A woman with blood group A gives birth to a child with blood group O. Which of the following blood groups cannot belong to the father?

Analyze the question: The mother is blood group A and the child is group O. We need the blood group that the father cannot have.

Concept: ABO blood groups are controlled by three alleles: I^A, I^B and i. I^A and I^B are codominant, and i is recessive.

• Group A: I^A I^A or I^A i
• Group B: I^B I^B or I^B i
• Group AB: I^A I^B
• Group O: ii

Solution:

1. The child is group O, so the genotype is ii. The child must have received one i allele from each parent.
2. The mother is group A and gave an i, so her genotype is I^A i.
3. The father must also have an i allele to pass on. Check each group: • A (I^A i): possible, since he can pass on i. • B (I^B i): possible. • O (ii): possible. • AB (I^A I^B): has no i allele, so he can only pass on I^A or I^B. The child could never be group O.
4. Option C ("C") is not an ABO blood group at all. It is a distractor, and the question is clearly asking about valid ABO groups. Among the real groups, only AB is impossible.

Answer: AB cannot be the father's blood group, which is option D.

Tip: For a group O child, both parents must carry at least one i allele. Group AB is the only group with no i allele, so an AB parent can never have a group O child. Remember: "AB can't make O."