Explain how the ABO blood groups are inherited in humans. Why is group O called the universal donor?

How to lay out the answer. Follow the frame, then write it in your own words.

1 · State the principle. Blood group depends on a pair of genes, one from each parent.
2 · Name the genes. IA and IB are dominant; i is recessive.
3 · Give the gene pairs.
- IA IA or IA i: group A.
- IB IB or IB i: group B.
- IA IB: group AB.
- ii: group O.
4 · Universal donor. O blood can be given to A, B, AB and O, so O is the universal donor; AB, which receives from all, is the universal recipient.
5 · Real life. Blood is matched before every transfusion; O negative is used when there is no time to test.
6 · Close in one line. So a child's blood group can differ from both parents' groups.

Scoring tip: Write the four gene pairs as a small table; it earns the marks quickly.