QUESTION

Why are my eyes green?

EXPERIMENT OVERVIEW

In this experiment you and your parents will complete a survey that asks about certain inherited traits, or traits that you have no control over. Then you'll pick two of them to complete a probability study using a tool called a Punnett square.

SCIENCE CONCEPT

Traits like hair or eye color, attached ear lobes, hitchhiker's thumb, and the ability to roll your tongue are called dominant or recessive. Each of us has two genes for each trait in us — one we inherited from our mother and one from our father. As you might expect, a person who has two dominant genes or two recessive genes will have that trait. However, a person with one of each will display the trait of the dominant gene even though they possess the recessive gene as well. Here's an example.

Suppose you own two black rabbits and they produce a baby rabbit. In rabbits, black fur is a dominant gene (shown as a capital letter) and brown is a recessive gene (shown as a lowercase letter). Let's suppose in this case that your two rabbits each have a black and a brown fur gene. Can you explain why they both have black fur? When they have a baby, the baby will inherit one of the many combinations of fur genes from its parents — either two blacks, a brown and a black, or two browns. In the first two cases, the baby will also have black fur. There is actually a 75 percent chance that this will happen. But there is a 25 percent chance that the baby will inherit both parents' brown fur genes and will be born with brown fur. That is, two black rabbits can produce a brown rabbit.

The Punnett square below shows how this could happen.

So if both of your parents have brown eyes but yours are green, that is a perfectly reasonable possibility.

MATERIALS

Survey (at the end of the chapter)

You and your biological parents. If this isn't possible, find someone who has access to his or her biological parents and ask him to help you.

Punnett square

2 coins

PROCEDURE

• Complete the survey; then ask each of your parents to complete it as well. If you can think of other traits to include, add them.

• Talk with your parents about the results. Discuss how many of the traits of each parent you have.

• Pick two of the traits for the second part of the experiment. One should be a trait that both parents have in common but you do not have, if possible. Otherwise, just pick a trait that you all have in common. The second should be a trait that your parents differ on.

• For each trait you select, build a Punnett square that could produce your family's results. A sample is shown below. Mother has green eyes, father has brown eyes, child has green eyes. In this case, the brown gene is dominant over the green gene. This could occur in either of the combinations on the right.

• Count the number of smaller squares that could produce your results. For the example above, there are two.

• Divide this number by four, the total number of squares, to determine the probability, or chance, of this result occurring. For this example, the probability is 50 percent.

• On a piece of paper, decide which gene will be represented by heads and which will be tails for each coin you will be flipping. It might be easier to use different coins to represent each parent.

• Toss both coins 20 times for each trait. Count the number of times you get a result that matches your own results (in our example, the result we're looking for is that the child has green eyes) and divide that number by 20. This is your experimental probability.

• Compare your experimental probability with your theoretical probability and present your findings.

QUESTIONS FOR THE SCIENTIST

• Are there any traits that your parents share but that you do not possess? What are they?

• Are there traits that all three of you share?

• Do you think these traits are carried by dominant or recessive genes?

• How close were your experimental results to the values you calculated from the Punnett square?

• What does it mean if your results don't match your predictions?

• Does the Punnett square mean that if a mother and father have four children each child will fit into one of the squares? Why or why not?

CONCLUSION

Genetics is one of the most fascinating and scary topics in biological research today. From cloning to disease prevention, doctors are searching for ways to improve our lives by understanding what it is we are made of. So far, there is no guaranteed way to predict the traits of one's children, and that's probably a good thing. However, an understanding of your past helps you prepare for your future and that's one of the many reasons why children who are adopted try to find their birth parents. No matter who we are, it's reassuring to know that we didn't happen by chance, and that there is a plan, however complex it may be, for our being who we are.

SURVEY

• Can you roll your tongue? Stick out your tongue. Try to curl it into a u. Write “yes” or “no.”

  You___________

  Mother___________

  Father___________

• Are you right or left thumbed? Put your hands together, interlocking your fingers. Which thumb is on top? Write “right” or “left.”

  You___________

  Mother___________

  Father___________

• Do you have dimples? Smile at a friend. Do they see any dimples? Write “yes” or “no.”

  You___________

  Mother___________

  Father___________

• Are your earlobes attached or unattached? Write “attached” or “unattached.”

  You_______________________

  Mother_______________________

  Father_______________________

• Do you have “hitchhiker's thumb” (curved thumb when you stick it straight out)? Write “yes” or “no.”

  You___________

  Mother___________

  Father___________