The Genetics of Blood:
Codominance and Multiple Alleles
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Background: In humans the ABO blood types are inherited in a codominant fashion. There are four
phenotypes: Type A, Type B, Type AB, and Type O. There are controlled by three alleles: the
codominant alleles A and B and the recessive allele O. Blood types can be determined with a simple test
that use antibodies that recognize the presence or absences of particular carbohydrates on the surface of
red blood cells. If a blood sample agglutinates (clumps) when treated with a given antibody, then those
blood cells have that carbohydrate. (Table 1). In this activity, you and your lab group will test simulated
blood and solve case studies based on your understanding of blood type inheritance.
Materials:
 Clean blood typing cards
 Toothpicks
 Artificial blood samples
 Dropper vials with the antibodies that bind to the “A” carbohydrate (anti – A,), the “B”
carbohydrate (anti-B:), and the Rh factor (anti-Rh or anti-D)
Procedure for testing blood:
1. Visit each of the four ‘scenario stations’ set up around the room. Use one card per person you are
testing. Don’t let samples mix to prevent contamination from affecting your results.
2. With the vials capped, gently invert (tip upside down several times) the artificial blood to
resuspend the plastic ‘cells.’
3. Place one drop of the blood on all three circles on the patient’s card. Please make sure you put
the caps back on the appropriate vials so you don’t cross contaminate the samples.
4. Add a drop of anti-A to the well labeled A on your slide. Replace the cap.
5. Add a drop of anti-B to the well labeled B on your slide. Replace the cap.
6. Add a drop of anti-Rh to the well labeled Rh on your slide. Replace the cap.
7. Use the toothpicks to stir the reactions for about 30 seconds each. Use a clean toothpick for each
reaction (you can use both sides if you flip it around).
8. Carefully examine each slide for results.
a. In each well, a thin film should appear on the surface of the liquid.
b. If the film remains uniform in appearance, no reaction or agglutination has occurred. This
is a negative (-) result.
c. If the film appears granular (chunky), a reaction has occurred. This is a positive result.
Scenario 1:
Mrs. Johnson was one of several mothers in a maternity ward at a hospital when a fire broke out in the records
room. When she was given baby #1, she denied it was hers, claiming baby #2 instead. Another new mother also
claimed baby #2 was hers.
Mrs. Johnson was blood type O. Unfortunately, Mr. Johnson died a month before the baby was born so we don't
know his phenotype or genotype, but the Johnsons had three other children whose phenotypes are known. Ryan is
blood type A, Logan is blood type B, and Jenna is blood type B.
Blood type baby #1 and #2 and use the results of the test to determine which baby (if any) belongs to Mrs. Johnson
Scenario 2:
Mr. and Mrs. Taylor adopted a baby from a foreign country 10 years ago. Now a couple has emerged claiming they
are the child’s biological parents and they never consented to the adoption. Both couples want custody.
The alleged biological parents have types O and AB, and the child in question has type A. Test the Taylors to
determine if this case has enough evidence to go to court.
Scenario 3:
A famous paternity battle arose when Joan Barry claimed that Charlie Chaplin, a star of silent movies, fathered her
child. Blood typing in paternity cases was gaining widespread use in the 1940s, so Charlie’s lawyers requested that
Joan and Charlie be blood typed. The child (a daughter named Carol Ann) had type B.
In the end, Chaplin was ordered by the jury to pay child support. Blood type both adults and determine whether the
verdict was appropriate.
Scenario 4:
The Duke and Duchess of Gulfcoastia are in possession of a large blue diamond that they plan to sell in order to
raise funds for their yearlong safari in Africa. They awake one morning to find that someone has broken into their
jewel sanctuary and has attempted to steal the diamond. They immediately suspect that one of their three children,
all of whom stand to lose if the jewel is sold, is behind the attempted theft. The would-be thief was injured at the
scene on a broken window and left behind blood determined to be type AB. Both the Duke and Duchess know their
blood types: the Duke is type O and the Duchess is type A. Their three children – Linda, Deborah, and Mona – each
display varying degrees of innocence, provide samples for testing.
Analysis:
Scenario 1: What must have been Mr. Johnson’s blood type? Which baby belongs to Mrs. Johnson?
Sample
Anti – A
Anti – B
Anti – Rh
Blood type
Baby #1
Baby #2
Scenario 2: Based on blood type, which set of parents have a biological claim for this child?
Sample
Anti – A
Anti – B
Anti – Rh
Blood type
Mr. Taylor
Mrs. Taylor
Scenario 3: What is the probability that Charlie is the father of Carol Ann?
Sample
Anti – A
Anti – B
Anti – Rh
Blood type
Joan Barry
Charlie Chaplin
Scenario 4: Who committed the break in? What was the other mystery uncovered by this scenario? Who
is the real heir to the diamond?
Sample
Deborah
Mona
Linda
Anti – A
Anti – B
Anti – Rh
Blood type
Teachers Notes:
Scenario 1: What must have been Mr. Johnson’s blood type?AB Which baby belongs to Mrs. Johnson?#1
Sample
Baby #1
Baby #2
Anti – A
Anti – B
Anti – Rh
Blood type
+
-
-
+
-
A+
O-
Scenario 2: Based on blood type, which set of parents have a biological claim for this child? both
Sample
Mr. Taylor
Mrs. Taylor
Anti – A
Anti – B
Anti – Rh
Blood type
+
+
-
+
AA+
Scenario 3: What is the probability that Charlie is the father of Carol Ann? Zero
Sample
Joan Barry
Charlie Chaplin
Anti – A
Anti – B
Anti – Rh
Blood type
+
-
-
+
-
A+
O-
Scenario 4: Who committed the break in? What was the other mystery uncovered by this scenario?Linda
is the thief, and is their biological child
Sample
Deborah
Mona
Linda
Anti – A
Anti – B
Anti – Rh
Blood type
+
+
+
+
+
-
AA+
AB-