Inheritance of ABO and Rh blood groups
Blood group depends on a pair of genes, one from each parent. IA and IB are dominant over i; Rh+ is dominant over Rh−.
How a blood group is inherited
Every person has one of four blood groups: A, B, AB or O. The group is decided by a pair of genes, one received from each parent. Three genes take part: IA, IB and i. When a foetus (the baby growing in the mother's womb) forms, it gets any two of these, one from each parent. IA and IB are both dominant over i, and i is recessive. When IA and IB come together, both show, giving group AB.
Gene pairs and blood groups
| Gene pair | Blood group |
|---|---|
| IA IA or IA i | A |
| IB IB or IB i | B |
| IA IB | AB |
| ii | O |
Activity: work out the Rh group from gene pairs
Besides the ABO group, everyone is either Rhesus positive (Rh+) or Rhesus negative (Rh−). Of the two Rh genes, Rh+ is dominant and Rh− is recessive.
- 1Gene pair Rh+ Rh+: both genes are positive, so the person is Rh positive.
- 2★Gene pair Rh+ Rh−: the dominant Rh+ gene shows, so the person is Rh positive; the Rh− gene is hidden.
- 3★Gene pair Rh− Rh−: no dominant gene is present, so the person is Rh negative.
- 4Conclusion: a person is Rh negative only with two Rh− genes, which is why most people are Rh positive.
★ key step
Who can give blood to whom (ABO groups)
| Blood group | Can give blood to | Can receive blood from |
|---|---|---|
| A | A and AB | A and O |
| B | B and AB | B and O |
| AB | AB only | A, B, AB and O (universal recipient) |
| O | A, B, AB and O (universal donor) | O only |
Worked example: a child's possible blood groups
A father has the gene pair IA i (group A) and a mother has IB i (group B). Which blood groups can their children have?
- Father's gametes: IA or i. Mother's gametes: IB or i.
- Possible pairs: IA IB, IA i, IB i, ii.
- Groups: IA IB = AB; IA i = A; IB i = B; ii = O.
- Each pair is equally likely, so each has a chance of 1 in 4.
A, B, AB or O, each with a chance of 1 in 4 in every birth
Why you should know your blood group
After an accident or during an operation, a person may need a blood transfusion (blood given from a donor). Only a matching group can be given. Knowing your group saves time. If there is no time to test, O negative blood is the safest to give. The answer key in your NIOS lesson says O positive, but O negative is correct: a patient who is Rh negative must not get Rh positive blood. All humans fall into the same four blood groups, whatever their caste, creed or gender. Science gives no basis for dividing people in those ways.
Blood banks and hospitals in India test the ABO and Rh group of every donor and patient before a transfusion. Knowing your own group, and keeping it written in your wallet or phone, can save time in an emergency.
1 marks: “What is the blood group of a person with the gene pair IB i?” Group B, because IB is dominant over i.
2 marks: “Why is group O called the universal donor and group AB the universal recipient?” O blood can be given to people of all four groups. A person of group AB can receive blood from all four groups.
2 marks: “Both parents have blood group A, but their child has group O. Explain how, using gene symbols.” Both parents are IA i. Each passes the recessive gene i to the child, who is ii, group O. The chance is 1 in 4.
Capital I wins over small i; IA and IB share the stage (AB); ii is O. O gives to all, AB takes from all.