Module 6 Lesson 3 - 5

ABO Blood Types

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Human blood type is determined by three alleles: I^A, I^B, and i. Because more than two alleles are involved in the blood type gene, it is considered multiple alleles inheritance pattern. At the same time, the I^A and I^B alleles are codominant and i allele is recessive. Four phenotypes are possible: blood types A, B, AB, and O.

Examine the sample blood type problems on the top of page 606 of your textbook. Notice how the textbook indicates alleles by using a capital I as the letter base, then adding a superscript A or B if indicating one of the codominant alleles, and using a lower case i with no superscript for the recessive O allele. The alleles can be expressed without using a capital I by using capital letters A, B, and O as shown in the table.

Both methods are acceptable, but you will see Method 1 on the Alberta Diploma Exams.

All possible genotype combinations for blood types:

	Method 1	Method 2
Blood Type A	IAIA      IAi	AA    AO
Blood Type B	IBIB     IBi	BB    BO
Blood Type AB	IAIB	AB
Blood Type O	ii	OO

Example 1

A man with blood type AB and a woman with blood type A have children. What possible blood types could the children have?

Solution

The man's blood type is AB. This means his genotype is I^AI^B. There are no other possibilities. The woman's blood type is A. Her genotype can be I^AI^A or  I^Ai. This gives two possibilities.


Three blood types are possible for the children: A, B, and AB.
( Note: When the genotype of individuals with blood type A or B is unknown, the better assumption is that they can be heterozygous, AO or BO. )

Example 2

Four children have the following blood types: A, B, AB, and O. Could they have the same two biological parents?

Solution

Yes, they can. Each child had to receive one copy of his or her allele from the mother and another copy from the father.

• The child with blood type AB indicates that one parent must be able to pass on the allele for A and the other parent must be able to pass on the allele for B.
  
• The child with blood type O indicates that both parents must be able to pass on the allele for O.

This means Parent 1 has AO genotype and Parent 2 has BO genotype. A Punnett square confirms this.

	A	O
B	AB	BO
O	AO	OO

The Punnett square confirms that it parental genotypes AO and BO can have children with blood types A, B, AB and O.

Rhesus Blood Factor

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Another possible blood typing system is the Rh blood group. The surface of the red blood cells of Rh-positive individuals have markers called antigens. Rh-negative individuals do not have these cell surface markers.

Rh-positive allele (D) is dominant over Rh-negative allele (d). A Rh-positive individual can have genotypes DD or Dd. A Rh-negative individual can have the genotype dd.

When a Rh-negative individual is exposed to Rh-positive blood, the Rh-negative person's immune system starts to make antibodies against the Rh-positive blood. This becomes very important during childbirth when a Rh-negative mother gives birth to a Rh-positive baby (the father is Rh-positive). At parturition, some of the baby's Rh-positive blood crosses the placenta to the mother's blood stream. At this point, the mother's immune system starts making antibodies against the Rh-positive blood. During her second pregnancy, if the second baby is also Rh-positive, the mother's antibodies can cross the placenta and start attacking the baby's blood cells, causing them to clump.

Example 3

A woman with blood type A with Rh-positive is having a baby with her partner who is blood type B with Rh-negative. What is the probability that the baby will have type O and Rh-negative blood?

Solution

	Phenotype	Possible Blood Type Genotypes	Possible Rh Factor Genotypes
Woman	Blood Type A, Rh+	AA or AO	DD or Dd
Man	Blood Type B, Rh-	BB or BO	dd

1. The woman must have the genotype AO and the man must have the genotype BO to have a baby with blood type O.
   
   

   	A	O
   B	AB	BO
   O	AO	OO

   
   There is a 25% chance that the baby will have blood type O if both parents are heterozygous for their blood type.



2. The woman must have the genotype Dd to have a baby with Rh-negative blood.
   
   

   	D	d
   d	Dd	dd
   d	Dd	dd

   
   There is a 50% chance that the baby will have Rh- blood.


3. Now, combine the two probabilities.
   0.25 (probability of having O type blood) x 0.50 (probability of having Rh-negative blood)  = 0.125
   

This couple has a probability of 0.125 to have a baby with blood type O and Rh-negative blood.