Bio A – Mendelian Genetics
Switched at Birth? Blood Typing Lab – Bio A
Copyright, 2005, by Jennifer Doherty and Ingrid Waldron, Department of Biology, University of Pennsylvania
I.
Basics of Blood – antigens and antibodies
There are many different ways to classify blood types, but the most common blood type classification system is
the ABO system. There are four types of blood in the ABO system: A, B, AB, and O. These blood types refer
to different versions of carbohydrate molecules, which are present on the surface of red blood cells. These
carbohydrate molecules are examples of molecules called Antigens. People with Type A blood have Type A
antigens on the surface of their red blood cells, people with Type B blood have Type B antigens on the surface
of their red blood cells, and people with Type AB blood have both type A and Type B antigens. People with
Type O blood do not have any antigens on their red blood cells.
The term antigen refers to any molecule that can react with another type of molecule called antibodies.
Antibodies are special proteins that our bodies make to protect us against molecules that our bodies do not
make. This includes molecules which are a part of the surface of bacteria and viruses as well as molecules
found on other people’s blood cells. Generally we think of antibodies as part of our immune system, but they
will basically prevent foreign molecules from entering the body. To do this each antibody is in charge of
recognizing a specific antigen. One antibody will only react with one type of antigen. This is why, once you’ve
had the chicken pox, you won’t get it again, but you can get other illnesses. Normally, our bodies do not make
antibodies against any molecules that are supposed to be part of our own bodies. For example, people with
Type A blood do not make antibodies against the Type A antigen, which is present on their red blood cells, but
they do make antibodies against the Type B antigen which is NOT found on their cells.
II.
Blood transfusions – who can receive blood from whom?
Remember, antibodies react with their matching antigens. If the antigen is on a bacterial cell this is a good
thing! The antibody will kill the bacterial cell. BUT if your blood antibodies attack your blood cells, they start
to form clumps or clots in your blood vessel and this can be fatal. If you are given a blood transfusion that does
not match your blood type, antibodies present in your blood will react with the antigens present on the donated
red blood cells. For example, if a person who has Type A blood is given a Type B cells during a blood
transfusion, then this person's natural anti-B antibodies will react with the Type B antigens on the donated red
blood cells and cause a harmful reaction.
The blood being given to you is actually only red blood cells. This means that the blood you receive has no
antibodies (which are normally found in the liquid part of the blood called plasma), that might attack your blood
cells. So your blood can attack “incoming blood cells” (your plasma does have antibodies) but the incoming
blood cells cannot “attack” your cells!
Because transfusions have the potential to be fatal (cause death from clotting), doctors test whether a person's
blood is compatible with the donated blood before they give a transfusion. The general principle is that a
person will be harmed if the person’s body makes an antibody against the antigen found on the donated red
blood cells. So, as long as your antibodies do not match up with the antigens on the donated blood, you’re safe!
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Bio A – Mendelian Genetics
III.
The genetics of blood type
Your blood type is established before you are born, by the specific alleles that you inherited from your parents.
You receive one allele from your mother and one from your father for blood type. These two alleles determine
your blood type by causing the presence or absence of the Type A and Type B antigen molecules on the red
blood cells.
A person’s blood type is determined by one gene that has three different versions or alleles: IA, IB, and i. Since
everybody has two copies of these genes, there are six possible combinations:
IA IA and IA i - both resulting in Type A blood,
IB IB and IB i - both resulting in Type B blood,
IA IB - resulting in Type AB blood,
i i - resulting in Type O blood.
IV.
Were the babies switched?
Two couples had babies in the same hospital at the same time. Michael and Danielle had twins, a boy, Michael,
Jr., and a girl, Michelle. Denise and Ernest had a girl, Tonya. After being home for a few days, Danielle was
convinced that she had the wrong baby. There must have been a mix-up at the hospital. After all, her kids were
twins, and even though they were fraternal twins, you would think that they would look a lot more alike than
they do—one is light-skinned and the other is dark-skinned. The blood types of the parents are all known:
Ernest is type A. Denise is type B, Michael is type AB and Danielle is type B. You must test the blood of the
three infants to determine whether the hospital made a terrible mistake.
Blood Typing Procedure
1. Place a drop of the blood of one baby on both the A and B wells of the dish.
2. Place three drops of A antiserum (antibody) in well A and three drops of antiserum B well B.
3. Mix the blood sample with the added A antiserum with one end of the toothpick. Mix the blood sample with
the added B antiserum with the other end of the toothpick. Discard each toothpick after you use it. Make sure
you mix each sample thoroughly.
4. The reaction is subtle. You are looking for the formation of “clumped” molecules or a white precipitate.
This is the same as creating a bad transfusion. So if you see clumping when the Anti-A antibody is added, it
means that the A antigen is present in that drop of blood.
5. Repeat the procedure, steps 1 through 4, for each baby blood sample (three in total).
6. Once you have identified the blood types, look back at the parent’s phenotypes to determine if there was a
switch.
Data chart:
Reacts with
Reacts with
anti-A antibody anti-B antibody
(Yes or No)
(Yes or No)
Blood type
(A, B, AB, O)
Tonya
Michelle
Michael, Jr.
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Bio A – Mendelian Genetics
Pre-Lab Questions: Answer in your OWN words (points will be taken off for verbatim quotes of the lab). If
these are not completed by the time you come to lab, you will be assigned a zero for this portion of the lab.
1. What is an antigen (please use your own words, and describe “generally” ?
2. What is an antibody?
3. What happens when an antibody detects the antigen against which it acts?
4. What is the definition of subtle?
5. What do you need to do to see an antigen-antibody reaction?
6. Fill in the chart below:
Blood Type
Antigens on Antibodies in
red blood cells
plasma
Possible
genotypes
Can give
blood to
Can receive
blood from
A
B
AB
O
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Bio A – Mendelian Genetics
Rewrite your Data Chart
Reacts with anti-A
antibody (Yes or No)
Reacts with anti-B
antibody (Yes or No)
Blood type
(A, B, AB, O)
Tonya
Michelle
Michael, Jr.
Discussion Questions
1. The gene that determines blood type has 3 different alleles. Two different inheritance patterns are
demonstrated by these three alleles. Which two patterns, discussed in class, (complete dominance,
incomplete dominance, etc.) are shown? Explain.
4. A. Was there a switch at the hospital? B. How do you know? C. Draw TWO Punnett squares (one for each
set of parents), that shows possible outcomes for each set of parents that would support what you have stated.
Remember that BOTH Punnett squares must be “true” in order to agree with your outcome. If you need to
work out other possibilities, do so on scrap paper and copy only the two correct squares below. You must
also include a verbal explanation of how you know there was a switch and which babies were switched.
A. Punnett Square for Michael and Danielle
B. Punnett square for Denise and Ernest
Explanation:
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Bio A – Mendelian Genetics
5. How would you explain to a set of parents why it is acceptable for fraternal twins to look so different.
Include an explanation of how both fraternal and identical twins are created.
6. When we donate blood, the blood bank separates the plasma of the blood from the blood cells themselves.
Each substance, the plasma and the cells, can then be stored and donated to different recipients. Considering
that the antibodies are in the plasma, first explain in general terms why this separation is necessary. Then
provide an example of a person who could receive cells, but not plasma, from a specific donor.
7. If you were a daredevil, prone to many, many accidents, which blood type would you want to be and why?
Be sure to address antibodies and antigens in your answer.
8. If you are in a terrible accident(and away from all your family and friends) and a blood transfusion needs to
happen immediately, what would the doctors do if they don’t have time to determine your blood type?
EXPLAIN in terms of antigens and antibodies why your solution would be an acceptable treatment.
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