Using Protists to Understand Evolution (Part 2/2)
Lesson 9 of 11
Objective: Students examine the life cycle of Plasmodium for use as an example of natural selection.
I begin the class by having students construct a K-W-L to answer
- What they know about sickle cell anemia?
- What they want to know about sickle cell anemia?
- What they learned about sickle cell anemia?
(Note: Many of my students know nothing about sickle cell anemia so I have them focus on the questions they might have about sickle cell anemia.)
Next show the video Living With and Managing Sickle Cell Anemia
While watching the video, have students brainstorm more questions that are not answered by the video. Students will write their questions in the W portion of the graphic organizer. Once the video is completed, have students share some of the questions they wrote. Have students add to any additional questions they might have.
Next, we start with a short video of what sickle cell anemia is. The video shows how red blood cells sickling.
Next have students read the case study about the first young man that was diagnosed with sickle cell anemia. This based on the scientific paper by James B. Herrick. Students should use this reading guide to help organized their thoughts. (Note: Here is a sample of a completed student work.)
As a review of Mendellian genetics, students are asked to complete several sample problems that might suggest the cause of the young man's disease based on clues from several studies and an except from Living with sickle cell anemia by the CDC. Tell students that sickle cell anemia is determined by one allele. Student should consider if sickle cell is a dominant or recessive trait. Ask them what Mendellian inheritance rules might be in place if it were dominant versus recessive. Remind students about homozygous and heterozygous individuals. Remind students that in one of the videos, the individual being interviewed stated that she knew her daughter would have sickle cell because the girl's father did not. However, she said that her daughter did have sickle cell trait. Ask students what that might mean. Finally, ask students to consider if it might be a co-dominant trait. Ask them what type of inheritance rules might be in place for this to be true.
Students will complete a case study that walks them through the body of knowledge that lead to our current understanding about sickle cell anemia. Primarily, students will explore how Anthony Allison and others determined the link between individuals in some African populations with gene called HbS/sickle gene anemia were more likely to survive a malarial infections. Students will use this handout to explore Allison's data. (Note: Here is an example of completed student work.)
Then students look at Pauling's study that determined that sickle cell anemia was a molecular disease. Students will use a case study from Natural Sciences Learning Center at Washington University - Biology. Students will take notes over the text of the case study and write a summary. They will also explore the Chime interactive on the case study page to better understand the differences between Hemoglobin S and normal hemoglobin. Students should also be given a copy of the graph from Pauling's study. They should write a caption for the graph summarizing the major trends seen. (Note: See these samples of student work: Student summary, p. 1,Student summary, p. 2, and Student summary, p. 3).
In this lecture, explain how finding the answers in a scientific study often leads to more questions. Use this powerpoint, Comparing Hemoglobin Variants, with the lecture.
First, we start with a world-wide comparison of other types of abnormal hemoglobins (there are hundreds) with a comparison of the distribution of the incidence of malaria. Students must consider if these abnormal hemoglobins arose independently or as a "offshoot" of sickle cell anemia. Then, students consider several other examples of balance polymorphism. Stress that these mutations arose randomly and are not consider beneficial. However, they provide some protection against other problems and therefore, remain in the gene pool due to to heterozygous individuals.
Students will watch the HHMI video, Malaria and Sickle Cell Anemia, to help them summarize how the distribution of sickle cell anemia demonstrates natural selection.
Using the student handout, Putting it all together (evidence for natural selection), student will look back on all of the examples we have studied so far to support natural selection.