Multiple Alleles and ABO Blood Grouping Explained

30 September 2026Biology Try the interactive 3D model →

Multiple alleles means that a single gene has more than two possible versions (alleles) in a population, even though one person carries only two copies of that gene. The ABO blood group system is the best example: the gene I on chromosome 9 has three alleles — IA, IB and i. Your two alleles together decide whether you have blood group A, B, AB or O.

Blood grouping matters a lot in medicine. During a blood transfusion, if the receiver's antibodies attack the donor's red blood cells, dangerous clumping can happen and even kill the patient. Knowing the ABO group also helps in organ transplants, pregnancy care and forensic work like paternity testing.

Key Concepts

The ABO gene controls which sugar antigens sit on the surface of red blood cells. Allele IA makes antigen A, allele IB makes antigen B, and allele i makes no antigen. IA and IB are both dominant over i, and they are codominant with each other, meaning both show up together when present.

Genotype: the two alleles a person has, like IAIB or ii
Phenotype: the visible blood group, like AB or O
Antigen: marker on red blood cells
Antibody: a protein in plasma that attacks foreign antigens. Group A people have anti-B antibodies, group B have anti-A, group O have both, and group AB have neither

There are 6 possible genotypes: IAIA, IAi, IBIB, IBi, IAIB and ii, but only 4 phenotypes.

How It Works

Follow these steps to find a child's blood group. First, write down each parent's genotype. Second, list the alleles each parent can pass in a gamete (egg or sperm). Third, combine them in a Punnett square. Fourth, read the phenotype from each combination.

A parent with group A could be IAIA (passes only IA) or IAi (passes IA or i with equal chance). This is why two group A parents can sometimes have a group O child.

Worked Example

A mother has blood group A with genotype IAi. A father has blood group B with genotype IBi. What groups can their children have?

Step 1: Mother's gametes are IA and i. Father's gametes are IB and i.

Step 2: Combine them: IAIB (group AB), IAi (group A), IBi (group B), and ii (group O).

Step 3: All four combinations are equally likely, so each child has a 25% chance of AB, 25% A, 25% B and 25% O. One couple can have children with four different blood groups.

Common Misconceptions

Misconception: Group O people cannot receive any blood. Correction: They can receive only O blood, but they can donate to everyone, so O negative is called the universal donor.
Misconception: A child always has a blood group matching one parent exactly. Correction: Two A parents with genotype IAi can have an O child (ii).
Misconception: AB is a 'mixed' weak group. Correction: AB shows codominance — both antigens fully appear, and AB people are universal receivers because they make no anti-A or anti-B antibodies.

Real-World Applications

Blood banks type every unit of donated blood before storage. Hospitals cross-match donor and patient blood before transfusion. In pregnancy, doctors watch for Rh incompatibility when an Rh-negative mother carries an Rh-positive baby. Police and courts use ABO typing along with DNA tests in identity and paternity cases. Population studies also use ABO frequencies to trace human migration history.

In short, multiple alleles expand the choices for one gene, and the ABO system shows this beautifully — three alleles, six genotypes, four blood groups that save lives every day.