A man's blood group is AB. What is the probability of the man giving birth to a child with blood group O?

Analyze the question The question asks about the chance that a man with blood group AB fathers a child with blood group O. (The word "giving birth" is just loose phrasing for "fathering".)

Concept: ABO blood group inheritance

• Blood groups are controlled by three alleles: IA, IB and i.
• IA and IB are codominant, and both are dominant over i.
• Group O has the genotype ii only, so a child must receive an i allele from each parent.

| Blood group | Possible genotypes | |---|---| | A | IAIA or IAi | | B | IBIB or IBi | | AB | IAIB | | O | ii |

Step-by-step solution

1. The man is AB, so his genotype is IAIB.
2. His gametes carry either IA or IB. Half carry IA and half carry IB.
3. He has no i allele to pass on.
4. A child of group O needs genotype ii, which requires an i allele from the father.
5. Since the father cannot supply i, the child cannot be group O, whatever the mother's blood group.

Example: if the mother is O (ii), the children are IAi (group A) or IBi (group B). If the mother is AB, the children are IAIA, IAIB or IBIB. In every case none are O.

Answer: The probability is 0%, which is option A.

Exam tip: An AB parent can never have an O child, and an O parent can never have an AB child. This is a frequently tested fact, so remember "AB cannot give O."