Question
Explain the genetic basis of ABO blood groups. How are blood groups inherited? What is the Rh factor? Describe the transfusion compatibility rules.
Solution — Step by Step
Blood group is determined by the gene I (isoagglutinin) on chromosome 9. This gene has three alleles: , , and . Since each person has two copies, there are six genotypes producing four phenotypes:
| Blood Group | Genotype | Antigens on RBC | Antibodies in Plasma |
|---|---|---|---|
| A | or | A antigen | Anti-B |
| B | or | B antigen | Anti-A |
| AB | A and B antigens | None | |
| O | No antigens | Anti-A and Anti-B |
and are codominant to each other (both expressed in AB). Both are dominant over .
The Rh factor is determined by the presence or absence of the Rh antigen (D antigen) on RBCs. Controlled by the RhD gene on chromosome 1. Rh-positive (Rh⁺) is dominant over Rh-negative (Rh⁻).
About 85% of the population is Rh-positive. Rh incompatibility matters most in pregnancy: if an Rh⁻ mother carries an Rh⁺ fetus, her immune system may produce anti-Rh antibodies — causing erythroblastosis fetalis (haemolytic disease of the newborn) in subsequent pregnancies.
The rule is simple: donor’s antigens must not match recipient’s antibodies.
| Recipient | Can Receive From |
|---|---|
| A (has Anti-B) | A, O |
| B (has Anti-A) | B, O |
| AB (no antibodies) | A, B, AB, O (universal acceptor) |
| O (has Anti-A, Anti-B) | O only (universal donor) |
flowchart TD
A[Blood Group Compatibility] --> B{Recipient group?}
B -->|Group A| C[Can receive A or O]
B -->|Group B| D[Can receive B or O]
B -->|Group AB| E[Can receive A, B, AB, O]
B -->|Group O| F[Can receive O only]
E --> G[Universal Acceptor]
F --> H[Universal Donor is O]Why This Works
The ABO system is an example of multiple allelism (3 alleles for one gene) and codominance ( and are expressed equally in the AB blood group). Transfusion rules follow antigen-antibody logic: if the donor has antigen X and the recipient has antibody anti-X, agglutination (clumping) occurs — which is fatal.
Common Mistake
Students say “O blood group has no antigens AND no antibodies.” Wrong — O has no antigens on RBCs but has both Anti-A and Anti-B antibodies in plasma. This is why O can donate to all (no antigens to trigger reaction) but can only receive from O (any donor antigen would react with O’s antibodies).
For NEET genetics problems: if both parents are heterozygous (), the children can have ALL four blood groups (A, B, AB, O) — with probability 1/4 each. This cross is a classic exam question.