Teaching Multiple Alleles
1994 Woodrow Wilson Collection
After leaving the protective womb of Punnett squares, students generally exhibit trepidation toward multiple alleles. A simple view of the ABO blood system is used for teaching this subject using concrete models of erythrocytes (red blood cells, or RBC's), antigens, and antibodies. Students can physically see the antigens that result from certain allele combinations and can move antibodies to determine whether agglutination will occur when two given blood types are mixed.
If no magnetic whiteboard and metallic plastic is available, this demonstration can still be performed using a blackboard and magnetic tape (sold in rolls at hardware stores, etc.). See "Alternative Construction" for more details.
Draw and cut out 33 RBC's (see Template 1) from red construction paper. Tape or glue the RBC's to a dark blue or black piece of construction paper, leaving eight (8) cm of space between RBC's. Laminate. Also laminate a separate piece (30 cm X 30 cm or so) of the dark blue or black background paper.
Red Blood Cell
Cut out the antigen templates (Templates 2 - 4) and the antibody templates (Templates 5 - 7) and paste to a piece of thin cardboard. Cut out the antigens and antibodies from the cardboard. (This makes tracing easier later.)
Using the antigen templates, trace antigens protruding from the RBC cell membranes (the antigens will be cut from the dark background material surrounding each RBC). Cut out four (4) RBC's with each of the following antigen combinations:
- No antigens (O-negative)
- 4 Rh antigens only (O-positive)
- 4 "A" antigens only (A-negative)
- 3 "A" antigens, 3 Rh antigens (A-positive)
- 4 "B" antigens only (B-negative)
- 3 "B" antigens, 3 Rh antigens (B-positive)
- 3 "A" antigens, 3 "B" antigens (AB-negative)
- 2 "A" antigens, 2 "B" antigens, 2 Rh antigens (AB-positive)
Also, make one (1) RBC with one of each of the antigens. This will be displayed as the MODEL for all antigen shapes at the beginning of the lesson.
Using the antibody templates (Template 5 = Anti-B; Template 6 = Anti-A; Template 7 = Anti-Rh), trace eight (8) of each antibody on the laminated dark blue or black paper. Cut out.
Attach a piece of metallic plastic to the backs of all cut-out pieces using tape. You will have to experiment with the amount of plastic needed, but make sure you use enough to make the RBC's and antibodies stick well to the whiteboard.
Alternative Construction: If you are using a blackboard, simply cut a length of magnetic tape and apply to the back of each RBC and antibody. The cut-out pieces will stick to the blackboard nicely.
If graphite-backed paper is available, make the following labels on it. If no such paper is available, make the labels on construction paper and attach magnetic tape.
Labels: "A Antibody," "B Antibody," "Rh Antibody," "Anti-A Antigen," "Anti-B Antigen," "Anti-Rh Antigen," "Agglutination."
Although there are numerous alleles which determine a person's blood type, a simplification to three alleles (IA, IB, and i) can effectively illustrate multiple allele systems to students. My students and I discuss genotypes that produce the various phenotypes (blood types A, B, AB, and O) with the understanding that
|Type A ||IAIA, IAi
|Type B ||IBIB, IBi
|Type AB ||IAIB
|Type O ||ii
Take out the MODEL red blood cell and place on the whiteboard. Explain that the allele IA will cause the formation of antigen A. Apply the label "Antigen A." Then explain that the allele IB causes the formation of antigen B. Apply the label "Antigen B." Reinforce to students that "O" is not an antigen type, but rather the absence of an antigen. Finally, explain to students that the third antigen is called the Rh antigen, and that more will be said about that one later. (NOTE: Students frequently and mistakenly think the allele i codes for a type of antigen. Reinforce that i codes for no antigens at all.)
Remove the MODEL and labels. (A suggestion: Put the MODEL somewhere students can still see it for reference.)
Take out two A-negative red blood cells and place them on one end of the whiteboard. Ask a student to come up and write, using an overhead marker, a genotype that would produce this blood type. Remind the students about the alleles which cause antigen formation. Usually, the student will write "IAIA" on the first RBC. Then ask if there are any other genotypes which would produce the phenotype. Have a student write the genotype "IA i" on the second RBC. Students can now see very clearly the relationship between genotype and phenotype.
Take out a couple of A-positive RBC's. Place them on the opposite end of the whiteboard. Ask students about the difference between these cells and the A-negative cells. Explain about Rh-antigens - history, genotypes that produce the antigens, etc. Students can clearly see that while both types of cells have the A antigens, one has the Rh antigens as well. The terms "A-negative" and "A-positive" now make sense to them.
Explain (or review) the relationship of antibodies and antigens. You may want to go into a good amount of detail about B-cells, etc., or you may just mention that antibodies are proteins produced by certain white blood cells and leave it at that. Display a few anti-A and a few anti-B antibodies on the whiteboard alongside the A-negative RBC's. Ask students if it would be wise for a person with type-A blood to produce anti-A antibodies. (They will normally slip into a moaning, sarcastic tone of voice and mutually groan, "Noooooo.") Have a student come up and show the rest of the class why the anti-A antibodies would not be produced by this person: She or he can physically move the anti-A antibodies into position on the antigens, showing an antibody-antigen complex. Introduce the term "agglutination." Ask students whether the anti-B antibodies could be safely produced by the A-negative person. Have a student try to form an antibody-antigen complex with the anti-B antibodies and the A-antigens. Explain that people with type A blood, whether Rh-positive or Rh-negative, will have anti-B antibodies in their blood stream. Finish by placing some anti-Rh antibodies up on the whiteboard and asking the students whether someone who is A-positive would produce such antibodies. ("Nooooooooo.....") Can an Rh-negative person produce anti-Rh antibodies? ("Yesssssss.....") Again, have students manipulate the anti-Rh antibodies and try to make them fit on antigens.
Remove the RBC's. Place RBC's of other blood types up and follow the procedures above: Students write genotypes on the cells, see antibodies produced by each blood type, and compare the Rh-positive and Rh-negative forms of each blood type. Have students fill out a table as they learn about the blood types to keep in their notes.
Hint: Students seem to pick up the concept of which antibodies are produced by which blood types if you write the following on the board:
People will produce antibodies to any antigens NOT found on their RBC's.
It seems simple, but it works.
Spend some time doing blood donation problems. Write "DONOR" and "RECIPIENT" on the whiteboard. Put various blood types under "RECIPIENT" along with the antibodies that a person will have/produce. Under "DONOR," place some RBC's of various blood types. Do not place any antibodies under "DONOR," to indicate that the donor's blood antibodies are so dilute that they have no real effect on the operation. Have students physically move the donor's RBC's into the recipient's "blood stream" and check if any of the recipient's antibodies will attack any antigens on the donor's cells. Set up several donor problems. Once students are comfortable, place an RBC of each blood type (A-negative, A-positive, B-negative, B-positive, etc.) on the whiteboard and ask the following questions:
- If you were director of the Red Cross, and wanted to collect blood which could help the most people, which blood type would you want to collect the most? (Remember: The fewer antigens on the donor's cells, the less likelihood that any antibodies in the recipient will cause agglutination.)
- If you were in a major accident and needed a large blood transfusion, which blood type would be best to have, in terms of being able to accept the widest range of donated cells? (Remember: The more antigens on your red blood cells, the fewer antibodies you will produce. The fewer antibodies produced, the more blood types you can accept.)
Students are usually very quick to figure out that O-negative and AB-positive are the correct responses, respectively. Introduce the terms "Universal Donor" and "Universal Recipient."
Finish this lesson with a discussion of Dr. Charles Drew and his work with plasma separation, especially if this lesson falls during African-American History month. Teachers unfamiliar with this great scientist can contact their local American Red Cross or consult any encyclopedia. (By the way, the story that Drew died of blood loss after an automobile accident is true. However, the tale that a whites-only hospital nearby would not treat him because he was black is untrue. His daughter recently said Dr. Drew received the best care possible but his trauma was simply too great to overcome.)
- Tour a Red Cross blood center.
- Display various blood collection bags (borrow from local Red Cross).
- Discuss HIV/AIDS and Hepatitis B. In particular, discuss history of virus in blood supply. (If you do this, be sure to reinforce that the nation's blood supply is EXTREMELY SAFE. Please do not neglect this!!!)
- Research new efforts to remove antigens from RBC's.
- Have your class organize a school-wide blood drive.