Biology · Principles of Inheritance and Variation · NEET
A "multiple allele" means one gene has more than two allele forms in the population. The ABO blood group gene, called gene I, has three alleles: IA, IB and i. Even though the gene has three alleles, one person still carries only two of them (one from each parent), because we are diploid. Memory hook: "Gene I = 3 keys (IA, IB, i), but each person holds only 2 keys."
Gene I has three alleles (IA, IB, i) making four blood groups. IA and IB are codominant in AB, i is recessive giving O. Six genotypes collapse into four phenotypes, and each diploid person holds only two of the three alleles.
Your doubts, answered
What are multiple alleles in simple words?
Multiple alleles means a single gene has MORE THAN TWO different allele forms present in the whole population. Normally we study a gene with just two alleles (like T and t). But the ABO blood group gene, gene I, has THREE alleles: IA, IB and i. This is why it is a classic example of multiple alleles for NEET.
How many alleles does the ABO gene have, and how many does one person have?
The gene I has THREE alleles in the population: IA, IB and i. But any ONE person carries only TWO of these three, because humans are diploid (one allele comes from the father, one from the mother). So the gene has 3 alleles, but a single individual has only 2. NEET repeatedly tests this exact point.
What does each allele (IA, IB, i) actually make?
Each allele codes for an enzyme that adds a sugar to the red blood cell surface. IA makes the A sugar (A antigen). IB makes a slightly different B sugar (B antigen). The i allele makes NO sugar at all. The type of sugar on the RBC decides the blood group.
Why is ABO blood group both multiple alleles AND codominance?
It is multiple alleles because the gene has three alleles (IA, IB, i). It shows codominance because when IA and IB are together (genotype IAIB), BOTH express their own sugar, giving blood group AB. It also shows dominance because IA and IB are each completely dominant over i. So ABO shows dominance, codominance and multiple alleles together, but NOT incomplete dominance and NOT polygenic inheritance.
How do three alleles give four blood groups?
Combine the alleles two at a time. IAIA or IAi give group A. IBIB or IBi give group B. IAIB gives group AB (codominance). ii gives group O (no sugar). So 3 alleles make 6 genotypes but only 4 phenotypes (A, B, AB, O). This genotype-to-phenotype jump is a favourite NEET trap.
Why is O the group when a person has ii?
The i allele makes no sugar. So genotype ii puts no A or B antigen on the red blood cells, which is blood group O. Because i is recessive, an O person can only be ii. That is why an O child (ii) needs each parent to carry at least one i allele.
⚠️ The NEET trap ✗ When IA and IB are present together, they express the same type of sugar. ✓ When IA and IB are together (IAIB), they each express their OWN, different sugar, so the RBC carries BOTH A and B antigens. This is codominance, not blending. Also remember: gene I has 3 alleles in the population but each person has only 2. 🧠 Codominance = both partners speak, neither is silenced. IAIB shows A sugar AND B sugar, never one mixed sugar.
Real NEET questions
NEET 2018
Which of the following characteristics represent 'Inheritance of blood groups' in humans?
a. Dominance
b. Co-dominance
c. Multiple allele
d. Incomplete dominance
e. Polygenic inheritance
A · b, d and e
B · a, b and c ✓
C · b, c and e
D · a, c and e
Solution: ABO inheritance shows dominance (IA and IB are dominant over i), co-dominance (IA and IB together both express their sugars in AB) and multiple alleles (gene I has three alleles IA, IB, i). It does NOT show incomplete dominance or polygenic inheritance. So a, b and c is correct.
NEET 2020
Identify the wrong statement with reference to the gene 'I' that controls ABO blood groups.
A · When IA and IB 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: When IA and IB are together they each express their OWN (different) sugar due to codominance, so the RBC carries both A and B sugars, not the 'same' sugar. That statement is wrong. The other three are correct: i makes no sugar, gene I has three alleles, and a diploid person carries only two of the three.
NEET 2026
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 is IAi and heterozygous B is IBi. Cross IAi x IBi gives 1 IAIB (AB) : 1 IAi (A) : 1 IBi (B) : 1 ii (O). The ii (O group) class is 1 out of 4, so the probability of an O child is 25%.
Solved Principles of Inheritance and Variation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is ABO blood group an example of multiple alleles or codominance?
Both. It is multiple alleles because gene I has three alleles (IA, IB, i), and it is codominance because IAIB expresses both A and B antigens. It also shows dominance of IA and IB over i.
How many alleles and how many genotypes are in the ABO system?
Three alleles (IA, IB, i) give six genotypes: IAIA, IAi, IBIB, IBi, IAIB, ii. These six genotypes make only four phenotypes: A, B, AB and O.
Why does one person have only two alleles if the gene has three?
Because humans are diploid. Each person gets one allele from each parent, so out of the three possible alleles a person carries only two.
What is the genotype of blood group O?
Only ii. The i allele makes no sugar and is recessive, so an O person must be homozygous ii.
Does ABO blood group show incomplete dominance?
No. IAIB does NOT give a blended intermediate group; it gives a distinct AB group where both sugars appear. That is codominance, not incomplete dominance. NEET often uses this as a trap.
