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NOVA scienceNOW: Risky Genetics

Classroom Activity


Activity Summary
Student teams analyze genetic test results for fictional characters, compare their characters' risks of developing specific diseases, and assess the risks for the next generation.

Learning Objectives
Students will be able to:

  • Define the terms genotype, homozygous, heterozygous, recessive, and dominant

  • Apply the above terms to diseases with different inheritance patterns

  • Differentiate between genetic risk and lifestyle risk, in the context of disease

  • Demonstrate the use of Punnett squares to predict the probabilities of particular genotypes in a future generation

  • Construct a three-generation pedigree

  • Discuss how disease can be affected by lifestyle

  • Debate the pros and cons of genetic testing

Suggested Time
One class period

Materials

Multimedia Resources

Additional Materials


Background

Can genetic testing predict who will get a particular disease? The answer depends on the disease, how it is inherited, and whether or not it is influenced by other factors. For every gene, we inherit one copy, known as an allele, from each parent. Alleles can come in different forms, and some are harmful enough to cause diseases.

Some diseases are simply caused by a defect in one gene. For example, a type of lung disease known as Alpha-1 Antitrypsin Deficiency is caused by a defective SERPINA1 gene. Normally, this gene codes for an enzyme called alpha-1 antitrypsin that protects the lungs against deterioration. In healthy people, two normal, dominant alleles produce plenty of the enzyme. But if two damaged, recessive alleles are inherited, then the person does not produce enough of the enzyme, and his or her lungs slowly deteriorate over a lifetime. If one copy of each type of allele is inherited (one normal-dominant and one damaged-recessive), then an intermediate amount of the enzyme is produced, which is usually enough to protect the lungs. However, if a person is a heavy smoker or works in a dust-filled factory, severe problems could start early. This is an example of when genetic testing cannot predict the severity of the disease or when it will strike, because environmental factors can have a powerful influence on human health.

Other genetically caused diseases are more complicated. A specific type of Alzheimer's disease, which causes mental deterioration, is strongly linked to the gene PSEN1. A person with a dominant disease allele will almost certainly get "early onset" Alzheimer's disease, which occurs before the age of 65, because a harmful protein builds up within the brain. However, Alzheimer's disease that develops later in life is still poorly understood. Scientists have discovered several genes involved, but details remain fuzzy.

In another example, two genes—BRCA1 and BRCA2—are strongly linked to breast and ovarian cancer. Dominant disease alleles for either gene lead to a fivefold increase in women's risk of the disease, and even increase cancer risk for men. However, not all women with the disease alleles will get breast cancer, and not all women with breast cancer have these specific alleles. What accounts for this variation? Many genes are involved in cancer, as are health and environmental factors such as smoking habits and toxic pollution. Scientists are still looking for answers.

People may find genetic testing valuable, depending on the specific diseases they are interest in and their motivations. For example, genetic testing for alpha-1 antitrypsin deficiency, or for breast cancer, could give a person important insights into his or her health, leading to more diligent screening and better prevention and treatment. However, testing for early-onset Alzheimer's disease could result in a person feeling despair over a condition for which there is currently no treatment or cure. Some people choose genetic testing for other reasons, such as for help in deciding whether to have children or to contribute to research focused on learning about a disease and developing cures.

In this activity, students play the role of doctors interpreting patients' genetic test results. They create characters, analyze genetic test results for three diseases, compare their characters' risks of developing specific diseases, and assess the risks for the next generation.


Procedure Before the Lesson The Lesson
  1. As a class, watch the NOVA scienceNOW segment Personal Genomes. You can stream it from the NOVA scienceNOW Web site at http://www.pbs.org/wgbh/nova/sciencenow/0406/01.html.

  2. Ask students what questions they have about the segment, and explain that they will explore genetic testing for specific diseases: breast cancer, lung disease, and Alzheimer's. Students will play the role of genetic counselors who counsel patients about their genetic test results for genes associated with the three diseases. Students will pair with a partner, and by rolling dice and following steps provided in the handout, each "councilor" will create a male or female character with a unique genotype. Then students will analyze genetic data for their "patients," compare their characters' risks of developing the diseases, and predict the disease risks for the next generation.

  3. If necessary, review relevant terms and concepts, including:

    1. Allele — one of the variant forms of a gene
    2. Gene — a functional portion of DNA that is passed from parent to offspring and which usually codes for a specific protein
    3. Dominant allele — an allele that almost always results in a specific trait, even if the person possesses only one copy
    4. Recessive allele — an allele that only results in a specific trait if the person possesses two copies of the recessive allele
    5. Genotype — the genetic identity of an individual
    6. Heterozygous — having two different forms of a particular gene
    7. Homozygous — having two of the same forms of a particular gene
    8. Punnett square — a tool used to predict the genetic outcome of a mating
    9. Pedigrees — a pictorial representation of a family's health history
    10. Demonstrate a sample Punnett square on the board, such as the following, which shows a heterozygous parent (Bb) mating with a homozygous recessive (bb) parent:

       bb
      BBbBb
      bbbbb
  4. Give each student a copy of the handouts and have students form pairs. Make sure each pair has at least one die.

  5. Instruct students to follow the procedure on the Risky Genetics Student Handout, including watching the videos and reading the text on the following sites:

  6. Before students do Steps 5 and 6, make sure they know how to construct pedigrees. When they are ready to begin Step 5, have them select a pedigree strip.

  7. When students are finished, lead a discussion. Ask the "councilors" to report their patients' results and recommendations. Note that students who drew the same pedigree will likely have made different recommendations. Ask:

    • Did the recommendations differ among the patients? (In some cases, students' recommendations will be strikingly similar, such as "make healthier choices." This highlights a limitation of genetic testing: testing can give us more information, but might not actually be able to help us in our lifetimes. In other cases, recommendations will differ. For example, a patient with a high risk of breast cancer will benefit from frequent mammograms, but for other patients, such action would be unnecessary.)
    • In terms of subsequent generations, do children inherit the same disease risks their parents have? (Sometimes. Each parent has a unique genetic makeup. Depending on whether parents carry dominant or recessive alleles and what combination a child receives, a child may or may not be at risk for a disease.)
    • Did the Punnett square help predict what genotype the child got? (A Punnett square only shows a child's theoretical probability of getting a particular combination of genes.)
    • What are the pros and cons of discovering a person's genotype for a particular disease? (Finding out makes great sense when there are therapies or lifestyle actions that can improve a person's quality of life. For diseases with no treatment, finding out can cause anxiety. Often, the decision to know or not know comes down to a person's temperament and approach to life.)

Extensions
  • Suppose that their patient couples had ten children. Have students roll the die to determine the children's genotypes for each disease. Ask students to determine the proportion of the children with each genotype. Did the proportions match the Punnett square predictions? Why or why not?

  • Have students consider personal and public genome sequencing. First, visit the Web site of a company that will perform DNA analysis, such as https://www.23andme.com/. Then, visit the Web site of a genetics research project, such as the Personal Genome Project at http://www.personalgenomes.org/ or The Genographic Project at https://genographic.nationalgeographic.com/genographic/participate.html. Ask students to discuss whether or not they would want to know their own genetic risks for particular diseases or if they would allow their genetic information to be used for research.

  • Have students research genetic counseling careers, starting with the Web page http://nsgc.org/career.

ASSESSMENT

Risky Genetics Student Handout Procedure

  1. Disease Chart from Section 4 of the Patient Record on the Student Handout (with answers)

    Breast Cancer
    • Cells in breast tissue divide uncontrollably and create tumors.
    • Prevention and treatment include: regular checkups to look for tumors, preventive surgery to remove breasts and ovaries, surgery to remove tumors, chemo and radiation therapy.
    Lung Disease
    (alpha-1 antitrypsin deficiency)
    • An enzyme that protects lung tissue is not present, therefore lung tissue slowly degrades over a lifetime.
    • Prevention is impossible, but avoiding smoking and illnesses and getting infusions to restore enzymes can help slow disease progress. If disease is advanced, lung transplants can extend life.
    Alzheimer's Disease
    • Brain function deteriorates.
    • No prevention or significant treatment is available. Doctors recommend staying active, and drugs can help slow disease progress.
  2. Character Analysis
    Answers will vary. For the Patient Record, check that the students have:

    • Correctly used the terms homozygous and heterozygous
      • Two dominant alleles → homozygous dominant
      • Two recessive alleles → homozygous recessive
      • One dominant and one recessive → heterozygous

    • Based on the genetic risk of disease on the correct genotype
      • BRCA2 → homozygous dominant and heterozygous genotypes will have high risk of disease
      • SERPINA1 → only the homozygous recessive genotype will have high risk of disease
      • PSEN1 → homozygous dominant and heterozygous genotypes will have high risk of disease

  3. Medical Recommendations.
    Answers will vary, but check that students have correctly identified the appropriate actions for each disease and interpreted the role of lifestyle choices for each disease genotype:

    • BRCA2 → High-risk patients should have regular checkups to look for tumors, and possibly consider having preventive surgery to remove breasts and ovaries. High-risk patients might want to encourage family members to be tested so that others can protect their health. Healthy living could slightly reduce risks, whereas unhealthy living could slightly increase risks
    • SERPINA1 → Homozygous recessive patients should take measures to protect their lung health, avoiding pollution, smoking, and contagious illnesses. Patients experiencing difficulty breathing can take medications and/or infusions. Family members might consider getting tested, particularly if they are having symptoms like difficulty breathing. Healthy living could reduce risks, whereas unhealthy living would definitely increase risks.
    • PSEN1 → High-risk patients have few options except to be aware of the disease. Lifestyle choices do not influence risk. Family members might want to consider getting tested.

  4. Punnett Squares and the Future Generation
    Answers will vary, but be sure that students have used and interpreted Punnett squares correctly. For example, for the BRCA2 gene, if the parents' genotypes are homozygous dominant and heterozygous, then:

    Punnett SquareProbabilities
    bb
    BBbBb
    bbbbb
    BB —  0%*
    Bb — 50%*
    bb — 50%
    * genotypes potentially affected by breast cancer

  5. Students should refer to their textbooks for how to construct a pedigree.

Risky Genetics Student Handout Questions

  1. What is the difference between a Punnett square and a pedigree? A Punnett square is a tool that shows a child's theoretical probability of getting a particular combination of genes. A pedigree shows the actual family history.

  2. Can doctors predict who will get a particular disease based on a genotype for one gene alone? Why or why not? Different diseases are inherited in different ways. Some diseases are based on a defect in one gene alone, and in that case, yes, doctors can predict who will be affected by the disease. But because many diseases are influenced by many genes and the relationships between all genes are unknown, the answer is usually no.

  3. Do factors other than genes—food choices, environmental pollution, smoking habits—have the same effect on all people? If not, why not? No. The influence of factors other than genes depends on each individual's genotype. In general, a healthy lifestyle is always a good idea. However, for most diseases, such as cancer or lung disease, whether or not disease strikes depends on a combination of both genetics and lifestyle.

  4. List some of the advantages and disadvantages of genetic testing. (See answer below.)

  5. Do the advantages of genetic testing outweigh the disadvantages? Form an opinion and support your response with facts and examples. Answers will vary but should be based on facts and examples. Advantages include: knowing about genetic-disease risks can encourage people to take measures to protect their health (frequent doctor visits) and reduce their other risk factors (stopping smoking or quitting a job in an unhealthy environment). For some diseases, such as lung disease, knowing the genotype can be very useful because specific treatments are available (such as enzyme infusions) that would not be used if the diagnosis were a different type of lung disease. Disadvantages include: genetic testing cannot predict the future; it can be expensive; and it could be used to discriminate against people. For diseases like early-onset Alzheimer's, knowing the genotype is not necessarily helpful and could be psychologically difficult to accept.

Use the following rubric to assess each team's work.

Excellent Satisfactory Needs improvement
Completing genetic analysis and answering questions on the Risky Genetics student handout
  • Students use Web resources effectively to determine the genetic risks of disease and to answer questions.
  • Students can apply patient information to make recommendations.
  • Students need assistance determining how specific genotypes relate to disease and are able to answer questions adequately.
  • Students have difficulty applying information to make recommendations.
  • Students use Web resources but cannot determine how genotypes relate to disease
  • Students do not accurately base recommendations on patient information.

Standards

The Risky Genetics activity aligns with the following National Science Education Standards (see books.nap.edu/html/nses).

Grades 9-12
Unifying Concepts Content Standard E

  • Understandings about scientific inquiry

Life Science Content Standard B

  • Molecular Basis of Heredity

Sciencee in Personal and Social Perspectives Content Standard F

  • Science and Technology in local, national, and global challenges



Classroom Activity Author

Alison Fromme

Teacher's Guide
NOVA scienceNOW: Risky Genetics
PROGRAM OVERVIEW CLASSROOM ACTIVITY


WebsitePublic Genomes QuickTime or Windows Media Video
WebsiteBreast Cancer Gene Mutation Video
WebsiteBreast Cancer Support Group Video
WebsiteLung Disease: Alpha-1 Antitrypsin Deficiency Text
WebsiteMedline Plus Alpha-1 Antitrypsin Deficiency Text
WebsiteAlzheimer's Disease Video