6.1: Blood Type Genetics Protocol

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Overview

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, since Michelle had light skin, while Michael Jr. and Tonja looked more like twins since they both had dark skin. Danielle insisted on blood type tests for both families to check whether there had been a mix-up. To interpret the results of these tests, you will need to understand the genetics of blood types. Genetics of Blood Types The ABO blood type system is the major blood type classification system that determines which type of blood can safely be used for a transfusion. The four blood types in the ABO system refer to different versions of carbohydrate molecules which are present on the surface of red blood cells.

People with:	Have:
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 B carbohydrate molecules on their red blood cells.
Type O Blood	Neither type A and B carbohydrate molecules on their red blood cells.

These different blood types result from different alleles of a gene in the DNA that 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:
I^A	Puts type A carbohydrate molecules on the surface of red blood cells
I^B	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 cell make:	Blood Type
I^AI^A	the version of the enzyme that puts type A carbohydrate molecules on the surface of red blood cells.
i i	the version of the enzyme that puts type B carbohydrate molecules on the surface of red blood cells.
I^Ai	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 I^Ai genotype, which allele is dominant, I^A or i? Explain your reasoning.

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 needed to put this carbohydrate on the surface of his/her red blood cells?	Blood Type
I^BI^B	Type A __ yes __ no; Type B __ yes __ no
I^B i	Type A __ yes __ no; Type B __ yes __ no
I^AI^B	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 will show how meiosis and fertilization result in the inheritance of a gene.

5a. The mother has type _____ blood and the father has type ______ blood.

5b. Complete the chart above 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.

5c. Draw a Punnett square in the usual format for this same mother and father.

5d. Write in the blood type for each child. Note that meiosis and fertilization can produce a child who has a different blood type than either parent.

Understanding Blood Type Tests

The type A and type B carbohydrate molecules 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 bind to type B antigens but not to type A antigens.

Normally, your body does not make antibodies against any antigens that are part of your body. For example, a person with type A blood

Does not make anti-A antibodies against the type A antigen on his/her red blood cells
Does make anti-B antibodies against the type B antigen which is not present on his/her red blood cells.

6. 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

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. This reaction can cause the donated red blood cells to burst and/or clump together and block blood vessels. A 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.

7. Explain how a type B blood transfusion could be fatal for a person with type A blood. Begin with the antibody-antigen reaction in a person with type A blood who has been given a type B blood transfusion. (Hint: See the figure at the top of the page.)

To test blood types, you will first mix a blood sample with a solution that contains anti-A antibodies. If the blood sample has type A antigens, they will react with the anti-A antibodies and this will result in clumping. Then, to test whether this blood has type B antigens, you will mix the second sample of this blood with a solution that contains anti-B antibodies.

8. To prepare to interpret the blood type tests, fill in the following chart.

Blood Type	Will this blood type clump if mixed with
Blood Type	anti-A antibody	anti-B antibody
A
B
AB
O

Procedure

Your group will need a blood-typing tray or another 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 and. 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
Michael (father of twins)
Danielle (mother of twins)
Earnest (father of daughter)
Denise (mother of daughter)
Michael Jr. (boy twin)
Baby Girl 1 (girl twin, according to the hospital)
Baby Girl 2 (daughter of Earnest and Denise, according to the hospital)

Interpretation

Now you can use the results of your blood tests to evaluate whether Michael and Danielle's baby girl was switched with Earnest and Denise's baby girl.

9a. Draw Punnett squares that show the possible genotypes for Michael and Danielle and their children. Write in the blood type for each genotype to show the possible blood types for Michael and Danielle’s children.

9b. Draw Punnett squares that show the possible genotypes for Earnest, Denise, and their children. Write in the blood type for each genotype to show the possible blood types for Earnest and Denise’s children.

9c. Who are the parents of each baby girl? How do you know?

9d. Were the babies switched?