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 to find which blood group the father cannot have.

Concept: ABO blood groups are controlled by three alleles: I^A, I^B and i (O). I^A and I^B are dominant over i. A child gets one allele from each parent.

• 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. One i allele came from the mother and one from the father.
2. The mother is group A and has passed on an i, so her genotype must be I^A i.
3. The father must also have an i allele to give. Check each option: • B (I^B i): can give i, so he is possible. • A (I^A i): can give i, so he is possible. • AB (I^A I^B): has no i allele, so he can only pass on I^A or I^B. He cannot have a group O child. • O (ii): can give i, so he is possible.

Cross example: I^A i × I^A I^B gives I^A I^A, I^A I^B, I^A i and I^B i. These are groups A, AB, A and B, and there is never an O child.

Answer: C (AB)

Tip: A group AB parent can never have a group O child, and a group O parent can never have an AB child. For "cannot be the parent" questions, look for the group that lacks the needed allele. The child's O means both parents must carry at least one i.