1. Have students work in pairs to explore the How Cells Divide: Mitosis vs. Meiosis Web activity. Ask students to focus primarily on the meiosis portion of the activity and to pay particular attention to the outcome of meiosis: the sperm or egg cell ends up with half the total number of chromosomes found in other types of cells. Take time to discuss any questions that come up during the 15 stages of the meiosis animation. After the activity, discuss the following:

• All humans have 23 pairs of chromosomes (or 46 total) in each of their cells.
• At the end of meiosis, the resulting cell -- sperm or egg -- has 23 chromosomes total, or half the number of chromosomes in other types of cells in the body.
• During fertilization, chromosomes from the sperm cell and the egg cell combine.
• Equal numbers of chromosomes, one from each of an individual's parent's 23 pairs.
• Since chromosomes occur in pairs and genes are on chromosomes, then genes occur in pairs.
• Genes determine an individual's traits.
• Some traits are determined by a single pair of genes -- one from your mother and one from your father.
• If a mutation occurs in a gene, that mutation can be passed on to offspring.

2. Distribute a copy of the Sickle vs. Normal Cell image and discuss the following:

• Normal blood cells are pictured in the large image. Sickle cells, which are caused by a genetic mutation, are pictured in the inset of the photograph.
• Sickle cells often cause blockages in the blood vessels of people who have them.
• This disease, called sickle cell anemia, is passed from parent to offspring.

3. Show the A Mutation Story video and discuss the following:

• What effect did the sickle cell gene have on the people who were carriers of the mutation?
• Why has the sickle cell gene persisted even when sickle cell anemia is so debilitating?
• What are the odds that the child of parents who each carry one normal gene and one sickle cell mutation gene will have sickle cell anemia?
• What are the odds that a child of two carrier parents will also be a carrier and, thus, be protected from malaria?

4. Ask students to choose a partner. Give each pair two pennies. Then write the following on the board:

• heads = normal gene (H)
• tails = mutated gene (h)

5. Introduce (or review) the following terms:

• genotype: the genes you have for a trait
• phenotype: the traits you show based on your genotype
• heterozygous: having two different forms of a gene for a trait (such as Hh)
• homozygous: having two of the same forms of a gene for a trait (such as HH and hh)

6. (Note: Be sure to stress to students that, unlike sickle cell anemia, which is determined by just a single pair of genes, many traits are determined by multiple pairs of genes. The genetic causes of such complex traits are therefore far more difficult to trace than single-gene traits.)

7. Review the possible genotypes and phenotypes from the A Mutation Story video:

• HH: normal, no mutation
• Hh: one normal gene, one mutated gene, protection from malaria
• hh: two mutated genes, sickle cell anemia

8. Tell students that one coin represents the genes of the mother and the other coin represents the genes of the father. Because each coin has two sides -- heads or tails -- the parents are heterozygous. That is, each parent has one normal gene (H) and one mutated gene (h). Therefore, the chance of the father passing on the mutation (the coin turning up tails) is 50%; the chance of the mother passing on the mutation is likewise 50%. Have students make a table like the one below, with space for 20 trials:

9. Tell each pair of students to flip their coins 20 times and record their results on the table. A sample of the possible combinations is provided below:

10. Have students analyze their data by calculating the percent of each phenotype -- the number of each phenotype divided by the number of trials (20), then multiplied by 100.

11. Draw a large table on the chalkboard and compile the class data in the chart. Have students calculate the percent of each phenotype for the class data.

12. Discuss the following questions:

• How does this activity illustrate how the mutated gene for sickle cell can survive in the population and be passed on from generation to generation?
• If being heterozygous for the mutation did not provide protection from malaria, do you think it would last very long in the population? Why or why not?
• How did the mutation in the gene in the video occur?
• Why do you think the mutation was "selected" in the population and was passed on to future generations in greater and greater numbers?
• What does it mean to be a "carrier" of a disease?

13. Show the One Wrong Letter video and discuss the following:

• What causes Tay-Sachs disease?
• Does the Tay-Sachs mutation provide a benefit to the population as a whole in the same way that the sickle cell mutation does?
• What are the odds that a child born to two carrier parents will inherit Tay-Sachs disease?