ABO Blood Group Inheritance and Codominance | Class 10

Quick answer: Human ABO blood groups are controlled by one gene with three alleles: $I^A$, $I^B$ and $i$. $I^A$ and $I^B$ are both dominant over $i$, and $I^A$ and $I^B$ are codominant (both show when present together). So group A is $I^AI^A$ or $I^Ai$, group B is $I^BI^B$ or $I^Bi$, group AB is $I^AI^B$, and group O is $ii$. Parents with groups A ($I^Ai$) and B ($I^Bi$) can have children of all four groups.

Syllabus note: The rationalised NCERT Class 10 Heredity chapter uses blood groups only in a short question about dominance. Multiple alleles and codominance are taught in Class 12 Biology. This article is enrichment that helps with Class 10 questions on dominant and recessive traits.

The gene and its three alleles

The ABO blood group is decided by which antigens (markers) are present on the red blood cells. These are controlled by one gene (called $I$), which has three alleles:

Each person has two alleles of this gene, one from each parent.

How the alleles interact

• $I^A$ is dominant over $i$, so $I^Ai$ gives group A.
• $I^B$ is dominant over $i$, so $I^Bi$ gives group B.
• $I^A$ and $I^B$ are codominant: in $I^AI^B$ both antigens are produced, giving group AB.
• $i$ is recessive: group O appears only as $ii$.

Genotypes and blood groups

So there are 6 genotypes but only 4 blood groups.

Problem 1: parents A and B, children of all four groups

A man with blood group A and a woman with blood group B have children with groups A, B, AB and O. Find the parents' genotypes.

Step 1: Use the group O child. A group O child is $ii$, so it received one $i$ from each parent.

Step 2: Find the parents' genotypes. The father is group A and carries $i$, so he is $I^Ai$. The mother is group B and carries $i$, so she is $I^Bi$.

Step 3: Punnett square.

Answer: Father $I^Ai$, mother $I^Bi$. Each child has a 1 in 4 chance of being A, B, AB or O.

Problem 2: can AB and O parents have a group O child?

Step 1: Genotypes. Group AB is $I^AI^B$; group O is $ii$.

Step 2: Gametes. The father gives $I^A$ or $I^B$; the mother gives only $i$.

Step 3: Children. $I^Ai$ (group A) or $I^Bi$ (group B), each with probability ½.

Answer: No. This couple cannot have a child with group O or group AB; their children can only be A or B.

Link to the NCERT question

NCERT asks: a man with blood group A marries a woman with group O, and their daughter has group O. Is this enough to tell which trait is dominant? No. The daughter could be O because O is recessive and the father is $I^Ai$; one family is not enough evidence on its own to decide which trait is dominant.

Summary table

Exam tips

• Write alleles as $I^A$, $I^B$ and $i$, not just A, B and O.
• In parentage questions, start with any group O child: it tells you each parent carries $i$.
• Always draw a Punnett square and list the genotypes and blood groups of the offspring.
• Remember that "1 in 4" is a probability for each child, not a fixed count in a family.

Common mistakes

• Saying group AB shows blending or incomplete dominance. It is codominance: both antigens are fully produced.
• Writing group A as only $I^AI^A$; it can also be $I^Ai$.
• Thinking AB parents can have an O child. They have no $i$ allele to pass on.
• Confusing antigens (on red cells) with antibodies (in plasma).

Frequently Asked Questions

How many alleles control the ABO blood group?

Three alleles control it: Iᴬ, Iᴮ and i. Each person carries two of them.

What is codominance in blood groups?

In a person with genotype IᴬIᴮ, both alleles are expressed fully, so the red cells carry both A and B antigens. This gives blood group AB.

Can parents with blood groups A and B have a child with blood group O?

Yes, if both parents are heterozygous (Iᴬi and Iᴮi). The child can then receive i from each parent and have genotype ii.

Can a child with blood group O be born to AB and O parents?

No. The AB parent passes on only Iᴬ or Iᴮ, so every child will be group A or group B.

Are blood group genetics in the Class 10 syllabus?

Class 10 NCERT uses blood groups only in a question on dominance. Multiple alleles and codominance are covered in Class 12 Biology.