ABO blood group inheritance: Class 12 practice
Alleles IA and IB are codominant, and i is recessive. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Monohybrid crosses
The idea behind abo blood group inheritance
Alleles IA and IB are codominant, and i is recessive. Genotypes: A = IAIA or IAi, B = IBIB or IBi, AB = IAIB, O = ii. List each parent's gametes to find possible children.
Step-by-step method
- Write the parents' genotypes.
- Combine every pair of gametes.
- Translate genotypes to blood groups and count.
Worked example 1 (Basic)
Parents IAIB × IAIB. Which blood groups can the children have, and in what proportion?
- Gametes IA/IB × IA/IB
- A 1/4, B 1/4, AB 2/4
- Answer: A 1/4, B 1/4, AB 1/2
Worked example 2 (Standard)
Parents IBi × IBi. Which blood groups can the children have, and in what proportion?
- Gametes IB/i × IB/i
- B 3/4, O 1/4
- Answer: B 3/4, O 1/4
Worked example 3 (Challenge)
Parents IBi × IBIB. Which blood groups can the children have, and in what proportion?
- Gametes IB/i × IB/IB
- B 4/4
- Answer: B 1
Common mistakes and misconceptions
- Treating A and B as dominant over each other
- Forgetting O needs ii
Practice questions
- (Basic) Parents IBi × IAi. Which blood groups can the children have, and in what proportion?
- (Basic) Parents IAIB × ii. Which blood groups can the children have, and in what proportion?
- (Standard) Parents IAi × IAIA. Which blood groups can the children have, and in what proportion?
- (Standard) Parents IBIB × IBIB. Which blood groups can the children have, and in what proportion?
- (Challenge) Parents ii × ii. Which blood groups can the children have, and in what proportion?
- (Challenge) Parents IBi × IAIA. Which blood groups can the children have, and in what proportion?
Answer key with full working
- Q1: Gametes IB/i × IA/i → A 1/4, B 1/4, AB 1/4, O 1/4 → Answer: A 1/4, B 1/4, AB 1/4, O 1/4
- Q2: Gametes IA/IB × i/i → A 2/4, B 2/4 → Answer: A 1/2, B 1/2
- Q3: Gametes IA/i × IA/IA → A 4/4 → Answer: A 1
- Q4: Gametes IB/IB × IB/IB → B 4/4 → Answer: B 1
- Q5: Gametes i/i × i/i → O 4/4 → Answer: O 1
- Q6: Gametes IB/i × IA/IA → A 2/4, AB 2/4 → Answer: A 1/2, AB 1/2
Syllabus note
This skill is listed under "Principles of Inheritance" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is abo blood group inheritance in?
"Principles of Inheritance", Class 12 Biology (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.