It Takes Two: How Does a Punnett Square Predict Possible Gene Combinations?

Problem

How does a Punnett square predict possible gene combinations?

Materials

• marking pen
• ruler
• typing paper (or any writing paper)

Procedure

1. Draw a 2-inch (5-cm) square on the paper.
2. Divide the square into four smaller squares by drawing two lines, one vertical and one horizontal, through the center of the square.



3. Label the squares E and e across the top and down the left side of the large square as shown.
4. In each small square, write the letters that appear above and to the left of the square, writing capital letter first.

Results

There are three different combinations of letters in the boxes: EE, Ee, and ee.

Why?

The letter combinations stand for combinations of genes. Genes (locations on a chromosome that determine inherited traits), located in chromosomes (threadlike structures in cells that carry instructions, much like a computer program, to make the cell function), determine inherited traits. Traits are characteristics, such as hair color, eye color, and height, that help to identify living organisms. For each inherited trait, the offspring has two genes, one from the father and one from the mother. The gene that determines the trait is called the dominant gene, represented by a capital letter. The other gene, called a recessive gene, is represented by a lowercase letter. If both genes are either dominant or recessive, the trait is called a pure trait. If only one gene is dominant, the combination is said to be a hybrid trait. Gene combinations are written with the dominant gene (represented by a capital letter) first.

In this activity, you will use a Punnett square to show all possible gene combinations that can be transferred from parents to offspring for the trait of earlobe attachment. E represents the dominant gene for unattached earlobes, and e represents the recessive gene for attached earlobes. Because unattached earlobes are dominant, the combination EE is a pure trait for unattached earlobes, while Ee is a hybrid for unattached earlobes. Both parents have the hybrid, as shown by the E and e along both the top and side of the large square. Three of the smaller squares show combinations—EE, Ee, and Ee—that would give the offspring unattached earlobes. The fourth square shows the combination ee, which would give the offspring attached earlobes, a pure trait determined by recessive genes.

Let's Explore

What are the possible combinations if one parent has pure trait genes (EE) and the other has hybrid genes (Ee)? Repeat the activity, using the letters E and E across the top and E and e down the side. Science Fair Hint: Use the Punnett squares as part of a project display.

Show Time!

1. Although the Punnett square shows the possible combinations, it cannot be used to predict

   

   what will actually happen to each child if four children are born to the same parents. Coins can be used to show how chance affects heredity (the passing on of traits from parents to offspring). Place a small piece of tape on the front and back of each of 2 pennies. Write a capital E on one side of each penny and a lower case e on the other side.



Stretch a small hand towel out on a table. Hold both coins in your hands and shake them back and forth several times, then toss the coins together over the towel. NOTE: The towel will prevent the coins from rolling off the table. Prepare a chart as shown to record the letter combinations. Toss the coins three more times, recording each letter combination. Compare the four combinations to those in the Punnett square of the original experiment.

2. Repeat the previous experiment twice, using different gene combinations for the father and mother. Prepare a chart as before to record the results of each experiment. First use pure trait from dominant genes (EE) and hybrid (Ee), then use pure trait from recessive genes (ee) and hybrid (Ee). Prepare a Punnett square for each gene combination. Use the charts and Punnett squares as part of a project display.



3. How are you similar to or different from each of your parents? Examine some of your genetic traits and the genetic traits of your parents. Design and prepare a chart showing the presence or absence of the traits being examined. Possible traits to look for are right-or left-handedness, eye color, dimples, freckles, ear lobes (attached or unattached), and eye shape (round or oval). Use the results to prepare a written and/or oral presentation. Provide proof that you do not receive all of your genes from only one parent.

Check it Out!

Inherited traits are traits passed from parents to offspring. For more information about heredity, see pages 193-201 of Janice VanCleave's The Human Body for Every Kid (New York: Wiley, 1995).

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