Blood groups — ABO and Rh system, multiple alleles, and transfusion rules

Question

Explain the ABO blood group system as an example of multiple alleles and codominance. What are the transfusion rules? How is the Rh factor inherited, and what causes erythroblastosis fetalis?

(NCERT Class 12 — high-weightage for NEET)

Solution — Step by Step

The ABO blood group is controlled by gene I (isoagglutinin) on chromosome 9 with three alleles: $I^A$ , $I^B$ , and $i$ .

Blood group	Genotype	Antigen on RBC	Antibody in plasma
A	$I^A I^A$ or $I^A i$	A	Anti-B
B	$I^B I^B$ or $I^B i$	B	Anti-A
AB	$I^A I^B$	A and B	None
O	$ii$	None	Anti-A and Anti-B

$I^A$ and $I^B$ are codominant — both express equally in AB individuals. Both are dominant over $i$ .

The rule: donor’s antigens must NOT match recipient’s antibodies (or agglutination/clumping occurs).

Recipient	Can receive from
A	A, O
B	B, O
AB	A, B, AB, O (universal recipient)
O	O only (universal donor)

AB is the universal recipient because it has no antibodies. O is the universal donor because it has no antigens on RBCs.

In practice, matched blood is always preferred. “Universal donor” is used only in emergencies.

The Rh factor is determined by the presence (+) or absence (-) of the D antigen on RBCs. It’s controlled by a separate gene (dominant inheritance: Rh+ is dominant over Rh-).

Erythroblastosis fetalis occurs when:

Mother is Rh-negative, father is Rh-positive
First child is Rh-positive — during delivery, fetal Rh+ blood enters mother’s circulation
Mother produces anti-Rh antibodies (sensitisation)
In the second pregnancy with an Rh+ fetus, these antibodies cross the placenta and destroy fetal RBCs → severe anaemia, jaundice, even death

Prevention: Rho(D) immunoglobulin (anti-D injection) given to the mother after the first delivery to prevent sensitisation.

Why This Works

The ABO system is a textbook example of multiple allelism (3 alleles for one gene in the population) and codominance ( $I^A$ and $I^B$ both fully express). It also demonstrates that while a population can have multiple alleles, any individual carries only two alleles (one from each parent).

The transfusion logic is simple immunology: if the recipient’s plasma contains antibodies against the donor’s RBC antigens, those antibodies will attack the donor cells, causing clumping (agglutination) and potentially fatal transfusion reactions.

Alternative Method — Cross Problems

NEET loves blood group genetics crosses. Common question: “Parents have blood groups A and B. What blood groups are possible in children?”

If parents are $I^A i$ and $I^B i$ : children can be $I^A I^B$ (AB), $I^A i$ (A), $I^B i$ (B), or $ii$ (O). All four groups are possible!

If parents are $I^A I^A$ and $I^B I^B$ : all children will be $I^A I^B$ (AB). Only AB is possible.

Always determine the exact genotype before doing the cross.

Common Mistake

Students say “O blood group has no alleles” or “O is recessive to everything.” The correct statement: blood group O has the genotype $ii$ — it carries two copies of the recessive allele $i$ , which doesn’t produce any antigen. The allele exists, it just doesn’t code for an antigen. Also, erythroblastosis fetalis does NOT occur in the first pregnancy — the first Rh+ baby is usually unaffected. The danger is in the second (or subsequent) pregnancies.

Question

Solution — Step by Step

Why This Works

Alternative Method — Cross Problems

Common Mistake

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