Fancy Fish: Gene Frequency Grade Ten

Transcription

1 Ohio Standards Connection: Life Sciences Benchmark H Describe a foundation of biological evolution as the change in gene frequency of a population over time. Explain the historical and current scientific developments, mechanisms and processes of biological evolution. Indicator 20 Recognize that a change in gene frequency (genetic composition) in a population over time is the foundation of biological evolution. Lesson Summary: The purpose of this lesson is to help students recognize that a change in gene frequency (genetic composition) in a population over time is biological evolution. Students will participate in two activities that model evolutionary change via the processes of gene flow and natural selection. Students will work in pairs to report on the activity, describing their data and explaining what they learned about genetic change. Students will explain their results referring to specific data and will turn in their data with their reports. Estimated Duration: Two hours and 15 minutes Commentary: This lesson helps students understand the effects of natural selection and gene flow on the genetic diversity of diploid populations. Students will model gene flow by holding colored fish cards and migrating around the room, in and around barriers to disperse. Students then model allele frequency change via natural selection using colored candies as alleles. As a reflection exercise, students then consider how natural selection and gene flow together affect the genetic diversity of populations. This last step provides a window into the real complexity of naturally-occurring populations and helps clarify how multiple processes affect genetic diversity at the same time. Pre-Assessment and Post-Assessment: Have students respond to the following questions: 1. Define allele. Give an example of a gene with two alleles. 2. Describe dominant and recessive alleles. 3. Define gene flow. Give a specific example of gene flow and describe how it causes changes in allele frequencies over time. 4. List and briefly describe examples of two real barriers to gene flow. 5. Describe how natural selection affects the allele frequencies of a population. 1

2 6. Give an example of a population of organisms that is affected by natural selection. Your example should include a description of the animal population, what trait is being selected, what alleles control the expression of that trait, and how the allele frequencies are affected by natural selection. 7. Imagine that individuals from another population migrate into the one that you described in the previous question. How would this affect the allele frequencies of your population? Scoring Guidelines: See Attachment A, Pre-and Post-Assessment Answers, for a key to assessing student work. Instructional Procedures: Engagement 1. Construct a set of cards, each with a drawing of a colored fish on it. 2. Pass one card out to each student. Have the class pretend to be a population of brightly colored fish. Instructional Tip: Initially, students near each other will have the same color card. Keep it simple, four colors is enough. 3. Divide the class into four sections. Have students determine the dominant color of fish in their section- just a quick count will do it! 4. Simulate gene flow by having students swim around the room for 10 seconds, repeat step above in new section. Do this two to three times until they understand that their mixing has homogenized the four populations. 5. Put up a pretend "dam" to separate off one section of the room; the other fish will continue to mix for a few more rounds. 6. Discuss the effects the swimming or wandering had on the characteristics of the group, and how the isolated group was affected. Help students see that the barrier prevented gene flow, and that the color frequency of the isolated population did not change. Exploration 7. Use licorice candies that are colored pink and white to simulate alleles for a gene in a fish population. The white candies simulate the normal dominant allele (A) and the pink candies represent a recessive allele (a) that is lethal. Instructional Tip: You can use a variety of objects for this exercise, game pieces, colored paper, or colored candies. Have contrasting colors so that one color will represent the recessive allele (a) and the other will represent the dominant allele (A). 8. Divide the class into two-person teams. 9. Distribute 80 white candies and 20 pink candies to each student team. Each team will begin the exercise with the same population. The candies represent alleles, so that each 2

3 beginning population has a gene pool of 100 alleles, with 80% dominant (white) alleles and 20% recessive (pink) alleles. 10. Tell students to mix the candies together well in a bowl and then spill the candies out on a table. Explain the allele frequencies that are present in each population, and have students record the number of each allele they have and the relative frequencies of each that are present. In the beginning, the numbers of each allele will be 80 and 20, and the relative frequencies will be 80% and 20%. 11. With their eyes closed, have each student choose two candies (two alleles) at random as one would find in a diploid individual. Then have them look at their choice. 12. Explain that the recessive (a) allele is lethal, so that if they picked two pink candies (aa genotype), then their individuals died. If they had a homozygous dominant (AA) or heterozygous (Aa) genotype, their individuals lived. 13. Tell students with "dead" alleles to discard them in the "graveyard" (a separate bowl at the front of the room, NOT returned to the gene pool bowl). Then have students figure out the surviving number of dominant and recessive alleles in each group. Have them record this number on their paper. 14. After this first round of selection, explain how to calculate the new relative frequencies of alleles in the population. This can be done by dividing the total number of remaining alleles (candies) into the number of each type of alleles (white, then pink candies). For example, if each student had picked two pink candies, then four recessive alleles (pink candies) would be removed in the first round, and the resulting gene pool would be 80 dominant (white candies) and 16 recessive (pink candies), for a total of 96 remaining alleles (candies). The percent of dominant alleles is then 80/96 = 83%, and the recessive alleles are 16/96 = 17%. Tell students to calculate the relative frequencies of alleles in the population. 15. To begin the next generation, the survivors must reproduce. We will assume that the reproductive capability of the candies keeps the population of alleles at 100 for each generation, and that the pink and white candies reproduce equally well. Using the percent values calculated, have students adjust the number of candies by adding white and pink candies (from a supply in the front of the room) to bring the total candies to 100 in the ratio calculated. In the example given, three white candies would be added to bring the total white candy count to 83, and one pink would be added to bring that count to 17. There are now 100 candies in the correct proportions to begin round two. 16. Have the students put the candies for the second generation in the bowl. Mix and spill them onto the table again. This is the second generation. Have students choose their diploid pairs, record allele numbers and relative frequencies. 17. Have students repeat the process until all pink candies are gone OR until they have sampled 10 generations. While repeating rounds of selection, have students record their results in a chart like the one in Attachment B, Data Sheet. Sample data are found in Attachment C, Data Sheet Example. Instructional Tip: It is highly unlikely that the recessive allele will be purged from the population, even though it is lethal. As long as the heterozygous carriers are not affected by the recessive allele, the 3

4 allele can "hide" in these individuals. 18. Circulate around the room while students complete the exercise. Students should see a trend in an increase in number of white and a decrease in number of pink alleles over time. 19. Have students write up the activity (students can work in pairs). Ask them What did you learn about the change in a population? Have students explain their results referring to specific data points. Have students turn in their data with their write-ups. Explanation: 20. Lead a student discussion about the results of the exercise. Help students understand that they modeled natural selection, with homozygous dominant and heterozygous individuals "selected" to survive and reproduce, and homozygous recessive individuals selected to die. 21. Challenge students to think about the effects of natural selection and gene flow occurring in a population at the same time. Explain that these two processes normally affect populations of organisms simultaneously. For example, if the population of (80 white to 20 pink) alleles experienced a constant emigration of individuals from a similar population, the frequency of recessive alleles would have remained higher, despite the die-off of homozygous recessive individuals. Differentiated Instruction: Instruction is differentiated according to learner needs, to help all learners either meet the intent of the specified indicator(s) or, if the indicator is already met, to advance beyond the specified indicator(s). Students are paired or grouped to complete the activities such as the gene frequency activity. Students are provided with opportunities to use a variety of skills to reinforce concepts such as organization, communication, observations, etc. Students are provided with access to a variety of resources both in and out of the classroom. Instructional procedures are delivered through multi-sensory modes and a variety of teaching styles. Extensions: Carry out the allele selection activity with gene flow (exchange of colored candy alleles) occurring among populations (two-student teams). Find news articles concerning man-made barriers to gene flow. Create visual representations of the data. Materials/Resources: The inclusion of a specific resource in any lesson formulated by the Ohio Department of Education should not be interpreted as an endorsement of that particular resource, or any of its contents, by the Ohio Department of Education. The Ohio Department of Education does 4

5 not endorse any particular resource. The Web addresses listed are for a given site s main page, therefore, it may be necessary to search within that site to find the specific information required for a given lesson. Please note that information published on the Internet changes over time, therefore the links provided may no longer contain the specific information related to a given lesson. Teachers are advised to preview all sites before using them with students. For the teacher: Copies of gene frequency datasheet, vocabulary list colored candies, colored cards, overhead to show sample data For the students: Copy of gene frequency datasheet, vocabulary list, calculators Key Vocabulary: allele barrier dominant emigration evolution gene flow gene frequency gene pool immigration population recessive species Technology Connections: Students can use computers to work on reports and data collection sheets. Students can use calculators to figure percentages of gene frequencies. Research Connections: Marzano, R. et al. Classroom Instruction that Works: Research-Based Strategies for Increasing Student Achievement. Alexandria: Association for Supervision and Curriculum Development, Cooperative learning grouping has a powerful effect on student learning. This type of grouping includes the following elements: Positive interdependence; Face-to-face promotive interaction; Individual and group accountability; Interpersonal and small group skills; Group processing. 5

6 Nonlinguistic representations or imagery mode help students think about and recall knowledge. This includes the following: Creating graphic representations (organizers); Making physical models; Generating mental pictures; Drawing pictures and pictographs; Engaging in kinesthetic activity. Attachments: Attachment A, Pre- and Post-assessment Answers Attachment B, Data Sheet Attachment C, Data Sheet Example 6

7 Attachment A Pre- and Post-Assessment Answers 1. Allele an alternative form of a gene (e.g., for plant height, T for tall, t for short). Each organism receives one from each parent. 2. Dominant the trait that is expressed when its allele is either homozygous dominant (TT) or heterozygous (Tt). If either parent passes on this allele, the trait will be expressed in the offspring. Recessive this allele must be received from BOTH parents in order to be expressed in the offspring, homozygous recessive (tt). 3. Gene flow the movement of alleles into or out of a population due to migration of individuals in and out of the population. If new individuals are introduced into a population, new alleles would be added. 4. The answer could include any of a wide variety of examples. As long as their examples reduce or impede gene flow, then they are acceptable. 5. Natural selection changes the frequency of alleles by allowing some organisms to survive and reproduce, while disallowing others. As a result, the allele frequency of a population may not remain the same. 6. The student answers may be creative and unusual. It is important that the student describes a population of organisms that has a trait controlled by at least two alleles, and that the alleles confer differential survival and reproduction to the organisms that possess them. 7. Students should state that gene flow affects the rate at which allele frequencies change in a population. Gene flow could slow down or speed up allele frequency changes that would otherwise occur in the population. 7

8 Attachment B Data Sheet Generation Number Dominant Alleles (White Recessive Alleles (Pink 1. Beginning Numbers Beginning Numbers a 3. Beginning Number. 4. Beginning Numbers 5. Beginning Numbers 6. Beginning Numbers 7. Beginning Numbers 8. Beginning Numbers 9. Beginning Numbers 10. Beginning Numbers Recessive Alleles (Pink Removed Total Alleles Remaining (100 - column 3) a Beginning numbers for the next generation are calculated by dividing the remaining alleles for each type (dominant/white, then recessive/pink) by the total remaining alleles. 8

9 Generation No. Fancy Fish: Gene Frequency Grade Ten Dominant Alleles (White Attachment C Data Sheet Example Recessive Alleles (Pink Recessive Alleles (Pink Removed Total Alleles Remaining (100 - column 3) 1. Beginning Numbers /96 = 83% 16/96 = 17% 2. Beginning Numbers a /98 = 85 15/98 = Beginning Numbers Beginning Numbers /98 = 87% 13/98 = 13% 5. Beginning Numbers Beginning Numbers 7. Beginning Numbers 8. Beginning Numbers 9. Beginning Numbers 10. Beginning Numbers a Beginning numbers for the next generation are calculated by dividing the remaining alleles for each type (dominant/white, then recessive/pink) by the total remaining alleles. 9