Bio H – Mendelian Genetics
Switched at Birth? Blood Typing Lab – Bio H
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 combined with the Rh factor. We’ll start with 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, which we’ll discuss later. 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 antigens refers to any molecule that reacts with certain proteins called antibodies. Antibodies are
special proteins that our bodies make to protect us against foreign molecules. This includes molecules found on
bacteria and viruses as well as molecules found on a splinter or, in this case, on foreign blood cells. Antibodies
are part of our immune system, designed specifically to recognize and react with one and only one antigen.
When antibodies react with their target antigen, those antigens, and whatever they are attached to are targeted
for destruction. For obvious reasons, it is not normal for our bodies to make antibodies against any antigens
that are supposed to be part of our own bodies or else we’d turn on ourselves. 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. This way their own blood cells
are safe, but if any foreign blood cells enter, they can be attacked.
As you have probably heard, we don’t designate blood types simply as “A” or “B” but rather “A negative” or
“B positive.” This second characteristic of blood is called the Rh factor and also influences the antigens that are
found on the surface of the red blood cells. The Rh trait can be complex with 45 possible antigens, however,
when we refer to blood as being “pos” or “neg” we are usually referring to only one of those 45 antigens:
antigen D which. Having the D antigen makes your blood “positive” and not having antigen D makes you
negative. Like the ABO system, individuals without antigen D on their cells produce an antibody against this
antigen while individuals who are positive do not produce this antibody.
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 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. It should be noted, however, that only red blood cells are donated during a
transfusion. The antibodies, which are normally found in the liquid part of the blood called plasma, are
separated before the transfusion. While some people do get plasma transfusions, the plasma is still separate
from blood cells.
Transfusion reactions can be fatal. To prevent this from happening, 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
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Bio H – Mendelian Genetics
harmed if they make 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!
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 the ABO antigens and one allele from
each for the Rh factor. In other words, we need to consider the alleles from two genes to figure out a complete
blood type.
The ABO antigens are determined by one gene that has three different versions or alleles: IA, IB, and i. Since
everybody has two copies of this gene, 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.
The Rh antigens are determined by one gene that has two different alleles: + and -. Since everyone has two
copies of this gene, three combinations are possible:
++ and +- both resulting in positive blood
- - resulting in negative blood.
IV.
Blood typing reaction
As we discussed above, antibodies react with their antigen. This reaction is visible and we can use it to help
identify which antigens are in a blood sample. Basically all we need are cells from the individual we want to
test and a bottle of each of the antibodies. Then, in a lab dish, we can mix the blood with each antibody and see
if there is a reaction. If a particular antibody reacts, it means that antibody has detected its antigen in the blood
sample. Once we know which antigens are in the blood, we should know what blood type we are dealing with.
V.
Lab scenario: 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 Earnest 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. The blood types of the parents are all known: Earnest 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.
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Bio H – Mendelian Genetics
Pre-Lab Questions: Answer in your OWN words. These will be collected at the beginning of lab.
1. What is the difference between the blood cells of different blood types?
2. What is the difference between the plasma of different blood types?
3. If antibody A mixes with a blood sample and we get a reaction, what does that tell us about the blood
sample?
Fill in the chart below:
Blood
Type
Antigens on red
blood cells
Antibodies in
plasma
Possible
genotypes
Can give
blood to
Can receive
blood from
A+
B-
AB+
O-
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Bio H – Mendelian Genetics
Blood Typing Procedure
1. Place one drop of the blood of one baby in both the A and B well of one plate.
2. Place two drops of A antiserum (antibody) in well A, two drops of antiserum B well
3. Mix one well thoroughly with a toothpick. Use the opposite end of the toothpick to mix the other well.
Repeat this for all three children.
4. Use a fresh toothpick to do the same with each of the remaining two wells.
5. Examine each well to see if anything happened. Did the well become opaque? Does it look like there are
particles floating in it? If so, it means that the antibody you added to that well has reacted with that antibody’s
antigen.
6. Repeat the procedure, steps 1 through 4, for each baby blood sample (three in total).
7. Use the reactions to determine the blood type of each of the babies and then use the genetics to figure out
which babies belong with which parents.
Data chart:
Reacts with
anti-A
antibody
(Yes or No)
Reacts with
anti-B
antibody
(Yes or No)
Reacts with
anti-Rh
antibody
(yes or no)
Blood type
(ABO and Rh)
Tonya
Michelle
Michael, Jr.
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Bio H – Mendelian Genetics
Discussion Questions
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 are shown? Explain.
What blood types did you get for each of the babies below? Describe how it was determined (positive reactions
with which antibodies?).
Tonya -
Michelle Michael, Jr. –
4. Was there a switch at the hospital? How do you know? Draw the TWO punnett squares that represent each
pair of parents. 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.
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Bio H – Mendelian Genetics
5. Explain to the 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 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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