Human Blood Type Sample Problems

Table 1: The following tables give information on human blood types needed in problems below. Alleles of types I_A and I_B are dominant over type i.

1. A patient is rushed to the emergency room and has suffered severe blood loss. Type AB blood is in short supply, but the nurse says "Don't worry, he's type AB positive. We can give him any kind of blood." Explain. (Why is type AB called the universal recipient?)

A person who is AB positive has every kind of protein in his blood. There is nothing in any blood type that would be "foreign" to his body, so rejection is not a problem.

2. On the battlefield, a medic is treating a soldier who has lost a great deal of blood. They are out of blood typing supplies so the medic, who is Type O negative, simply donates his own blood to the patient. Why could this work? (Why is Type O called the universal donor?)

A type O negative person has none of the proteins that can cause rejection in his blood, so his blood can be accepted by anyone.

3. There is a practical joker in the maternity ward who removed all the baby id bracelets. There are three babies that cannot be easily distinguished and the parents want to be sure they get the right ones back so the doctors do a blood test. A particular mom is homozygous type A and the dad is type O. The babies have blood types AB, A, and O. Show your work below and indicate which baby must be theirs.

4. The police have rounded up the usual suspects in the latest rash of bookstore robberies. The thief got a nasty paper cut at the scene of the crime. The suspects are of blood types O, A, B and AB. The blood at the crime scene contained i alleles. Which suspect therefore cannot have been involved? Explain.

Release Suspect #4 because his blood contains no i alleles.

5. In a paternity case, a single mother claimed that a certain man was the father of her baby. The man denied it, claiming that her current boyfriend was the father. The court ordered a blood test (much cheaper than DNA testing) to see if he could be ruled out as the father. The mother was Type O and the baby was Type O. The man was Type AB. Is it possible that he was the father? Why or why not?

The man's genotype is I_AI_B and the mother's ii. The baby's genotype is also ii. This makes it impossible for the man with type AB blood to be the father of this child.

6. Why is it that a blood type test can only disprove but never prove paternity? Why are DNA tests used to "prove" paternity instead?

A baby can, in some circumstances, have a blood type that could not possibly be produced by a mating of two particular individuals, as shown in the example above. This would be sufficient to disprove paternity. However, there are only a handful of blood types, and many people have them, so while there are cases where a man of a certain blood type could be the father of a child, so could anyone else with that same blood type. DNA tests can be used more effectively to "prove" paternity, because the possibility of a child having a close DNA match to a non-relative is extremely rare. However, a good lawyer knows that nothing in science can be proven absolutely. Samples sometimes do get contaminated and chance events, though rare, do sometimes happen. The lottery odds may be one in a million, but that's different from zero.

 7. (True Story) In Denmark, a husband and wife who had been unsuccessfully trying to have a baby went to a fertility clinic. Sperm and eggs were collected from father and mother, and combined in a petri dish, creating several "test-tube babies". These babies were implanted in the mother and 9 months later she delivered twins, one with light skin and one with dark skin. Because this seemed strange, a DNA test was conducted and it was found that both children were related to the mother, but only the light skinned child was related to the father. How can this be explained?

Infidelity seems unlikely because of the timing of conception; she would have to have slept with another man at very close to the same time that she was implanted with her husband's sperm at the fertility clinic. A more likely explanation is that a test tube or dropper was accidentally used repeatedly without being sterilized, causing sample cross-contamination. DNA tests in this case did reveal that the father of the second child was another client of the fertility clinic who had never met the mother.

8. Rh factors are proteins that were first discovered in the blood of Rhesus Monkeys, but humans have them too. If you are Rh positive, it means that there are Rh type proteins in your blood. If you are Rh negative, there are no Rh type proteins in your blood. Positive is dominant over negative, so heterozygous individuals are Rh positive. Problems can arise when an Rh negative mother has a child who is Rh positive. Why does the mother's body attack her own baby in this situation? Why does the situation get worse for the second pregnancy?

The mother's body sees the foreign proteins produced by her baby as a threat. She builds up antibodies against them. The baby is delivered before the situation gets too out of hand, but the next pregnancy will be more problematic because her immune system is sensitized and fully prepared at the onset of the next pregnancy. Her immune system attacks the baby, often resulting in a miscarriage. Immune suppressing drugs can be given to the mom, but then she's more likely to get sick because her body's germ fighting ability is diminished.

9. The father of two children is type O+, and the mother is type A+. The children are O- and A+. Given this information, what can you say about the genotypes of father and mother?

If the father is O+, his ABO genotype is ii, and he may be either Rh +/+ or +/-. If the mother is Type A+, her ABO genotype is either I_AI_A or I_Ai and her Rh type is either +/+ or +/-.

However, if either of the children is Rh-, then both of the parents must be Rh +/- because, in order to get a -/- child, each parent must have had one -.

If one of the children is type O, then the mother must have been I_Ai, because otherwise the child could only be type A.

So, the father is ABO genotype ii, Rh type +/-, and the mother is ABO genotype I_Ai and Rh type +/-.