A child has blood group O. Her mother has blood group A and her father has blood group B. What does this tell you about whether blood group O is dominant or recessive? Explain your reasoning.
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
Blood group O is recessive.
Reasoning: The mother (blood group A) must have genotype I^A i, and the father (blood group B) must have genotype I^B i. The child inherited one 'i' allele from each parent, giving genotype 'ii' (blood group O). Since O only appears when both copies are 'i', it is recessive — a single copy of I^A or I^B is enough to suppress it.
Explanation
- The key logic: if O were dominant, at least one parent would show blood group O — but neither does. So O must be recessive, hidden in both parents as a single copy (i).
- Examiners expect you to state the conclusion clearly and give the reasoning using the concept of dominant/recessive traits.
- You don't need to write full genotype notation, but it strengthens the answer.