1. Give each student two index cards, one of each color. Tell them that these cards represent genes for an unknown trait. Assign an allele to each card (for example, the blue card is "A" and the white card is "a"), and have students write the appropriate symbols on their cards. Ask:

• What would the genotype for this trait be called?

Explain that each student is heterozygous for this trait because he or she has one dominant and one recessive gene.

2. Ask students to pair up. Have the students in each pair hold their gene cards behind their back, shuffle them, count to three, and then put one card in front of them. Tell them to pretend that each card is a gamete, and together the two cards shown represent a possible genotype of the first offspring of parents with these alleles. Ask students to write down the genotype of this offspring. Then have students repeat this process until they have determined and written down the possible genotypes for a total of four offspring in this family.

3. Show the One Wrong Letter video about Tay-Sachs disease. Ask:

• How can a mutation in a single DNA base affect the production of normal proteins, as it does for Tay-Sachs disease?
• How can mutations be passed on to offspring?
• What does it mean to be a "carrier" of a disease?

4. Ask students:

• Assume that the "a" allele in your gene cards was a gene for Tay-Sachs disease. How many of your offspring would have the disease?
• How does the proportion of each kind of genotype in the offspring compare for different pairs of students? What is responsible for this difference?
• How is this exercise different from or similar to what occurs in a real population?
• How would the frequency of the Tay-Sachs recessive gene in the general population compare to that in the population of Ashkenazi Jews (Jews with ancestors from Central and Eastern Europe)?

Show the video A Mutation Story and ask:

• What if the recessive gene in your imaginary family of four were a gene for sickle cell anemia? How many of these offspring would have the disease? How many would be protected against malaria?
• How is this pattern of inheritance similar to and different from that of Tay-Sachs disease?
• What influences the ability of this gene to survive in the gene pool of the population?
• Would you expect the frequency of this gene to vary in different areas of the world? Why?

5. Have students look at the Genetic Drift and the Founder Effect image and read the background essay. Ask:

• What factors can create a high incidence of genetic diseases linked to recessive genes in a population?

6. Point out that Tay-Sachs disease, sickle cell anemia, and Ellis-van Creveld syndrome are three examples of genetic disorders caused by recessive genes. Some genetic disorders, such as Huntington disease, are caused by a dominant gene. Huntington disease is not usually expressed until the person is between thirty and forty years of age. It causes degeneration of the nerves, dementia, and eventually death. Ask:

• In the case of a genetic disorder caused by a dominant gene, what would be the pattern of inheritance in the imaginary four offspring?

Show the How Genetic Disorders Are Inherited transparency or give it to students as a handout. Ask:

• Why would a disease caused by a dominant gene persist in a population?

7. Also emphasize that some genetic disorders, like Tay-Sachs disease and sickle cell anemia, are determined by a single gene. These are known as Mendelian disorders. Other diseases, such as breast cancer and diabetes, can be influenced by the interaction of several genes as well as by factors in the environment. They are known as multifactorial diseases.

8. Show the video Double Immunity and discuss the following:

• Are mutations are always harmful?
• When might mutations become advantageous?

9. Have students look at the Chromosome Viewer Web activity to identify the specific chromosomes on which the four disorders mentioned above (Tay-Sachs, sickle cell anemia, Ellis-van Creveld, and Huntington's) are located. Then have each student review the twenty-three chromosomes and identify five diseases that they would like to know more about. Make a class list, then choose the top five genetic disorders students would like to research. Point out that some genetic disorders are caused not by mutations but by extra or missing chromosomes. Nondisjunction is the failure of chromosome pairs to separate during meiosis I anaphase, and it can cause some eggs/sperm to gain an extra chromosome and others to lose one. For example, Down's syndrome is caused by an extra copy of chromosome 21. (To illustrate nondisjunction, see the How Cells Divide: Mitosis vs. Meiosis Web activity.)

10. Using the student-generated list of five genetic disorders to research, have the class brainstorm questions they want to answer about these diseases. If the following topics are not suggested, add them to the list: genetic cause, location on chromosomes (or which chromosomes have an extra copy or are missing), Mendelian or multifactorial, rate of occurrence in different populations, treatment, etc.

11. Divide the class into five teams. Have each team research a different disease and prepare a presentation using library and multimedia Web resources (see Cracking the Code of Life, http://www.pbs.org/wgbh/nova/genome/resources.html, to get started). Then have each team present information about their disease to the class.

12. Summarize by asking teams to identify the similarities and differences in inheritance patterns of the five different diseases.