In human ABO blood groups, the alleles I^A and I^B are codominant and the allele i is recessive. A father of blood group AB and a mother of blood group O have children. Which blood groups can their children have?
Answer and explanation
Correct answer: B. A or B only
- Step 1: The father (I^A I^B) passes on either I^A or I^B; the mother (ii) can only pass on i. Step 2. The children are therefore I^A i (group A) or I^B i (group B). Answer: A or B only. Why not the others: AB or O only repeats the parents' own groups, but a child cannot be AB because it receives only one allele from the father, and cannot be O because it always receives I^A or I^B from him. A, B, AB or O would need both parents to carry i and both to carry a dominant allele. A only ignores the I^B allele that the father can pass on.
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