To prepare, make copies of the Mutations Worksheet (S-B-8-3_Mutations Worksheet and KEY.doc) and the Mix and Match cards (S-B-8-3_Mix and Match Mutations-PDF.pdf). Cut the cards between each type of mutation and the names of the mutations, so that there are 14 cards total. Make the Huntington’s disease Wanted Poster (S-B-8-3_Huntington's Disease Poster.doc) for the gallery walk activity and make the Four Corner Signs (S-B-8-3_Four Corner Signs.pdf). Gather resource material of genetic diseases and materials to make “Wanted” posters. If a poster machine is unavailable, divide the poster page and enlarge and paste sections to a poster board or butcher paper.
Introducing Changes in the Genetic Code
Have students brainstorm what they think of when they hear the word mutations. Write all ideas on the board. Before distinguishing between the “Hollywood” version of mutations and biological mutations, play the Grapevine Game.
Have students form a line around the room. Explain that you are going to whisper a phrase to the first student, who will then whisper it to the next student, etc., until it reaches the last student. Students are not allowed to repeat the phrase. Caution students to whisper so that only the student being spoken to can hear the phrase. Use a phrase like, “The secret of life is the double helix.” After students pass the phrase, compare the final phrase to the original. Explain that mutations have a similar outcome as a phrase in the Grapevine game.
Part 1: Changes in Gene Structure
- “There is redundancy built into the genetic code which protects against errors because of what type of mutation?” (It is less likely to have errors in the polypeptide sequence due to point mutations.)
- “Nonsense, missense, and silent mutations are called point mutations. Why is it a good name for these types of mutations?” (These types of mutations only change one nucleotide, or point, on the DNA sequence.)
An example of a disease (disorder) caused by a change in the amino acid sequence is sickle cell anemia. Hemoglobin is made up of 4 proteins. The change occurs on the β-chain with the 6th amino acid. Normal hemoglobin has glutamate while sickle cell has valine. Ask:
- “What are the codons for glutamate?” (GAA, GAG)
- “What are the codons for valine?” (GUA, GUU, GUC, GUG)
- “Where do you think the point mutation occurred?” (The first A in GAA was changed to a U to make GUA.)
Explain that although sickle cell disease is serious, people heterozygous for sickle cell are slightly resistant to malaria. This is called a “heterozygous advantage,” and it is the reason that the disease continues to exist. The following diagram can be used to explain sickle-cell inheritance.
Has Sickle Cell Disease
Heterozygous = Healthy
Remind students that changes in the DNA might cause a change in the phenotype.
- “What mode of inheritance would cause a genetic disorder with just one mutated allele?” (autosomal dominant inheritance)
- “Where does the mutation have to occur to be passed to the offspring?” (A mutation in the gametes or sperm or egg of the organism can be passed to offspring and future generations.)
Tell students, “Let’s say that a mutation occurs in an egg cell that causes a protein to be defected but remains hidden in the individual that inherited the gene. What kind of allele is it?” (The allele must be recessive.)
Explain that if the homologous chromosome (in this case, the chromosome that came from the father) carries the good gene, the individual can make the proper protein.
Ask, “What if a nonsilent mutation occurs in the DNA of a somatic cell?” (If the mutation is expressed, the cell could die or lead to a cancerous tumor.)
Part 2: Chromosomal Changes
Give student pairs Mix-N-Match cards (S-B-8-3_Mix and Match Mutations-PDF.pdf). Before they match the cards, ask:
- “What do you think the segments A, B, C, etc. mean?” (genes found on the chromosome)
- “How many chromosomes do humans have?” (46)
- “How many chromosomes does a human zygote receive from each parent?” (23)
Allow students to match the cards with the names. When cards and names are matched, have the group classify the mutation as chromosomal or nondisjunction mutations. You may need to remind students that disjunction occurs when sister chromatids are pulled apart, so nondisjunction occurs when they fail to separate into two nuclei. Have them write an operational definition of the mutation type in their journal. Ask:
- “when do you think chromosomal mutations occur?” (Lead students to see that these mutations are likely to occur during mitosis or meiosis when a piece of chromosome breaks off and reattaches to a different chromosome or rejoins backwards.)
- “If an individual has more than 46 chromosomes, such as trisomy 21 (Down Syndrome), where did the mistake likely occur?” (Again, the mistake probably occurred when chromosome pairs failed to separate during meiosis in a fertilized egg.)
- “What is the difference between trisomy and triploidy?” (Triploidy occurs when there are three complete sets of homologous chromosomes. Trisomy occurs when there is a triplicate set of homologous chromosomes.)
This would be a good time to explain that polyploidy is rare and lethal in most animals, but occurs quite frequently in plants. Fruit produced by triploid plants are larger and more uniform. Seedless watermelons are triploids; some types of wheat and kiwifruit have six sets of chromosomes. Certain varieties of strawberries have ten sets of chromosomes. There are a few fish, like goldfish, and frogs that have more than two sets of chromosomes.
Diploid organisms need two copies of each gene, therefore monosomy is often lethal. However, individuals with Turners syndrome, have only one copy of the X chromosome. Ask:
- “Why do you think that it is possible to survive with only one copy of the X chromosome?” (Males only have one X and the extra X chromosome in females becomes and inactive Barr body. A barr body is a microscopic feature of female cells due to the presence of two X chromosomes in the female. One of these X chromosomes is inactive and is crumpled up to form the Barr body.)
- “When can a mutation become an inherited trait?” (when the mutation occurs in the DNA of gamete cells)
- “What do you think can happen to reproducing somatic cells that get an extra copy of chromosomes?” (Lead students to understand that the cell can die. It seems that in humans, a nucleus that is too tightly packed with DNA doesn’t function well, or in some cases, can cause cancer. There is evidence that hard tumors have 60 to 90 chromosomes, although the extra copies may be due to cells rapidly dividing.)
Explain that mutations are common in living things. Mutations can occur randomly and are the basis for evolution. However, some environmental agents can cause mutations such as high energy waves (gamma, X-rays and UV rays) and certain chemicals.
Finally, revisit their original idea of mutation. Students should write in their journal comparing the real effects of mutations and Hollywood mutations.
Part 3: Inherited Disorders in Humans
Introduce the lesson with a scenario:
Two people take their dog to the veterinarian for a yearly check-up. Everything goes well, but the doctor asks to take a blood sample of the dog for research. The veterinarian explains that a colleague, Dr. Emma Law, received a grant for genetics research and the blood is for her study of genetics. There is no cost for drawing the blood sample, and the people might find out if the dog is a carrier for a particular gene.
Based on the scenario, ask students to respond to these questions:
- “Should the people allow the veterinarian to draw blood from the dog to be studied? Why or why not?” (Accept all responses with reasons.)
- “What if you were asked to give blood for human genetics research, would you give blood?” (Accept all reasoned responses. Some may say “no” because they are afraid of needles. Some may not want to know if they have a particular gene.)
- “What if the particular human gene is for red hair, does it matter? Would you be curious?” (Accept all opinions.)
- “What if Dr. Law is studying a genetic disorder for which there is no cure, a gene that could pass to future children, does it matter then?” (Accept all opinions.)
Tell students, “Today, thanks to the Human Genome Project, tests can see if you have the allele for many genetic disorders.” Extend the discussion for any of the questions above. A summary of the Human Genome Project is available (S-B-8-3_Human Genome Project-PDF.pdf).
Wanted Poster/Gallery Walk
Show students an example of a Wanted Poster for a genetic disease (S-B-8-3_Huntington's Disease Poster.doc). Go over the requirements for the poster (S-B-8-3_Gallery Walk and KEY.doc), and review how to draw a pedigree chart. Ask questions such as:
- “What is the shape of a male on the chart?” (square)
- “What is the shape of a female on the chart?” (circle)
- “How do you distinguish between affected individuals and a normal individual?” (Individuals with the affliction or trait are colored.)
- “When do you have carriers?” (When the trait is recessive or sex-linked. If they are known, they are colored halfway.)
- “How can you distinguish between a dominant and recessive trait on a pedigree chart?” (Recessive traits skip generations, but dominant traits appear in each generation.)
- “How do you distinguish between an autosomal recessive and a sex-linked trait on a pedigree chart?” (Sex-link traits skip generations on the female-side but appear in almost every generation of the males.)
- “How do you distinguish between a dominant and a codominant trait on a pedigree chart?” (Heterozygous individuals of a codominant gene are colored like carriers and both genes are expressed in all the generations.)
Assign students a disease, or have them choose a disorder that another group has not chosen. Have students work in groups to create their own “Wanted” poster for a genetic disorder. Allow students to use information from the text or print information from the Internet.
Have students walk around the classroom, reading the posters and summarizing information from the posters to write in their science journals. Have students use the Gallery Walk Science Journal (S-B-8-3_Gallery Walk and KEY.doc) as a template for writing information in their science journals.
Listen carefully for students’ misconceptions and misunderstandings about genetic disorders. For instance, it is a common misconception that disorders are contagious. Before changing places or before the activity ends, correct any flawed thinking about genetic disorders.
Part 4: Four Corners Closure
Teacher Note: Gene therapy and the ethical considerations surrounding it can be a controversial topic. This optional activity offers two different methods to discuss advancements in modern science and society’s reaction.
Hang signs with “Strongly Agree,” “Agree,” “Disagree,” and “Strongly Disagree” (S-B-8-3_Four Corner Signs.pdf) in each of the corners of your classroom before the beginning of class.
Have students watch the scene from “Gataca” where Vincent is born and his parents get the list of all his genetic disorders. (Note: You may want to make mention that some genetic “disorders” are not a hindrance to one’s life. They may produce increased intelligence, athleticism, etc.) Source: http://www.youtube.com/watch?v=eB8QMWeDjSk&feature=fvw
Make the statement: “I would want to know if I were a carrier of a genetic disease, whatever the cost.” Allow students 3 minutes to think before moving to a corner that best fits their opinion.
Allow students 5 minutes to discuss with other members of their group reasons for their choice of strongly agreeing, agreeing, disagreeing, or strongly disagreeing with the statement. Assign one person to be note taker, while others suggest topics.
After 5 minutes, ask each group to share some reasons that their group discussed. Give students a few minutes to change sides if they changed their minds. Close this activity by having students write a paragraph, stating their opinion and their reasons. Students’ paragraphs should start off with “I strongly agree,” or “I strongly disagree,” or with whatever opinion they hold.
- Students who may be moving beyond the standards can research gene therapy and present information to the class. Students should choose one genetic disorder where gene therapy is currently being used for treatment, cure, or research. Students should describe the process, ethical considerations, and promise for gene therapy.
- Students who might need an opportunity for additional learning should create a game of chance that reinforces the concepts of heredity and changes in heredity.