ABO blood groups are controlled by ONE gene 'I' that has THREE alleles: I^A, I^B and i. I^A and I^B are codominant (both show), and both are dominant over i. So the four blood groups come from six genotypes: A = I^A I^A or I^A i; B = I^B I^B or I^B i; AB = I^A I^B; O = i i. Memory hook: "I has 3 alleles, YOU carry only 2" — one from each parent.
The single gene 'I' has three alleles (I^A, I^B, i). I^A and I^B are codominant and both dominate i, giving six genotypes that map to four blood groups (A, B, AB, O) and their RBC antigens.
Your doubts, answered
How many alleles does the gene 'I' have, and how many can one person carry?
The gene 'I' has THREE alleles in the population: I^A, I^B and i (this is multiple allelism). But any single person carries only TWO of them — one allele from the mother and one from the father. NEET 2020 tested exactly this: the statement 'a person will have only two of the three alleles' is CORRECT.
What are the genotypes for each blood group?
Group A = I^A I^A or I^A i. Group B = I^B I^B or I^B i. Group AB = I^A I^B (only one genotype). Group O = i i (only one genotype). So four phenotypes come from six genotypes. A and B each have two genotypes (one homozygous, one heterozygous).
Why are I^A and I^B called codominant and not just dominant?
In an AB person both alleles express their OWN antigen: I^A makes antigen A and I^B makes antigen B, and BOTH appear on the RBC. Neither hides the other, so this is codominance. If it were simple dominance, only one antigen would show. NEET 2020 marked 'when I^A and I^B are present together they express same type of sugar' as WRONG — they express two DIFFERENT sugars/antigens.
Can two blood-group-A parents ever have an O child?
Yes, if BOTH parents are heterozygous A (I^A i). The cross I^A i × I^A i gives I^A I^A, I^A i, I^A i and i i — so 1 in 4 (25%) children can be O group (ii). This surprises students because both parents 'look' like A. The i allele is hidden (recessive) in the parents but can pair up in the child.
A parent shows blood group A but child is O — is this possible?
Yes. The A parent must be heterozygous I^A i and pass the i allele. If the other parent also passes an i, the child is i i (O group). NEET 2024 used this idea: a B+ father and A+ mother can have an O child only if both are heterozygous (I^B i and I^A i), so the child is ii.
What does allele i do?
Allele i produces NO sugar and NO antigen. That is why O group RBCs have neither antigen A nor antigen B. Because i makes nothing, it behaves as recessive — it is hidden whenever I^A or I^B is present.
⚠️ The NEET trap ✗ AB blood group shows dominance, so one antigen masks the other. ✓ AB shows CODOMINANCE — I^A and I^B are expressed together, so BOTH antigen A and antigen B appear on the RBC. Neither masks the other. 🧠 AB = A-and-B both visible = codominance, never dominance. O = ii = no antigen at all.
Real NEET questions
NEET 2017
The genotypes of a Husband and Wife are I^A I^B and I^A i. Among the blood types of their children, how many different genotypes and phenotypes are possible?
A · 3 genotypes ; 3 phenotypes
B · 3 genotypes ; 4 phenotypes
C · 4 genotypes ; 3 phenotypes ✓
D · 4 genotypes ; 4 phenotypes
Solution: Cross I^A I^B × I^A i gives offspring I^A I^A, I^A i, I^A I^B and I^B i — that is 4 different genotypes. Their phenotypes are A (from I^A I^A and I^A i), AB (from I^A I^B) and B (from I^B i) — only 3 phenotypes. So 4 genotypes and 3 phenotypes.
NEET 2026 (1)
What is the probability of having children with 'O' blood group, where both mother and father are heterozygous for 'A' and 'B' blood group, respectively?
A · 25% ✓
B · 0%
C · 75%
D · 50%
Solution: Heterozygous A = I^A i and heterozygous B = I^B i. The cross I^A i × I^B i gives I^A I^B (AB), I^A i (A), I^B i (B) and i i (O) in 1:1:1:1. Only the ii child is O group, i.e. 1/4 = 25%.
NEET 2020
Identify the wrong statement with reference to the gene 'I' that controls ABO blood groups.
A · When I^A and I^B are present together, they express same type of sugar ✓
B · Allele 'i' does not produce any sugar
C · The gene (I) has three alleles
D · A person will have only two of the three alleles
Solution: This is the WRONG statement. I^A and I^B are codominant; in an AB individual each allele expresses its OWN distinct sugar (antigen A and antigen B), not the same sugar. The other three statements are all correct: i produces no sugar, gene I has three alleles, and any person carries only two of them.
Solved Body Fluids and Circulation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A single gene called 'I' controls ABO blood groups. It is an example of multiple allelism (three alleles) and codominance (I^A and I^B).
How many genotypes and phenotypes are there in ABO system?
There are SIX genotypes (I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B, i i) and FOUR phenotypes (A, B, AB, O).
Is O blood group dominant or recessive?
O group genotype is i i, which is homozygous recessive. The i allele makes no antigen and is hidden whenever I^A or I^B is present.
Why can heterozygous parents give a different blood group child?
Because heterozygous parents (like I^A i or I^B i) carry a hidden i allele. When both pass on their alleles, new combinations such as ii (O) or I^A I^B (AB) can appear in the child.
Is ABO blood grouping in the NEET syllabus?
Yes. It is in Class 11 Chapter 'Body Fluids and Circulation' and is also a classic example of codominance and multiple allelism in Genetics — so it is tested from two chapters.
