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Lesson 2


Investigating Lactose Intolerance and Evolution



Explore


At a Glance

Overview

Lesson 2 is an Explore activity that begins with a laboratory investigation of lactase. Students examine their results and the results from a larger sample of individuals across Africa, Europe, the Middle East, and Asia, and then make initial explanations of the patterns they see for lactase persistence. Students investigate the genetic basis for lactase persistence and learn that different mutations are responsible for lactase persistence in different populations and that each mutation arose independently. Students then use data to analyze two alternative hypotheses based on natural selection that attempt to explain the global patterns in lactase persistence.


Major Concepts

Objectives

After completing this lesson, students will begin to understand

Teacher Background

Consult the following sections in Information about Evolution and Medicine:
1.0 Fundamentals of Evolution and Medicine
3.0 Specific Applications of Evolution in Medicine
4.0 Students’ Prior Conceptions about Evolution here
5.0 Featured Examples of Evolution and Medicine


In Advance

Web-Based Activities
Activity Web Component?
1 Yes
Photocopies, Transparencies, Equipment, and Materials
Photocopies and Transparencies
For Classes Using Web-Based Activity:
1 copy each of Masters 2.1, 2.3, 2.6, and 2.10 for each student
1 transparency each of Masters 2.2, 2.4, 2.5, and 2.7
1 copy of Master 2.8, Data from Europe, Part A, for each student in half of the class
1 copy of Master 2.9, Data from Europe, Part B, for each student in the other half of the class

For Classes Using Print-Based Activity:
1 copy each of Masters 2.1, 2.6, 2.10, and 2.13 for each student
1 transparency each of Masters 2.2, 2.4, 2.5, 2.7, 2.11, 2.12, and 2.14
1 copy of Master 2.8, Data from Europe, Part A, for each student in half of the class
1 copy of Master 2.9, Data from Europe, Part B, for each student in the other half of the class
Equipment and Materials
1 lactase tablet
100 milliliters (mL) skim milk water
72 glucose test strips
2 250-mL beakers
2 test-tube racks
70 test tubes (18 × 150-millimeters, or large enough to hold 3 mL of fluid)
4 100-mL beakers
4 graduated cylinders (5- or 10-mL, or other means of measuring 2 mL)
Preparation

In the lab-based portion of the activity, students simulate the testing of samples from people around the world. Students determine whether the sample is from someone who is lactase persistent or not. Students receive a sample that either does or does not contain lactase. The frequency of lactase persistence matches the observed frequency of lactose tolerance in different parts of the world.

Decide how many samples you want your class to examine. The activity is designed to have each student test two samples. The following description is for the full set of 70 samples. We chose to use 70 samples because this allows students to see the results for 10 individuals from 7 different geographic regions. Students can then start to visualize the differences in lactase persistence in different geographic regions.

To limit the use of supplies, you may want to reduce the number of samples. For example, you may choose to simply assign one sample per student. Students will still be able to see the full set of results from other researchers, so limiting the number of samples examined should not cause any problems.

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Tip from the field test: Some field-test teachers used 25 or fewer samples, and the activity still worked well. At a minimum, you could perform a demonstration with four samples (two samples with lactase, two samples without) so students see the process.






To further reduce costs, you may cut the glucose test strips in half lengthwise. You may also use simpler materials than those included in the materials section. For example, you could replace the test tubes with small plastic cups.

Use the following steps to prepare for the lab:

Once students finish the investigation, have them return the test tubes to the proper rack. This will make it easier for you to get ready for the next class.

Make the necessary photocopies and transparencies.

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For classes using the Web version, verify that the computer lab is reserved for your classes or that the classroom computers are set up for the activities.

Refer to Using the Web Site for details about hardware and software requirements. Check that the Internet connection is working properly.

Set the computers to the opening screen for the activity. Log on to the “Student Activities” section of the Web site by entering the following URL:

http://science.education.nih.gov/supplements/evolution/student

Select “Lesson 2: Investigating Lactose Intolerance and Evolution.” This allows students to begin the activity directly.



Procedure

Activity 1: Investigating Lactose Intolerance and Evolution

Estimated time: 100 minutes

Note: This lesson is an Explore activity. We designed it to give students a common experience for building a full explanation for how natural selection affects human health and why understanding common ancestry is important for medicine. The lab experience at the beginning of the activity gives students a brief physical experience to better comprehend the role of lactase and to provide a link to other laboratory experiences they may have had with enzymes. This lab experience sets an important context for the lesson. If you find the costs and preparation prohibitive, you could skip this section and still achieve many of the student learning outcomes, but many field-test teachers commented that they felt the laboratory investigation was valuable. In the remainder of the activity, students practice important inquiry skills such as analyzing and comparing data and exploring alternative explanations. Students may still struggle with some of the basic aspects of natural selection. Research suggests that students need to practice using examples of natural selection in multiple contexts before they can generalize the concept. They will further build their understandings of evolution in later activities in the supplement.

1.

Begin the lesson by reading the following short scenario to students:

Even as a young child, Chang remembered not liking milk very much. At home in China, this did not cause a problem. Her parents did not keep milk around the house, and nobody in her family drank milk regularly. However, things changed when Chang came to the United States on a one-year high school exchange program.

At her new American school, she tried drinking milk at lunch, like many of the other students. This is when her problems started. Chang was miserable. She felt bloated and had painful cramps and diarrhea. She also seemed to be passing a lot of gas, which was embarrassing. Eventually, Chang went to the doctor. After a few tests, Chang learned that she was lactose intolerant. This means she wasn’t able to digest lactose, a sugar found in milk. The doctor explained that many people cannot digest lactose, especially people from certain geographic regions. Chang wondered why she was susceptible to this condition and why it was more common in certain parts of the world.

2.

Ask the class if they know anyone who is lactose intolerant. Then ask students for their ideas about the cause of lactose intolerance. Explain that the goal of this activity is to understand the questions that Chang asked: “Why are humans susceptible to lactose intolerance?” and “Why is it more common in certain parts of the world?”

Many students have heard of lactose intolerance. Encourage them to discuss their best ideas and not worry about whether their initial ideas are right or wrong at this stage in the activity. They will learn more about lactose intolerance.

3.

Explain that students will investigate patterns of lactose intolerance across the world. They will start by examining simulated samples from patients in different parts of the world. Give one copy of Master 2.1, Lactase Investigation, to each student. Ask students to read the introduction; then, answer any questions they have about it.

It is helpful for students to understand that “lactase persistent” means the same thing as “lactose tolerant” and “lactase nonpersistent” means the same as “lactose intolerant.” We use the terms “lactase persistence” and “lactase nonpersistence” in this activity because the scientific literature uses them.

Note: If students ask what is in the samples they are testing, explain that the samples simulate small intestine fluid. Lactase is secreted by cells lining the small intestine, and intestinal biopsies are required to test directly for enzyme function. However, direct testing for lactase activity from intestinal samples is impractical for large-scale screening, so indirect lactose tolerance tests are typically used. A common, fairly reliable method involves testing the amount of hydrogen in the breath of individuals after they have consumed lactose. When adults who are lactase nonpersistent consume lactose, bacteria in the colon digest the lactose and produce hydrogen gas, which can appear in the breath.

4.

Write on the board the summary equation for the breakdown of lactose. Ask students how monitoring glucose provides evidence of lactase activity. Remind them that the goal is to determine whether each patient sample shows evidence for lactase persistence.

The summary equation for the breakdown of lactose follows:

Lactose does not split into glucose and galactose at an appreciable rate without an enzyme. So the accumulation of glucose, monitored using glucose test strips, is evidence that the enzyme is working. Understanding the equation for the breakdown of lactose is important because students will measure glucose levels in the samples.

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Content Standard C: Most cell functions involve chemical reactions. Food molecules taken into cells react to provide the chemical constituents needed to synthesize other molecules. Both breakdown and synthesis are made possible by a large set of protein catalysts, called enzymes. The breakdown of some of the food molecules enables the cell to store energy in specific chemicals that are used to carry out the many functions of the cell.

5.

Hand out the labeled sample test tubes to students. Ask them to record the sample numbers on the table on Master 2.1. Then project Master 2.2, Lactase Results from Other Researchers, but hide the “lactase persistence (Yes/No)” column. Ask students to record the region and country of the samples.

6.

Perform a short demonstration to establish the amount of glucose present in the milk and to help the students understand the procedure. Pour approximately 200 mL of skim milk into a beaker. Transfer 2 mL of milk into a test tube, and then add 1 mL of water. Demonstrate to students how to use the glucose test strips to measure the amount of glucose in the diluted milk in the test tube. From this demonstration, students should record on Master 2.1 the color of the glucose test strip.

Examine your glucose test strips to determine how long you need to wait to record the color of the strip. Some strips allow you to deduce the concentration of glucose in the milk, which you can do if students are interested.

7.

Pour 50 mL of milk into four separate 100-mL beakers, and create four workstations at which students can access materials. Each workstation should have one beaker of milk, one 5- or 10-mL graduated cylinder (or other means for measuring 2 mL), and glucose test strips. Students will record whether or not the amount of glucose increased in the samples by using the procedures on Master 2.1. Students should record the results on the handout. Ask them to make a claim about whether the person who provided the sample was lactase persistent.

Explain to students that they will learn later if their claims about their samples were confirmed by other researchers.

8.

After students complete the lab investigation, ask them to clean up according to your directions.

Have students return the test tubes to either the test-tube rack for the samples that have lactase or the rack for samples that have water. This will make it easier for you to get ready for the next class.

9.

Explain to students that in this simulation, other researchers also analyzed the samples they investigated. Project Master 2.2. Previously, you hid the “lactase persistence (Yes/No)” column from students. Now reveal it. Hold a brief class discussion about any discrepancies in the results.

Emphasize that many kinds of errors, including human error, can cause problems with lab results. Therefore, important medical tests are often repeated by separate researchers.

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(For print version, skip to Step 10-p.)

In classrooms using the Web version of this activity:

10-w.

Instruct students to proceed to their computers and click on “Lesson 2: Investigating Lactose Intolerance and Evolution.” Students should then click on “Fill in samples from other researchers.” Ask, “Does the percentage of people who are lactase persistent vary in different parts of the world?”

Computers should be at http://science.education.nih.gov/supplements/evolution/student

Students will see differences in lactase persistence depending on geography. Map 1 plots the results that the other researchers obtained for the simulated samples students studied. The samples are organized into seven geographic regions. The frequencies of lactase persistence in the regions are based on frequencies found in the scientific literature (Enattah et al., 2007; Enattah et al., 2008; Gerbault et al., 2009; Tishkoff et al., 2007).

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Tip from the field test: Students are generally very interested in the patterns shown on the map. However, they will explore these patterns in more detail in future steps. Keep the discussion on Step 10 very brief.

11-w.

Explain that students will access a second map that has results from 300 individuals in the seven geographic regions found in Map 1. Then break the class into groups of four. Hand out 1 copy of Master 2.3, Investigating Patterns in Lactase Persistence, to each student. Ask students to work as a group to accomplish all the tasks on the handout, using Map 2 as a reference. Students access Map 2 by clicking on “View Map 2.”

The tasks described on the handout vary in difficulty. You may wish to have more-advanced students complete the tasks related to exploring geographic patterns of lactase persistence.

Students may wonder why the columns in the data table for infants (Table 3) are labeled “lactase activity” and “no lactase activity.” The answer is that the terms “lactase persistent” and “lactase nonpersistent” apply only to adults.

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Tip from the field test: If you have a limited number of computers available for students, assign one computer per group and ask the students to complete all three parts of the activity together. Groups should switch the person “driving” the computer between tasks to allow wider participation.

Continue with Step 12.


In classrooms using the print version of this activity: Logo5.eps

10-p.

Project Master 2.11, Map of Lactase Test Results. Ask, “Does the percentage of people who are lactase persistent vary in different parts of the world?”

Students will see differences in lactase persistence depending on geography. The map plots the simulated results that other researchers obtained for the simulated samples students studied. The samples are organized into seven different geographic regions. The frequencies of lactase persistence in the regions are based on frequencies found in the scientific literature (Enattah et al., 2007; Enattah et al., 2008; Gerbault et al., 2009; Tishkoff et al., 2007).

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Tip from the field test: Students are generally very interested in the patterns shown on the map. However, they will explore these patterns in more detail in future steps. Keep the discussion on Step 10 very brief.


11-p.

Project Master 2.12, Expanded Map of Lactase Test Results. This second map shows results from 300 individuals in the seven geographic regions found in the previous map. Break the class into groups of four. Hand out one copy of Master 2.13, Exploring Patterns of Lactase Persistence, to each student. Ask students to work as a group to accomplish all the tasks on the handout.

The tasks described on the handout vary in difficulty. You may wish to have more-advanced students complete the tasks related to exploring geographic patterns of lactase persistence.

Students may wonder why the columns in the data table for infants (Table 3) are labeled “lactase activity” or “no lactase activity.” The answer is that the terms “lactase persistent” and “lactase nonpersistent” apply only to adults.

12.

Hold a class discussion in which you summarize what the students learned from exploring patterns of lactase persistence.

Students should recognize that there are dramatic differences in the frequency of lactase persistence in different geographic regions, even in areas that are relatively close geographically. East Asia, parts of the Middle East, West Africa, and Southern Europe have relatively few people who are lactase persistent. East Africa, Saudi Arabia (Middle East 2), and Northern Europe have relatively high proportions of people who are lactase persistent.

Lactase persistence is not more common in one gender than the other. Some students may focus on the fact that the numbers of males and females who show lactase persistence are not identical. This is a good opportunity to discuss reasonable expectations for the numbers in each category.

Students will recognize that the vast majority of infants have functional lactase enzymes. Infants of all mammals use lactase to digest milk. Help students understand that this means that something happens during development to “turn off” the production of lactase in many adults.

13.

Lead a short class discussion about the three main causes of health conditions: infectious, genetic, and environmental. Ask students to use what they know and what they learned from the investigation to rule out possible causes for lactose intolerance.

The fact that lactase nonpersistence (lactose intolerance) occurs in clusters could support the hypothesis that the cause is any of the three main causes of health conditions. If students do not mention it, ask them what would happen when people move from one region of the world to another if lactase nonpersistence were caused by environmental or infectious agents. This would be an ideal time to bring up the fictional student Chang from the scenario in Step 1. If lactase nonpersistence had an environmental or infectious cause, we would not necessarily expect Chang to show the symptoms she did when she came to the United States. Help students understand that the evidence and their prior experiences and knowledge point to a genetic cause for lactase nonpersistence.

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Tip from the field test: Many students quickly eliminated genetics as a factor in lactase persistence because nearly every infant showed evidence of lactase activity. This indicates that students do not recognize that some mutations cause changes in when and where other proteins are made. The next step helps make this point for students.


14.

Explain to students that scientists have identified the gene (DNA) that codes for the lactase enzyme (which is a protein). Project Master 2.4, Questions about the Genetic Basis for Lactase Persistence. Go over the answers in a brief discussion.

Answer key for questions on Master 2.4, Questions about the Genetic Basis for Lactase Persistence

  1. Do you think you would find the gene that codes for lactase in both people who are lactase persistent and people who are lactase nonpersistent? Explain your answer.

    Nearly all infants can digest lactose, meaning that they must make lactase at this point in life. Therefore, individuals who are lactase persistent and those who are nonpersistent must both have the gene that codes for lactase.
  2. Scientists recognize that two major types of changes in DNA sequences can affect the phenotype of organisms. One type changes the coding sequence of a gene. These changes can affect the amino acids that form the protein, which can affect the protein’s shape and function. The second type of change affects when a gene gets “turned on” or “turned off.” What type of change do you think causes the difference between lactase persistence and lactase nonpersistence?

    Infants use lactase to digest the lactose in breast milk or infant formula, so changes to the coding sequence for lactase would likely have a detrimental effect on an individual. Because lactase is turned on in infants but turned off in people who are lactase nonpersistent, the change is more likely to be in a region that regulates other genes. In fact, scientists have identified a growing number of different mutations to a regulatory region for the lactase gene that cause lactase persistence in people from different geographic regions. Students learn about these mutations in the next steps in the activity.

15.

Explain to students that in 2002, scientists discovered a mutation in a segment of DNA that controls the production of lactase. Individuals with one copy of the mutation are lactase persistent. Individuals with two copies of the typical form of DNA are lactase nonpersistent.

Students may recognize that this means that the allele formed from the mutation that causes lactase persistence is dominant over the other allele.

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Content Standard C: Cells store and use information to guide their functions. The genetic information stored in DNA is used to direct the synthesis of the thousands of proteins that each cell requires.


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Content Standard C: Changes in DNA (mutations) occur spontaneously at low rates. Some of these changes make no dif-ference to the organism, whereas others can change cells and organisms. Only changes in germ cells can create the variation that changes an organism’s offspring.


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(For print version, skip to Step 16-p.)

In classrooms using the Web version of the activity:

16-w.

Direct students to Map 2 on the Web site. Tell them to click on “Explore Lactase Persistence by Mutations,” then “Mutation 1” to see the percentage of this lactase persistence mutation in different human populations. Ask students to explain the pattern they observe.

Students will notice that mutation 1 is prevalent in Northern Europe but is missing in other areas that also have a high amount of lactase persistence such as East Africa and Saudi Arabia (Middle East 2).

Note: The table students see on the Web site summarizes the percentage of the DNA regions with a specific mutation in each geographic region. To help visualize this amount, the same percentage of individuals in each geographic region is highlighted.

17-w.

Ask, “What does it mean if all the people who are lactase persistent do not have mutation 1?”

Different mutations that cause lactase persistence must be present in other populations.

18-w.

Explain to students that scientists discovered additional mutations causing lactase persistence by studying different groups of people around the world. Ask students to use map 2 to explore the percentages of the other two mutations by clicking on “Mutation 2” and “Mutation 3.”

Scientists have found additional mutations related to lactase persistence, but only the three mutations that are found in relatively high frequencies in different geographic regions are included in this activity. In fact, Enattah et al. (2007) suggest that the mutation called “mutation 1” in this activity was independently introduced into humans more than once. Other researchers suggest that the data can be explained in other ways. The independent evolution of this mutation is a hypothesis that can be further explored with new data.

Continue with Step 19 below.




For classes using the print version of this activity: Logo5.eps

16-p.

Project Master 2.14, Lactase Persistence and Mutation, and show students the “Mutation 1 (%)” column in the table. Hide the other -two columns. Ask students to explain the pattern they observe.

Students will notice that mutation 1 is prevalent in Northern Europe but is missing in other areas that also have a high amount of lactase persistence such as East Africa and Saudi Arabia (Middle East 2).

17-p.

Ask, “What does it mean if all the people who are lactase persistent do not have mutation 1?”

Different mutations that cause lactase persistence must be present in other populations.

18-p.

Explain to students that scientists discovered additional mutations causing lactase persistence by studying different groups of people around the world. Show students the remaining columns in the table on Master 2.14.

Scientists have found additional mutations related to lactase persistence, but only the three mutations that are found in relatively high frequencies in different geographic regions are included in this activity. In fact, Enattah et al. (2007) suggest that the mutation called “mutation 1” in this activity was independently introduced into humans more than once. Other researchers suggest that the data can be explained in other ways. The independent evolution of this mutation is a hypothesis that can be further explored with new data.

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


19.

Project Master 2.5, Lactase and Human Evolution. Ask students to answer these questions in their notebooks. Hold a class discussion on the answers.

Answer key for questions on Master 2.5, Lactase and Human Evolution

  1. Nonhuman primates and all other mammals are lactase nonpersistent. Do you think the common ancestor of humans was lactase persistent or lactase nonpersistent? Explain your answer.

    The common ancestor of all humans was very likely lactase nonpersistent.

Note: Studies of ancient DNA from Northern European skeletons 7,000–8,000 years old support the hypothesis that the mutation for lactase persistence that is common in Northern Europe today was absent or at a low frequency in the early Neolithic period (Burger et al., 2007).

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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.


  1. Genetic studies show that different mutations cause lactase persistence in humans from different geographic regions. Does this evidence suggest that lactase persistence evolved once or more than once in humans? Explain your answer.

    Evidence supports the conclusion that lactase persistence evolved more than once in humans.
  2. Did the mutation that causes lactase persistence first come about because people needed the mutation? Explain your answer.

    Mutations are random with respect to the needs of the organism. The mutations that cause lactase persistence came about in individuals through random processes. However, a very common misconception is that mutations come about because individuals want or need them. We designed this question to help students address this misconception directly. It is important to emphasize that although mutation is random, the remaining aspects of natural selection are not random. Individuals that possess genetic variants that help them survive and reproduce in a particular environment leave more offspring. This portion of the process is not random.

20.

Hand out one copy of Master 2.6, Explaining the Evolution of Lactase, to each student. After students complete the reading, project Master 2.7, Data from Africa. Lead a class discussion about their conclusions after they finish analyzing the data.

The data in this example come from Gerbault et al. (2009) and show a pattern that is consistent with the culture-historical hypothesis in Africa. However, these data merely show a correlation. You should emphasize that correlations do not establish cause. Use this opportunity to emphasize that identifying the factors that affected populations in the past is important. In the case of the evolution of antibiotic resistance in MRSA, the factor in the environment that affects the survival and reproduction of individuals in the population is clear. However, other cases, such as the evolution of lactase persistence, are not as simple.

Scientists collect a broad range of different types of data to test hypotheses about natural selection in the past. You may wish to ask students what types of data they would like to have to further test the culture-historical hypothesis in Africa. Students may mention that it would be important to know if lactase-persistent individuals in pastoralist populations really did survive and reproduce at higher rates. They may also want to know if the increase in frequency of the lactase-persistence alleles coincided in time with dairying. In fact, estimates of the age of specific mutations in Africa are also consistent with the culture-historical hypothesis.

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Master 2.10 can help you assess how well students understand the process of natural selection and can apply it to link evidence with explanations. As the class discusses the responses to each question, ask whether everyone agrees or anyone has anything else to add. Use student responses to inform your teaching of Lesson 3.


21.

Tell students that they will now consider data from Europe. Ask students to get back into their groups of four. Hand out

  • four copies per group of Master 2.8, Data from Europe, Part A, to half the groups;
  • four copies per group of Master 2.9, Data from Europe, Part B, to the other half the groups; and
  • one copy of Master 2.10, Summing Up Lactase Persistence and Nonpersistence, to each student.

Ask students to follow the directions on the handouts. Again, after students analyze the data, lead a class discussion about their conclusions.

The data in Figure 2 on Master 2.8 come from Gerbault et al. (2009). Ask students who received these data to summarize their conclusions. Many students who examine these data will claim that the data support the calcium-absorption hypothesis. Some students may recognize that these data are also correlations and that correlations do not necessarily mean causation. Students who analyze the data in Master 2.9 should offer counterarguments to students with data from Master 2.8.

This exercise is important because it helps reinforce the search for alternative explanations, a critical aspect of science. The frequency of rickets from the data in Table 1 on Master 2.9 is lower than expected if rickets were responsible for driving the evolution of the lactase alleles. Therefore, these data do not support the calcium-absorption hypothesis. The data in Master 2.9 come from Simoons (2001) and Itan et al. (2009). If your students are particularly advanced, you may wish to explain that both hypotheses may, in fact, be partially correct because they do not logically exclude each other. You may also want to ask interested students to investigate other hypotheses for the evolution of lactase persistence.

22.

Conclude the lesson by having a class discussion about students’ summary explanations for the evolution of lactase in human populations.

Students should begin to recognize that there are common elements to every case of evolution by natural selection, as mentioned in Lesson 1.

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Content Standard C:

  • Species evolve over time. Evolution is the consequence of (1) the potential for a species to increase its numbers, (2) the genetic variability of the offspring due to mutation and recombination of genes, (3) a finite supply of the resources required for life, and (4) the ensuing selection by the environment of those offspring better able to survive and leave offspring.
  • Recognize and analyze alternative explanations and models.
  • Understandings about scientific inquiry: Scientific explanations must adhere to criteria such as: a proposed explanation must be logically consistent; it must abide by the rules of evidence; it must be open to questions and possible modification; and it must be based on historical and current scientific knowledge.

Answer key for questions on Master 2.10, Summing Up Lactase Persistence and Nonpersistence

  1. Do humans vary in their ability to digest lactose? What is the evidence for your answer?

    Yes; the maps and tables of data presented to students all demonstrate that there is variation among humans for the ability to digest lactose.
  2. Can the ability to digest lactose as an adult be passed from parents to offspring? What is the evidence for your answer?

    Yes, the ability to digest lactose as an adult can be passed from parent to offspring. Researchers have identified mutations to DNA that keep the lactase gene turned on in some adults.
  3. Describe how mutations to DNA are important in lactase persistence and nonpersistence.

    Students should recognize that mutations to DNA cause the variability among different people for lactase persistence or nonpersistence. This is a good time to reemphasize that not all mutations are “bad” and that mutations provide the variation on which natural selection acts.
  4. In certain environments, did digesting lactose seem to affect an individual’s ability to survive and reproduce? Explain.

    Yes, it is very likely that lactose digestion affected fitness. Students should mention that scientists have different hypotheses to explain specifically how digesting lactose affected an individual’s ability to survive or reproduce in certain environments and cultural contexts.
  5. What is the evidence that the frequency of the mutation that causes lactase persistence changed in certain groups of people over time?

    The common ancestor of all humans was lactase nonpersistent. The fact that some groups of humans have high frequencies of the mutation that causes lactase persistence is evidence that the frequency of this mutation has changed over time.

Lesson 2 Organizer: Web Version

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Activity 1: Investigating Lactose Intolerance and Evolution
Estimated time: 100 minutes
Page and Step
Read to the class the scenario about a student who is lactose intolerant. Page 72
Step 1
Ask students if they know anyone who is lactose intolerant. Explain that the activity’s goal is to understand why humans are susceptible to lactose intolerance and why it is more common in certain parts of the world. Page 73
Step 2
Explain to students that they will investigate patterns of lactose intolerance across the world.
  • Give each student one copy of Master 2.1 and answer any questions they have about the introduction.
  • Describe the summary equation for the breakdown of lactose:
Pages 73 and 74
Steps 3 and 4
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Hand out the labeled sample test tubes to students. Project Master 2.2 but hide the “lactase persistence (Yes/No)” column. Page 74
Step 5
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Demonstrate the procedure for measuring glucose:
  • Pour about 200 milliliters (mL) of skim milk into a beaker.
  • Transfer 2 mL of milk into a test tube, and then add 1 mL of water.
  • Demonstrate to students how to use the glucose test strips to measure the amount of glucose in the diluted milk in the test tube.
Students should record on Master 2.1 the color of the glucose test strip.
Page 74
Step 6
Create four workstations. Each should have
  • one beaker with 50 mL of milk,
  • one 5- or 10-mL graduated cylinder (or other means for measuring 2 mL), and
  • glucose test strips.
After the investigation, students should make a claim about whether the person who provided the sample was lactase persistent. Then direct them to clean up.
Pages 74–75
Steps 7 and 8
Project all of Master 2.2. Hold a brief class discussion about any discrepancies in the results. Page 75
Step 9
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Have students log on to the Web site and click on “Lesson 2: Investigating Lactose Intolerance and Evolution.” They should then click on the “Fill in samples from other researchers” button.
  • Ask, “Does the percentage of people who are lactase persistent vary in different parts of the world?”
Page 75
Step 10-w
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Explain to students that they will access Map 2, which has additional samples, by clicking on “View Map 2.”
  • Divide the class into groups of four.
  • Hand out one copy of Master 2.3 to each student.
After groups accomplish the tasks on the handout, hold a class discussion to summarize what they learned.
Pages 76 and 77
Steps 11-w and 12
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Ask students if they can use evidence to help decide if lactase persistence has an infectious, genetic, or environmental` cause. Page 77
Step 13
Explain that scientists identified the gene that codes for lactase.
  • Project Master 2.4 and have students answer the questions on their own.
  • Go over the answers in a brief discussion.
Page 78
Step 14
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Describe the discovery of a mutation that leads to lactase persistence.
  • Have students revisit the Web site and click on “Explore Lactase Persistence by Mutation,” then “Mutation 1.”
  • Discuss the patterns that students observe.
  • Ask, “What does it mean if all the people who are lactase persistent do not have mutation 1?”
Page 79
Steps 15, 16-w, and 17-w
Direct students to the Web site. Explain that they will explore the percentages of the other two mutations that result in lactase persistence by clicking on “Mutation 2” and “Mutation 3.” Page 79
Step 18-w
Project Master 2.5. Ask students to record their answers in their notebooks. Then lead a class discussion about the questions. Page 80
Step 19
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Hand out one copy of Master 2.6 to each student and ask students to read it. Project Master 2.7. Discuss the data as a class. Page 81
Step 20
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Hand out
  • four copies per group of Master 2.8 to half of the groups;
  • four copies per group of Master 2.9 to the other half of the groups; and
  • one copy of Master 2.10 to each student.
Discuss the data from Europe as a class.
Page 82
Step 21
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Lead a class discussion about students’ summary explanations for the evolution of lactase in human populations. Page 82
Step 22

Logo6.eps = Involves copying a master.       Logo7.eps = Involves making a transparency.        Logo3.eps = Involves using the Internet.  


Lesson 2 Organizer: Print Version

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Activity 1: Investigating Lactose Intolerance and Evolution
Estimated time: 100 minutes
Page and Step
Read to the class the scenario about a student who is lactose intolerant. Page 72
Step 1
Ask students if they know anyone who is lactose intolerant. Explain that the activity’s goal is to understand why humans are susceptible to lactose intolerance and why it is more common in certain parts of the world. Page 73
Step 2
Explain to students that they will investigate patterns of lactose intolerance across the world.
  • Give each student one copy of Master 2.1 and answer any questions they have about the introduction.
  • Describe the summary equation for the breakdown of lactose:
Pages 73 and 74
Steps 3 and 4
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Hand out the labeled sample test tubes to students. Project Master 2.2 but hide the “lactase persistence (Yes/No)” column. Page 74
Step 5
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Demonstrate the procedure for measuring glucose:
  • Pour about 200 milliliters of skim milk into a beaker.
  • Transfer 2 milliliters of milk into a test tube, and then add 1 milliliter of water.
  • Demonstrate to students how to use the glucose test strips to measure the amount of glucose in the diluted milk in the test tube.
Students should record on Master 2.1 the color of the glucose test strip.
Page 74
Step 6
Create four workstations. Each should have
  • one beaker with 50 milliliters of milk,
  • one 5- or 10-milliliter graduated cylinder (or other means for measuring 2 milliliters), and
  • glucose test strips.
After the investigation, students should make a claim about whether the person who provided the sample was lactase persistent. Then direct them to clean up.
Pages 74–75
Steps 7 and 8
Project all of Master 2.2. Hold a brief class discussion about any discrepancies in the results. Page 75
Step 9
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Project Master 2.11. Ask, “Does the percentage of people who are lactase persistent vary?” Page 76
Step 10-p
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Project Master 2.12, which shows a second map with additional samples. Divide the class into groups of 4. Pass out 1 copy of Master 2.13 to each student. After groups accomplish the tasks on the handout, hold a class discussion to summarize what they learned. Page 77
Steps 11-p and 12
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Ask students if they can use evidence to help them decide if lactase persistence has an infectious, genetic, or environmental cause. Page 77
Step 13
Explain that scientists identified the gene that codes for lactase.
  • Project Master 2.4 and have students answer the questions on their own.
  • Go over the answers in a brief discussion.
Page 78
Step 14
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Describe the discovery of a mutation that leads to lactase persistence.
  • Project Master 2.14 and show only the “Mutation 1 (%)” column in the table.
  • Discuss the patterns that students observe.
  • Ask, “What does it mean if all the people who are lactase persistent do not have mutation 1?”
Pages 79 and 80
Steps 15, 16-p, and 17-p
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Show students the percentages of two additional mutations for lactase by revealing the last two columns on Master 2.14. Page 80
Step 18-p
Project Master 2.5. Students should record their answers in their notebooks. Next, lead a class discussion about the questions. Page 80
Step 19
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Hand out one copy of Master 2.6 to each student. Then project Master 2.7. Discuss the data as a class. Page 81
Step 20
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Hand out
  • four copies per group of iMaster 2.8 to half of the groups,
  • four copies per group of Master 2.9 to the other half of the groups, and
  • one copy of Master 2.10 to each student.
Discuss the data from Europe as a class.
Page 82
Step 21
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Lead a class discussion about students’ summary explanations for the evolution of lactase in human populations. Page 82
Step 22

 

Logo6.eps = Involves copying a master.        Logo7.eps = Involves making a transparency.

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