What is the probability? of a man of blood group AB married to a woman of blood group O producing a child of blood group O?

Analyze the question We need the chance that a man of blood group AB and a woman of blood group O have a child of blood group O.

Concept ABO blood groups are controlled by three alleles: Iᴬ, Iᴮ and i.

• Iᴬ and Iᴮ are codominant, and both are dominant over i.
• Group A = IᴬIᴬ or Iᴬi
• Group B = IᴮIᴮ or Iᴮi
• Group AB = IᴬIᴮ
• Group O = ii

Solution

• Man (AB): IᴬIᴮ, so his gametes are Iᴬ or Iᴮ.
• Woman (O): ii, so her gametes are i or i.

Punnett square:

| | i | i | |---|---|---| | Iᴬ | Iᴬi | Iᴬi | | Iᴮ | Iᴮi | Iᴮi |

Offspring: 50% Iᴬi (group A) and 50% Iᴮi (group B).

For a child to be group O, the genotype must be ii. The AB father has no i allele to pass on, so this cannot happen. The probability of group O is 0%.

Answer: A. 0%

Tip: An AB parent can never have a group O child, and an O parent can never have an AB child. In ABO questions, check which alleles each parent can pass on. If a parent has no i allele, a group O child is impossible.