world history
Were the babies switched? – The Genetics of Blood Typesi
Two couples had babies on the same day in the same hospital. Denise and Earnest had a girl, Tonja. Danielle and Michael had twins, a boy, Michael, Jr., and a girl,
Michelle.
Danielle was convinced that there had been a mix-up and she had the wrong baby girl. Tonja and Michael Jr. looked more like twins since they both had darker skin,
while Michelle had lighter skin. Danielle insisted on blood type tests for both families to check whether there had been a mix-up.
Blood Types and Genetics Each person has one of the blood types shown in this chart. Your blood type is determined by whether your red blood cells have type A and/or
type B carbohydrate molecules on the surface. A Person With: Has:
Type A blood
Type A carbohydrate molecules on their red blood cells
Type B blood
Type B carbohydrate molecules on their red blood cells
Type AB blood
Both type A and type B carbohydrate molecules on their red blood cells
Type O blood
Neither type A nor type B carbohydrate molecules on their red blood cells
These four different blood types result from different alleles of a single gene in the DNA. These alleles give the directions for making different versions of a
protein enzyme that puts different types of carbohydrate molecules on the surface of red blood cells. Allele Gives the directions for making a version of the enzyme
that: IA puts type A carbohydrate molecules on the surface of red blood cells IB puts type B carbohydrate molecules on the surface of red blood cells i is
inactive; doesn’t put either type of carbohydrate molecule on the surface of red blood cells 1. Each person has two copies of this gene, one inherited from his/her
mother and the other inherited from his/her father. Complete the following table to relate genotypes to blood types.
Genotype This person’s cells make: Blood Type IA IA the version of the enzyme that puts type A carbohydrate molecules on the surface of red blood cells. i i
the inactive protein that doesn’t put either type A or type B carbohydrate molecules on the surface of red blood cells. IA i both the version of the enzyme that
puts type A carbohydrate molecules on the surface of red blood cells and the inactive protein A
2. In a person with the IA i genotype, which allele is dominant, IA or i? Explain your reasoning.
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3. For the genotypes listed below, which type(s) of enzyme would this person’s cells make? What blood type would the person have? Genotype Will this person’s cells
make the version of the enzyme that puts this carbohydrate on the surface of his/her red blood cells? Blood Type IB IB Type A __ yes __ no; Type B __
yes __ no IB i Type A __ yes __ no; Type B __ yes __ no IA IB Type A __ yes __ no; Type B __ yes __ no AB
Codominance refers to inheritance in which two alleles of a gene each have a different observable effect on the phenotype of a heterozygous individual. Thus, in
codominance, neither allele is recessive — both alleles are dominant.
4. Which of the genotypes listed above results in a blood type that provides clear evidence of codominance? Explain your reasoning.
This expanded version of a Punnett square shows how meiosis and fertilization result in the inheritance of genes.
5. Complete this chart to show the genetic makeup of each zygote produced by fertilization.
A baby develops from a zygote by repeated rounds of mitosis, so each cell in a baby’s body has the same genetic makeup as the zygote. This is the genotype of the
child.
6a. Draw a Punnett square in the usual format for this gene for the same mother and father.
6b. Write in the blood type for each parent and each child.
Note that meiosis and fertilization can produce a child who has a different blood type than either parent.
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Using Immune Responses to Test Blood Types
The type A and type B carbohydrate molecules on the surface of red blood cells are called antigens because they can stimulate the body to produce an immune response,
including antibodies. Each specific type of antibody binds to a specific antigen. For example, anti-B antibodies in the blood bind to type B antigens, but not to type
A antigens.
When an antibody binds with an antigen on the surface of a cell, this can result in damage to the cell. Fortunately, your body generally does not make antibodies
against any antigens that are part of your body.
7. Do you think that a person with type A blood makes anti-A antibodies or anti-B antibodies? Explain your reasoning.
8. Fill in the blanks in this chart. If you have type A blood, you have: type A antigens on the surface of your red blood cells and ________ antibodies in your
blood. If you have type B blood, you have: type B antigens on the surface of your red blood cells and ________ antibodies in your blood.
If you have type AB blood, you have: both type ___ and type ___ antigens on the surface of your red blood cells and neither anti-A nor anti-B antibodies in your
blood.
If you have type O blood, you have: neither type of antigen on the surface of your red blood cells and both ________ and ________ antibodies in your blood.
If you are given a blood transfusion that does not match your blood type, antibodies in your blood can react with the antigens on the donated red blood cells and cause
them to burst and/or clump together and block blood vessels. This type of transfusion reaction can be fatal. To prevent this from happening, doctors test whether
donated blood is compatible with a person’s blood before they give a blood transfusion.
9. Explain how a type B blood transfusion could be fatal for a person with type A blood. Be specific.
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10. To test the blood type of each parent and each baby, you will: mix one sample of their blood with anti-A antibodies to test whether their blood has the type
____ antigen mix a second sample of their blood with anti-B antibodies to test whether their blood has the type ____ antigen. If the sample has antigens that match
the antibodies, this will result in clumping.
11. To prepare to interpret the blood type tests, fill in the following chart.
Blood type
Will this blood type clump if mixed with anti-A antibody? anti-B antibody? A B AB O
Procedure Your group will need a blood-typing tray or other testing surface for each person listed below. For each person, you will use two separate blood samples
to test for the A antigen and test for the B antigen. Place two drops of the person’s blood in each of the testing locations on the testing surface. Place two drops
of anti-A antibody solution on the appropriate blood sample and place two drops of anti-B antibody solution on the other blood sample. Mix each blood sample with the
antibody solution with a clean toothpick. Discard each toothpick after you have used it. If your testing surfaces are transparent, place them on a white background
so you can more easily see whether there was a clumping reaction. For each person, record the results of both tests in the table below. Write in the blood type and
possible genotypes of each person.
Results
Reacts with anti-A antibody (Yes or No)
Reacts with anti-B antibody (Yes or No)
Blood type (A, B, AB, O)
Possible genotype or genotypes
Danielle (mother of twins)
Michael (father of twins)
Denise (mother of daughter)
Earnest (father of daughter)
Michael Jr. (boy twin)
Baby girl 1 (girl twin, according to hospital)
Baby girl 2 (daughter of Earnest and Denise, according to hospital)
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Interpretation Next, you will use the results of your blood tests to evaluate whether Danielle and Michael’s baby girl was switched with Denise and Earnest’s baby
girl.
12a. Draw Punnett squares that show the possible genotypes for Danielle and Michael and their children. Write in the blood type for each genotype to show the possible
blood types for Danielle and Michael’s children.
12b. Draw Punnett squares that show the possible genotypes for Denise and Earnest and their children. Write in the blood type for each genotype to show the possible
blood types for Denise and Earnest’s children.
12c. Who are the parents of each baby girl? How do you know?
12d. Were the babies switched?
i By Drs. Jennifer Doherty and Ingrid Waldron, Dept Biology, Univ Pennsylvania, © 2016. Teachers are encouraged to copy this Student Handout for classroom use. A Word
file of the Student Handout and Teacher Preparation Notes with background information and teaching suggestions are available at
http://serendip.brynmawr.edu/exchange/waldron/bloodtests.
