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Lesson 1


Ideas about the Role of Evolution in Medicine

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Engage


At a Glance

Overview

Lesson 1 consists of two activities and should take two 50-minute class periods to complete. In the first activity, students offer explanations for the evolution of antibiotic-resistant bacteria. These initial explanations give you an idea of your students’ ideas about evolution by natural selection. In the second activity, students examine data from the Pax6 gene in different species used as models in medical studies. Students suggest how an understanding of common ancestry enables scientists to use model species to learn about human health.


Activity 1: Outbreak!

Estimated time: 30 minutes

Major Concepts

  • Mutation is the source of the genetic variation that is acted on by natural selection.
  • Natural selection is a powerful process of evolution and is the only mechanism to consistently yield adaptations.
  • Understanding mechanisms of evolution, particularly adaptation by natural selection, provides many insights that enhance medical practice and understanding.

Objectives

After completing this lesson, students will

  • have expressed their ideas about the source of genetic variation,
  • have described their thoughts about the role of evolution in antibiotic resistance, and
  • appreciate that evolution and medicine are relevant in their lives.

Activity 2: Models and Medicine

Estimated time: 70 minutes

Major Concepts

  • “Descent with modification” suggests that modern organisms inherited their traits from ancestors and that modern species all share common ancestors at some point in time. The characteristics of living organisms are shaped by this history.
  • Phylogenetic information from diverse organisms across the “Tree of Life” offers many insights that inform medicine.

Objectives

After completing this lesson, students will

  • have expressed their ideas about why various organisms can serve as model species that help scientists learn about health-related issues in humans and
  • have expressed their ideas about interpreting an evolutionary tree.

Teacher Background

Consult the following sections in Information about Evolution and Medicine:

1.0 Fundamentals of Evolution and Medicine
4.0 Students’ Prior Conceptions about Evolution
5.0 Featured Examples of Evolution and Medicine


In Advance

Web-Based Activities
Activity Web Component?
1 No
2 No
Photocopies, Transparencies, Equipment, and Materials
Photocopies and Transparencies Equipment and Materials
Activity 1: Outbreak!
1 copy of Master 1.1 for each student
Activity 1
None
Activity 2: Models and Medicine
1 transparency each of Masters 1.2 and 1.5
1 color transparency of Master 1.4
1 copy each of Masters 1.3 and 1.5 for each student
Activity 2
Different-colored pens or pencils
Preparation

Activity 1

Each student will need to maintain a notebook or folder dedicated to this supplement for handouts and a record of their ideas. The lessons include opportunities for students to record their initial ideas and answers to questions about evolution and medicine. To help monitor their own understandings and track how their thinking has changed, students will frequently refer back to their previous work and their initial understandings. Decide what format will work best for your students. If your students normally use bound composition books, they can continue to use these and tape or staple handouts into the book. Alternatively, students can record their notes on notebook paper and keep their notes and their handouts in binders or folders.

Activity 2

No other preparations are needed except for making copies and transparencies.


Procedure

Activity 1: Outbreak!

Estimated time: 30 minutes

Note: During Activity 1 of this Engage lesson, students have an opportunity to express their initial thoughts about the source of genetic variation and the role of natural selection in the context of the spread of antibiotic resistance in bacteria. The activity centers on an outbreak of illness caused by a strain of bacteria that is resistant to one class of antibiotic drugs. The intent of the activity is not to teach content related to genetic variation and natural selection but rather to elicit students’ prior knowledge about these concepts. We chose the antibiotic-resistance scenario because it represents an example of natural selection in action that is relevant to students’ lives. Responses to the questions posed during the activity can help you assess students’ relative familiarity with the concepts and identify misconceptions. Intended to be brief, this initial assessment of preconceptions can help you adjust your teaching of the remaining lessons of the supplement.

We do not intend for the lessons in this supplement to be students’ only exposure to the concepts of genetic variation, natural selection, and common ancestry. Ideally, before beginning the supplement, students already will have been introduced to

  • Mendel’s laws;
  • the central dogma that DNA codes for mRNA, which directs the synthesis of proteins;
  • types of mutations;
  • natural selection and adaptation; and
  • evolution and common ancestry.

1.

Begin the lesson by explaining that you saw a news story about a disease outbreak at two local high schools. Read the following news story to the students:

“News flash: This just in from our News 8 Action Alert Team. Two local high schools are reporting cases of students infected with the ‘superbug’ called ‘methicillin-resistant Staphylococcus aureus,’ or ‘MRSA’ [pronounced mer-sah] for short. At one school, four members of the football team were diagnosed and treated for MRSA infections. Three friends at a second school are also reported to have MRSA infections. School officials are telling parents and students not to panic. Cleaning crews at both schools are sanitizing locker rooms and classrooms. Students are being told to pay better attention to hygiene. They should wash their hands frequently, keep cuts and scrapes clean and covered with bandages, and not share water bottles or towels.”

This step is designed to grab students’ attention and help make the connection between their lives and evolution and medicine immediately apparent. Many communities are experiencing MRSA outbreaks in schools.

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Tip from the field test: You may consider supplementing this step by using a local MRSA-related story.


2.

Ask students if they know anyone who has been affected by MRSA. Ask one or two students to share their experiences.

If nobody volunteers, bring up any familiar local cases of MRSA infections. If you are unaware of any recent outbreaks, consider sharing the following story. The outbreak of MRSA among the St. Louis Rams is described in a 2005 article by Kazakova et al.

Near the end of October 2007, six high school football players from North Carolina had infections that looked like spider bites or large pimples. The infections grew larger and were identified as MRSA. All the students were eventually treated successfully with a class of antibiotics different from methicillin. The school started sanitizing lockers once a week and wrestling mats and gym towels every day. Similar cases occur across the country. One case that received a lot of attention involved a 2003 MRSA outbreak among eight players on the St. Louis Rams professional football team.

3.

Hand out one copy of Master 1.1, Information about MRSA, to each student. Instruct students to read the information. Provide an opportunity for students to ask about any unfamiliar terms.

Encourage students to keep a list of unfamiliar and important terms on a separate page in their notebooks.

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Tip from the field test: If your class has a large number of English language learners, consider having volunteers read Master 1.1 aloud to the class.


4.

Direct students to work independently to answer the questions at the end of Master 1.1.

Give students a few minutes to write their best ideas at this point. The questions in this step are designed to elicit the students’ initial ideas about the role of evolution in infectious diseases, including the evolution of antibiotic and antiviral resistance. You may need to reassure students that these questions are not part of a “test.” Do not correct individual answers or go over the correct answers with students at this point in the learning cycle. They will revisit their answers to these questions later in the supplement.

Examine students’ answers for common misconceptions such as ideas that the bacterial population purposefully changed or that it changed simply because it “needed to” or “wanted to.” Expect many students to be confused about mutations and the origin of genetic variation. Section 4.0 of the Information chapter contains a further explanation of students’ common misconceptions about evolution (here).

Optimally, answers to these questions should touch on the five principles of natural selection described in Box 1 (see also Section 1.0 of the Information chapter, here). However, it is likely that students won’t have this level of understanding yet. For your background information, the answers in the answer key incorporate the five principles.

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Students’ responses to these questions will help you assess their current understandings about genetic variation and adaptation by natural selection.



Box 1. Five Major Principles of Natural Selection

Throughout this curriculum supplement, you will ask students to focus on and practice using five principles related to natural selection. Students will consider different examples and situations and how each principle is represented. As students continue building their understandings, their explanations of why each principle is important for understanding natural selection and evolution should become clearer and more complete.

  1. Variation: Individuals within a population vary in many traits, including physical and biochemical ones.
  2. Inheritance: Some of the differences in traits among individuals can be passed from parents to offspring. (They are heritable.)
  3. Origin of variation: Some of the variation in traits among individuals has a genetic basis. This variation originated, often many generations ago, as mutations—changes in the genetic information that are random with respect to the needs of the organism.
  4. Fitness: Both the environment and the traits individuals possess affect survival and reproduction. Individuals with heritable traits that enable them to better survive and reproduce in a particular environment will leave more offspring.
  5. Evolutionary change in populations: The frequency of traits and the alleles that affect those traits change in a population over time.
 

Answer key for questions on Master 1.1, Information about MRSA

  1. Researchers developed the antibiotic methicillin to treat people with infections of S. aureus that are resistant to penicillin. Within two years, populations of S. aureus that were resistant to methicillin started showing up in hospitals. How would a scientist explain how the change may have occurred in S. aureus populations?
  2. Typical student responses (ranging from partially accurate to inaccurate) may include the following:

    • The genetic information in the populations mutated.
    • The populations changed or evolved.
    • The population got stronger.
    • Individuals in the population changed because they needed or wanted to.

A complete explanation of the evolution of resistance in S. aureus populations includes five major principles:

1)

Variation: Within a population of S. aureus, some individuals are resistant to methicillin and some are not.

2)

Inheritance: Resistance to methicillin in S. aureus has a genetic basis and is passed from parent to offspring.

3)

Origin of variation: Random mutations in the genetic information of S. aureus led to differences in traits among individuals in the population.

4)

Fitness: When exposed to methicillin, individual S. aureus that were resistant to methicillin left more offspring than those that were not resistant.

5)

Evolutionary change in populations: The frequency of the allele that causes resistance to methicillin increased in the S. aureus population over time.

Note: A common misconception about antibiotic resistance is that the people who have the infection are resistant to antibiotics. In reality, it is the bacteria that are resistant, and these resistant bacteria are passed from person to person.

  1. How can the study of evolution (such as the adaptation of bacterial populations to an antibiotic) help researchers improve people’s health? Explain your initial ideas.

    Students’ answers will vary. Use this question as a formative assessment of how students view the role of evolution within medicine.

5.

Wrap up this activity by holding a brief class discussion about students’ ideas.

Do not provide the correct answers. The point of this discussion is to focus on differences of opinion among the students so that students recognize that their classmates may have different ideas.


Activity 2: Models and Medicine

Estimated time: 70 minutes

Note: This activity prompts students to begin thinking about how common ancestry enables scientists to use model organisms to learn about health conditions. Students will consider the Pax6 gene, which is involved in eye development during embryogenesis. We chose this gene because its sequence is very similar in a wide variety of species; it was inherited from a distant common ancestor and has not changed much in any of the species. In addition, students can see examples of phenotypic changes that occur when mutations occur. This activity also introduces students to interpreting evolutionary trees, which are useful tools for understanding relationships among species.

1.

Ask students to describe what they learned about mutations and disease from the MRSA case. Then ask them to consider how mutations in people might affect health.

This is an opportunity to further assess students’ developing understandings from the previous activity. In the case of MRSA, the mutations occurred in the S. aureus bacteria (not in the humans who developed the disease). The mutations in the bacteria (often in previous generations) enabled some of them to be resistant to methicillin.

Step 1 helps students transition from the idea of a mutation in a pathogen causing a human disease to the idea that mutations in people can cause a disease. Students will likely say that mutations will cause disease or even death. Step 1 enables you to gauge their understandings of mutations.

2.

Explain to the class that they will now consider a human disease. The goal is for students to express ideas about how scientists can use their knowledge of evolution to inform medical research. Project Master 1.2, Aniridia: An Eye Disease. Ask for a volunteer to read the information aloud to the class. Then give students a few minutes to discuss the questions with a partner before participating in a class discussion.

In this brief vignette, students will read about a family affected by an eye disease called aniridia and deduce that it may have a genetic basis.

Answer key for questions on Master 1.2, Aniridia: An Eye Disease

  1. Do you think the parents’ concern for their baby’s health is justified? Why?

    Most expectant parents are concerned about the baby’s health.
    These parents are understandably more anxious than normal because of the family history of aniridia (baby’s father and grandmother).
  2. From the information provided, would you expect aniridia to be caused by an infection, an environmental factor, or a genetic factor? Explain your answer.

    The family history is a clue that aniridia has a genetic basis.

3.

Confirm for students that aniridia is a genetic disease. Scientists have identified and isolated the gene that is mutated in aniridia. The gene is called “Pax6” and is important for the development of eyes in an embryo.

Pax6 plays an important role in the development of the brain and pancreas as well as the eyes. However, for the purpose of this activity, students will focus on the gene’s role in eye development.

4.

Explain that students will now examine data about aniridia and consider how an understanding of evolution can inform medicine.

5.

Give each student one copy of Master 1.3, Learning about Human Health from Other Organisms. Inform students that they will work in groups of three to four to analyze the information provided and respond to the questions.

Part 1 of the handout includes a portion of the amino acid sequence for the protein encoded by Pax6. This sequence uses the one-letter codes for the amino acids. Students may be more accustomed to the three-letter codes. If so, explain that this code is just a different abbreviation for the amino acids and is more commonly used by scientists. For the Pax6 example, there are six differences in this part of the fruit fly protein compared with the other organisms on Master 1.3.

Students simply need to recognize that the sequences for the Pax6 protein are very similar among these organisms. It is not essential that students know the specific changes in the protein sequence.

Part 2 on the handout requires looking at evidence captured in photographs. When students reach Part 2, project a color version of Master 1.4, Photographs of Pax6 Mutations. As students work, circulate among groups to monitor students’ discussions and ask guiding questions if necessary.

Because of the orientation of the photomicrographs, you may need to help students understand the images. The picture of the human eye will probably be understandable to students. The pictures of the mouse eye and zebrafish eye may confuse students. Instead of looking at the eye from the front (as in the picture of the human eye), the mouse and zebrafish images are cross sections of the developing eye; they show the eye from the side. The fruit fly (Drosophila) has compound eyes that look very different from the other organisms’ eyes. The purpose of students making these observations is to look at evidence for gene function (not to learn the anatomy of the eye). The main thing that students should observe is that, for all these species, eye development is abnormal in Pax6 mutants.

6.

Hold a brief class discussion to go over the answers that students gave to the questions on Master 1.3.

Simply allow a few students to express their ideas. Do not go through the correct answers at this point. Students will revise their answers later.

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Content Standard A: Formulate and revise scientific explanations and models using logic and evidence.
Content Standard A: Recognize and analyze alternative explanations and models.

Answer key for questions on Master 1.3, Learning about Human Health from Other Organisms

Part 1

  1. What do you notice about the amino acid sequences in the different species?

    In all four species, the amino acid sequences are very similar. In fact, for this part of the protein, the sequences are identical in zebrafish, mice, and humans. The fruit fly sequence differs by six amino acids.
  2. What can you infer about the Pax6 gene from the protein sequences from these four species?

    Because the protein sequences are so similar, students should expect the gene sequences also to be very similar in these organisms. (If students have trouble making this connection, remind them that the DNA sequence codes for the protein sequence.)

Part 2

  1. On the basis of the pictures, do you think the function of the Pax6 gene is similar in all four species? Explain your reasoning.

    In each of the different species, individuals with mutations in the Pax6 gene have abnormalities in the eye structure. This suggests that Pax6 plays a role in the formation of the eye in each species.

Part 3

  1. What do these experiments suggest about the function of the Pax6 gene? Explain your thinking.

    These experiments suggest that the Pax6 gene is similar enough in all these species that the gene can still function in a different species.

Note: Although students don’t need to know this level of detail, they may still have questions. When either the squid Pax6 gene or the mouse Pax6 gene is expressed in a fruit fly, the eye that develops is a fly eye—not an eye of the original species from which the gene was taken. Although Pax6 is critical for eye development, many other genes also play an important role in specifying eye anatomy. The combination of these genetic influences with the Pax6 gene determines the kind of eye that forms. So in a normal mouse, the Pax6 gene interacts with other genes involved in eye development to form a mouse eye, and in a squid, the Pax6 gene interacts with other genes involved in eye development to form a squid eye. When the Pax6 gene from either of these species is inserted into and expressed in a fruit fly, Pax6 interacts with fruit fly genes involved in eye development to form a fly eye.

Summary Question

  1. In the sequence data, you saw that the protein coded by the Pax6 gene is very similar in fruit flies, zebrafish, mice, and humans. In the other experiments, you examined evidence related to the gene’s function. Why might many species have almost exactly the same gene that has a similar function?

    Students may answer in a variety of ways, depending on their prior knowledge. They may express ideas about common ancestry or they may phrase the answer in terms of the gene being selected for throughout evolution.

7.

Continue the discussion by asking students, “Do the Pax6 experiments support the idea that medical scientists can learn about gene function in humans by studying different organisms?”

Yes, these experiments do support the idea that medical scientists can learn about human health by studying other organisms. The Pax6 gene is a good example because

  • it is conserved across a wide variety of organisms;
  • when mutated, it causes abnormalities in eye development in all species; and
  • the gene can be expressed in other organisms and produce the same feature that it normally would.

This leads scientists to the conclusion that the mechanisms that regulate this gene are probably very similar in a wide variety of organisms, including humans.

8.

After students express their ideas, inform them that species commonly used in scientific research are called “model species” or “model organisms.” Briefly discuss why model organisms might be valuable for scientific research. Prompt the discussion by asking, “Why would scientists want to study a gene and its function in a mouse, a fruit fly, a zebrafish, or even a bacterium when they already know the gene is present in humans?”

It can be difficult to study some issues in humans. Not only are the generation times very long, but some investigations would be unethical to do in humans. For example, it would never be appropriate to induce mutations intentionally in humans because the potential consequences of this manipulation are unknown.

The organisms that students considered in the Pax6 example are very common model species, but many others exist. Which species a scientist uses for a model depends on the question being investigated. For example, scientists’ extensive knowledge of the fruit fly’s genetic makeup and its relatively short generation time are advantages. For some questions, a scientist would want to study a mammalian system, making the mouse a good choice.

This discussion does not need to be long, but use it as an opportunity to help students understand why medical researchers rely on model organisms to learn about human health issues.

9.

Tell students that understanding evolution helps us explain why model species are useful for understanding human disease. Introduce students to evolutionary trees by explaining that these diagrams show relationships among species. Hand out one copy of Master 1.5, An Evolutionary Tree, to each student. Ask the students to use the evolutionary tree to answer the questions on the handout.

Again, you may need to reassure students that these questions are not part of a test. Instead, the questions are designed so that they can express their initial ideas about how to interpret an evolutionary tree diagram. They will revisit their answers to these questions in Steps 11 and 12. Give students about five minutes to complete this task.

This is a chance to look for prominent misconceptions that students may have (which will inform your teaching for Step 11; see Section 4.0 in the Information chapter). Do not go over the correct answers with students at this point in the learning cycle; the correct answers are included with Step 12. The information below points out some common responses, problems, and misconceptions that students may have when interpreting trees:

  1. What part of the evolutionary tree diagram represents the common ancestor of humans, mice, and zebrafish (but not fruit flies)? Why did you identify this part of the diagram?

    Students’ responses to this will indicate their current understandings of how a common ancestor is represented on an evolutionary tree. Students may not, at this point, understand that the line leading up to the splitting point labeled “2” represents a lineage that is the common ancestor of humans, mice, and zebrafish.
  2. Does the evolutionary tree suggest that the mouse is more closely related to the zebrafish or the fruit fly?

    The common incorrect response is that the mouse is more closely related to the fruit fly than to the zebrafish because the two organisms are next to each other on the evolutionary tree. This does not take into account that the mouse and the zebrafish share a more recent common ancestor than do the mouse and the fruit fly. This response may indicate that students do not understand what the splitting points signify or how time is represented on the evolutionary tree.
  3. Is the fruit fly the ancestor of all the other species on the evolutionary tree? Explain your answer.

    No. Many students assume that the species on the left part of the diagram or the species that branched off from the other species in the most distant past is the ancestor of the other species on the tree. Again, this may indicate a lack of understanding of the splitting points or of the representation of time on the evolutionary tree.
  4. What does the vertical line beneath Point 1 represent?

    The vertical line beneath Point 1 represents the root of this evolutionary tree. This line is the lineage that leads to the most recent common ancestor of the four species shown on the tree.

10.

Project Master 1.5. This evolutionary tree shows the relationships among the four species in the Pax6 case. Go over the following information with students. Ask them to record notes on their handouts.

The information in bold below is essential for students to understand. The information in parentheses is additional information for you to use if questions arise.

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Content Standard C: The millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors.


The evolutionary tree in Master 1.5 shows the relationships among four different species.

  • The vertical lines represent separate lineages. A lineage is the line of species and populations for a group of organisms over time. The group may contain a single species, a single population, or a group of species. (A common misconception is that the vertical branches, or lineages, on the tree indicate that no change occurred over time (that is, that the lines are simply connections between one ancestor species and the present-day species). In actuality, change occurs throughout time. There were certainly other organisms that went extinct on the lineage between the common ancestor and the zebrafish, for example.)
  • The vertical line below Point 1 represents the root of the tree, or the lineage leading to the common ancestor shown on this tree.
  • The splitting points show where one lineage splits into two new lineages. The organism at the splitting point is the common ancestor for the two new lineages.
    • Point 1 is the most recent common ancestor of all the organisms on the tree.
    • Point 2 is the most recent common ancestor of zebrafish, humans, and mice. (This ancestor can be described as “fishlike,” but it was not the same as today’s fish.)
    • Point 3 is the most recent common ancestor of humans and mice.
  • Time is represented on the vertical axis of the diagram. The present time is at the top. More-distant historical times are toward the bottom.
  • To answer the question, Who is more closely related to whom? compare the most recent point where the different organisms share a common ancestor. An organism is more closely related to the organisms with which it shares more recent common ancestors. If students understand this, they should understand that the sequence of organisms from left to right on the diagram is not informative. It may help to describe the evolutionary tree as a mobile that can be picked up by the root and twirled without changing any of the relationships among the species.

11.

Ask students to work with a partner to use the information they just learned to revise their answers to the questions on Master 1.5. Ask students to use a different-colored pen or pencil to make their revisions. Encourage students to simply put a single line through any text they wish to delete instead of erasing or scribbling it out.

Individual students should describe why they answered the questions as they did and should change their answers based on what they learned in Step 10. Using a different color for revisions makes it easier for students to see when they changed their minds. This reinforces the importance of students monitoring their own understandings.

12.

Conduct a class discussion to review students’ revised answers to the questions on Master 1.5. Ask students to explain their answers and compare them with their initial ideas.

Answer key for questions on Master 1.5, An Evolutionary Tree

  1. What part of the evolutionary tree diagram represents the common ancestor of humans, mice, and zebrafish (but not fruit flies)? Why did you identify this part of the diagram?

    The common ancestor for humans, mice, and zebrafish is at Point 2 on the evolutionary tree. Point 3 represents the common ancestor for mice and humans, and Point 1 is the common ancestor for all four species.
  2. Does the evolutionary tree suggest that the mouse is more closely related to the zebrafish or the fruit fly?

    The mouse is more closely related to the zebrafish than the fruit fly. To answer this question, students need to find the most recent common ancestor of the mouse and the zebrafish and then of the mouse and the fruit fly. Once they identify the most recent common ancestor, they can mark the time frame for the existence of that common ancestor on the timeline. In this case, the most recent common ancestor for the mouse and the zebrafish occurred more recently than did the most recent common ancestor for the mouse and the fruit fly.
  3. Is the fruit fly the ancestor of all the other species on the evolutionary tree? Explain your answer.

    No. The fruit fly is a modern, living species. Fruit flies share a common ancestor with all the other species on the diagram.
  4. What does the vertical line beneath Point 1 represent?

    The vertical line beneath Point 1 represents the root of this evolutionary tree. It is the lineage that represents the common ancestor of the four species shown on the tree.

13.

Conclude the activity by asking students to consider the following questions:

  • Do you think that the common ancestor of fruit flies, zebrafish, mice, and humans had a gene similar to Pax6? Explain your answer.
    If students understand that the Pax6 gene is very similar in these four species and that the four species all share a common ancestor, then students should respond that Pax6 was present in the common ancestor. Students should cite the highly similar amino acid sequences for the Pax6 protein in the four species and the evolutionary tree as evidence.
  • How does shared ancestry explain why scientists can use model organisms to learn about human health?
    Common (shared) ancestry allows scientists to study genes in model organisms that are similar to those in humans and make inferences to how the gene and its products function in humans. This allows scientists to conduct studies that may not be feasible in humans. If a gene had a different function in a different species or if there were a large number of significant changes in the gene in a given species, then that species might not be a good candidate as a model organism for that particular trait. Pax6 is a very strong example of the conservation of both gene sequence and gene function; not only is the sequence highly conserved in a wide variety of species, but it has a similar function in all of these species. Therefore, learning how it functions in a model organism (descended from the same common ancestor as humans) can provide valuable information for scientists about how it functions in humans.

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Before discussing the answers to the questions in Step 13, you may want to ask students to write their answers first. You can then collect their responses to assess their understandings of common ancestry and its relationship to the value of model organisms. Students will revise their answers to these questions later in the supplement.

 

Lesson 1 Organizer

Activity 1: Outbreak!
Estimated time: 30 minutes

Page and Step

Read to the class the news story about an outbreak of methicillin-resistant Staphylococcus aureus (MRSA). Page 54
Step 1
Ask one or two students who have had experiences with MRSA to share them with the class. Page 54
Step 2
Give each student 1 copy of Master 1.1.
  • Instruct them to read and answer the questions on the master.
  • Conduct a brief discussion about students’ answers.
Steps 3–5
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Activity 2: Models and Medicine
Estimated time: 70 minutes

Page and Step

Ask students to describe what they learned about mutations and disease from the MRSA case. Ask how mutations in people might affect health. Page 57
Step 1
Explain that students will learn more about how evolution informs medical research.
  • Project Master 1.2.
  • Have a volunteer read the information aloud.
  • Allow time for students to discuss the questions on the handout with a partner before holding a class discussion.
Page 58
Step 2
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Confirm for students that aniridia is a genetic disease. Explain that the Pax6 gene is responsible for aniridia and it is involved in eye development. Page 58
Step 3
Explain that students will analyze data about aniridia.
  • Give each student 1 copy of Master 1.3. Explain that students will work in groups of three to four to analyze data and respond to questions.
  • When students reach Part 2 of the master, project a color version of Master 1.4.
Page 59
Step 4–5
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  • Hold a class discussion about students’ answers to questions on Master 1.3.
Page 59
Step 6
Ask, “Do the Pax6 experiments support the idea that medical scientists can learn about gene function in humans by studying different organisms?” Briefly discuss this as a class. Page 61
Step 7
Inform students that species commonly used in scientific research are called model species or model organisms.
  • Ask, “Why would scientists want to study a gene and its function in a mouse, a fruit fly, or even a bacterium when they already know the gene is present in humans?”
Page 61
Step 8
Introduce students to evolutionary trees by explaining that these diagrams show relationships among species.
  • Give each student 1 copy of Master 1.5.
  • Allow time for students to answer the questions. Watch for difficulties and misconceptions but do not provide the correct answers.
Page 62
Step 9
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Project Master 1.5. Explain that the evolutionary tree shows the relationships among four different species:
  • The vertical lines represent separate lineages.
  • The vertical line below Point 1 represents the root of the tree, or the lineage leading to the common ancestor.
  • The splitting points show where one lineage splits into two new lineages. The organism at the splitting point is the common ancestor for the two new lineages.
    • Point 1 is the most recent common ancestor of all the organisms on the tree.
    • Point 2 is the most recent common ancestor of zebrafish, humans, and mice.
    • Point 3 is the most recent common ancestor of humans and mice.
  • Time is represented on the vertical axis, with present time at the top and more-distant historical times toward the bottom.
  • To answer this question, Who is more closely related to whom? compare the most recent point where the different organisms share a common ancestor. An organism is more closely related to the organisms with which it shares more recent common ancestors.
Page 63
Step 10
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Ask students to revisit the questions on Master 1.5 with a partner. Instruct them to revise their answers using a different-colored pen or pencil and not to erase their earlier answers. Page 64
Step 11
As a class, discuss the revised answers to the questions on Master 1.5. Page 64
Step 12
Conclude the activity by asking,
  • “Do you think that the common ancestor of fruit flies, zebrafish, mice, and humans had a gene similar to Pax6? Explain your answer.”
  • “How does shared ancestry explain why scientists can use model organisms to learn about human health?”
Page 65
Step 13

Logo6.eps = Involves copying a master.       = Involves making a transparency.

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