New! Lesson 10: Sickle Cell Trait
Through this two-part lesson, students will learn about the symptoms, risk factors, and diagnostic criteria for sickle cell trait.
- conduct background research on sickle cell trait
- distinguish between sickle cell trait and sickle cell disease (also called sickle cell anemia)
- describe the risk factors and diagnostic criteria for sickle cell trait
- Sickle Cell Trait | Case and Research Worksheet 10
- Internet access and library databases
- Gel electrophoresis equipment for eight groups of four students, including:
− electrophoresis gel box and power supplies
− one gel tray with six-tooth comb
− 50-ml beaker or graduated cylinder
− disposable plastic pipette
− 250-ml beaker
− 20-μl micropipette with tips or 10-μl fixed volume micropipette
− safety glasses
− 0.8% agarose solution (enough for 25 ml per team)
− plastic carboy with TAE buffer (1X)
− distilled water
This lesson has two parts and is meant to familiarize students with the symptoms, risk factors, and diagnostic criteria for sickle cell trait. On day one, students are first given a brief case study to read, in which a high school football player is discussing his collapse at an August practice with his family and doctor. It comes up in discussion that the athlete may have sickle cell trait. After reading the case study, students should generate questions they have about sickle cell trait and sickle cell disease. These questions can be shared either on a whiteboard or chart paper, or electronically using an interactive software program such as Socrative (www.socrative.com). Students should then organize their questions into specific categories. Teachers may give students the question categories, which may include:
- symptoms or health risks
- diagnostic criteria or testing
- inheritance patterns or risk factors
Alternatively, teachers may ask students to create their own categories in groups, then collectively agree on the categories as a whole class. However, the above listed categories should be included and students should be guided in that direction if they are not asking appropriate questions. Students should then conduct background research (see suggested references below) to answer their own questions. Questions should be split equally among group members, but each group member should research and answer at least two questions, listing all references on the attached research worksheet.
On day two, groups should present their research findings to the whole class and discuss the case scenario again. Students should then work in groups to conduct gel electrophoresis to determine whether Garrett Anderson has sickle cell trait (is a carrier for one copy of the gene that causes sickle cell disease). Standard gel electrophoresis protocols using 1% agarose mini-gels should be used. The samples given to students are simply a mix of dyes: blue dye No. 1 for the normal hemoglobin (negative control), blue dye No. 2 for the sickle hemoglobin, also known as hemoglobin S (positive control), and a mix of the same two dyes for Garrett (sickle cell trait-positive). The gels can be run for 20 minutes using 1X TAE buffer at 100 volts. The voltage can be modified to adjust the time it takes for the dyes to separate. See reference list for gel electrophoresis instructions and available kits or equipment. If gel electrophoresis equipment is not available, an alternative is to give students a sketch or diagram of the results and have them interpret the gel in order to make a diagnosis. The sickle (abnormal) hemoglobin “band” should be closer to the positive electrode, since it has a neutral amino acid (valine) in place of the normal negatively charged amino acid (glutamic acid). Students then complete a genetic testing report (see worksheet) summarizing their observations and the clinical diagnosis for Garrett Anderson. This final assignment can be completed in class or as homework. If the genetic testing report is completed in class, students should be given at least 20 minutes to complete their reports.
The following answer key includes some frequently asked questions students may pose, along with the answers. If students fail to pose most of these questions, the teacher should guide them in that direction during small-group or whole-class discussions. For example, if a student simply asks “What is sickle cell disease?” based on the reading, the teacher may pose the question “Does Garrett actually have this disease?”
Answer Key for Common Student Questions:
What is the function of hemoglobin in the body?
Hemoglobin is a protein present in red blood cells that binds to oxygen in order to deliver it to the tissues. It is a tetramer made up of four polypeptide chains (two alpha chains and two beta chains). Oxygen binding results in a conformational change (or change in shape) in the protein.
What happens to hemoglobin in sickle cell disease?
In sickle cell disease, a single amino acid in the beta chain is incorrect. This is due to a single base substitution in both alleles of the gene encoding beta globin. Individuals with sickle cell trait have this mutation in only one of the alleles for beta globin. This mutation (or change in the DNA sequence) results in the replacement of the amino acid glutamic acid with valine. Because of this change in the amino acid sequence, the abnormal hemoglobin (HbS) tends to polymerize when it is in the deoxygenated state, thereby deforming the red blood cell in which it resides, forming so-called “sickle-shaped” cells. Though the cells usually can return to their normal shape when oxygen levels increase and the sickle hemoglobin polymers resolve, the continual changes in conformation damage the red blood cells and decrease their life span. Note: Some cells may sustain such significant damage that they become irreversibly “sickled”
What is the difference between sickle cell disease and sickle cell trait?
Sickle cell disease is an inherited blood disorder in which the red blood cells, due to abnormalities in the hemoglobin molecule, become misshapen. Individuals with this disease inherit TWO abnormal alleles for the gene encoding the beta chain of the hemoglobin protein. Individuals with sickle cell trait inherit ONE abnormal allele and are considered “carriers” of the disease. Though some of the hemoglobin in each of their red blood cells is abnormal, enough normal hemoglobin is present in the red blood cells to prevent the health problems associated with sickle cell disease.
How is sickle cell disease inherited?
Sickle cell disease is an inherited disorder. All of us have two copies of every gene (except for some genes on the X and Y chromosomes). If both copies of the gene encoding the beta chain of hemoglobin are abnormal, then all of the individual’s hemoglobin will be the abnormal HbS, or sickle hemoglobin. This person is considered to have sickle cell disease (or sickle cell anemia), and will experience its symptoms. (It is important to note that mutations in the genes encoding the alpha chains of hemoglobin can also result in a variety of other genetic disorders.)
Some individuals have two different forms (alleles) of the gene encoding the beta chain of hemoglobin (i.e., one normal and one mutated). Individuals who carry one abnormal allele have “sickle cell trait.” In this case, about half of the individual’s hemoglobin is normal and functions properly while the remainder is the HbS form. The carrier’s red blood cells typically function normally though they can become sickle shaped under extremely low oxygen conditions, such as might be encountered in an unpressurized airplane or with very extreme exertion such as in the case described of the football player. This person is considered a carrier since enough normal hemoglobin is present to prevent the symptoms of the disease.
However, a carrier can pass the abnormal allele to his or her children. If two people with sickle cell trait have a child, there is a 25% chance that the child will have sickle cell disease. The gene encoding the beta chain of hemoglobin is inherited in an autosomal recessive fashion. In other words, the gene is not located on a sex chromosome (X or Y), and one normal copy of the gene is enough to prevent the symptoms of the disease. Only individuals with two abnormal alleles have the disease.
For a helpful diagram depicting the inheritance of sickle cell disease, see: www.cdc.gov/ncbddd/sicklecell/documents/sicklecelltraitfactsheet_english.pdf.
What are the signs and symptoms of sickle cell disease? Sickle cell trait?
Due to the presence of an abnormal form of the protein hemoglobin, the red blood cells of individuals with sickle cell disease become misshapen and sticky when the HbS within them is not bound to oxygen. These cells have a shorter life span, resulting in a constant shortage of red blood cells and decreased oxygen delivery to the tissues. In addition, the cells can clump together and block small blood vessels, causing blockage of blood flow and pain. Patients may experience acute pain crises, an infarcted spleen (since the RBCs get stuck in the spleen); stroke, reduced kidney function (due to the “clumping” of the abnormal RBCs, which can reduce or completely block blood flow); and increased risk of infections (due to reduced immune function). For more advanced students (in an honors or advanced placement course), this is an opportunity to distinguish between signs of a disease versus symptoms of a disease. Signs are objective manifestations of a disease (e.g., sickled red blood cells on a blood smear) perceived by an examiner, while symptoms (pain, shortness of breath) are subjective indications of disease or changes perceived by a patient. Because individuals with sickle cell trait are considered “carriers,” they do not usually show any symptoms of sickle cell disease.
What does it mean to be a carrier for sickle cell disease?
Carriers for sickle cell disease have one abnormal and one normal copy of the hemoglobin gene. Such individuals are often said to have sickle cell trait. This condition is not a disease, and the person generally does not have any symptoms of sickle cell disease. However, a carrier can pass the abnormal gene to his or her children. Therefore, if both parents are at risk for being carriers, it is a good idea to be screened and receive genetic counseling.
Who is affected by sickle cell disease?
Sickle cell disease only occurs in individuals who have two abnormal copies of the hemoglobin gene. According to the Centers for Disease Control and Prevention (CDC), sickle cell disease affects between 90,000 and 100,000 Americans and occurs in 1 in 500 African-American births and 1 in 36,000 Hispanic-American births. (http://www.cdc.gov/ncbddd/sicklecell/documents/scd-factsheet_what-is-scd.pdf)
How common is sickle cell trait?
Sickle cell trait can occur in individuals of any race or ethnicity though it is much more common in individuals of African descent. According to the Centers for Disease Control and Prevention, approximately 1 in 12 African Americans in the United States have sickle cell trait (http://www.cdc.gov/ncbddd/sicklecell/documents/scd-factsheet_what-is-scd.pdf and www.cdc.gov/ncbddd/sicklecell/data.html).
How are individuals tested for sickle cell trait?
The individual gives a blood sample, which is screened for the presence of abnormal hemoglobin. This screening is performed at birth in all 50 states and the District of Columbia. If the person has sickle cell trait, a small percentage of the hemoglobin will be abnormal. A negative test indicates the sickle cell gene is not present. People with two copies of the abnormal gene show a much higher percentage of abnormal hemoglobin in the screening test (and have sickle cell disease). Though one can examine the individual’s blood smear under the microscope for the presence of sickled red blood cells, this is not a definitive way to distinguish between a person who has sickle cell disease and someone who has sickle cell trait (since either individual could have some normal and some sickled red blood cells). Hemoglobin electrophoresis (to detect the presence of the HbS form of hemoglobin) is the most commonly used confirmatory test.