draw a circle and a square to represent your parents. Leave about a quarter inch space between each shape. Color in the shape brown, blue or green to represent their eye color. Connect your parents with a horizontal line (about half an inch wide) from the center of each shape. Inside your mother's circle, write "mom" and inside your father's square write "dad".

5. If you've picked your mother's side, add her siblings next to her as circles or squares and color their shapes the corresponding color. If you've picked your father's side, do the same as you would for your mother. Again, leave about a quarter inch spacing between shapes. Connect the siblings with a horizontal line from the center of each shape. Label this line "Grandchildren".

6. Repeat this process by adding your grandparents (chose only your grandmother or grandfather) and their siblings to the right or left of your parents. Label each shape grandma and grandpa. Label this line "Parents".

7. Repeat again by adding the last of four generations, which would be your great-grandparents. Label this line "Grandparents".

8. Using the chart below, analyze why you have brown, blue or green eyes. Write a statement determining why your eye color is brown, blue or green.

#Make It Your Own:

In this experiment, you drew and examined your family tree on one side of your family going back four generations. Expand this study to both your mother and your father's side. You will need a wider piece of paper to do so. If there are unknown eye colors from your family tree, can you make an education guess on what their eye color could have been?

Use what you learned about eye color and reproduce a similar family tree using other inherited traits such as having freckles, red hair, dimples, ear lobes, or the ability to curl your tongue.

#