Question
Explain the genetic basis of ABO blood groups, the Rh factor, and the rules for safe blood transfusion.
Solution — Step by Step
flowchart TD
A[ABO Blood Groups] --> B[Gene I with 3 alleles]
B --> C[IA - produces antigen A]
B --> D[IB - produces antigen B]
B --> E[i - no antigen]
C --> F[IA IA or IA i = Type A]
D --> G[IB IB or IB i = Type B]
C --> H[IA IB = Type AB - Codominance]
E --> I[ii = Type O]
F --> J[Has anti-B antibodies]
G --> K[Has anti-A antibodies]
H --> L[No antibodies - Universal recipient]
I --> M[Has anti-A and anti-B - Universal donor]Blood type is controlled by the gene I (isoagglutinin) with three alleles: I, I, and i. I and I are codominant to each other (both expressed simultaneously in II individuals = Type AB). Both I and I are dominant over i. Since humans are diploid, each person has two alleles — giving 6 possible genotypes but 4 phenotypes.
| Blood Group | Genotype | Antigen on RBCs | Antibodies in Plasma |
|---|---|---|---|
| A | II or Ii | A | Anti-B |
| B | II or Ii | B | Anti-A |
| AB | II | A and B | None |
| O | ii | None | Anti-A and Anti-B |
The antibodies are naturally present (not produced by prior exposure). They attack foreign antigens during mismatched transfusions, causing agglutination (clumping of RBCs).
Type O is the universal donor (no antigens to trigger a reaction) and Type AB is the universal recipient (no antibodies to attack donor blood). The Rh factor is determined by another gene: Rh individuals have the Rh antigen (D antigen) on their RBCs; Rh individuals lack it. An Rh person receiving Rh blood will develop anti-Rh antibodies — a second exposure causes severe agglutination.
Why This Works
ABO blood groups are a classic example of multiple allelism (3 alleles of one gene) and codominance (I and I are both fully expressed in heterozygotes). The transfusion rules follow directly from the antigen-antibody interaction: donor antigens must not match recipient antibodies.
Erythroblastosis fetalis: When an Rh mother carries an Rh baby (father is Rh), fetal Rh antigens may enter the mother’s blood during delivery, triggering anti-Rh antibody production. In the second Rh pregnancy, these maternal antibodies cross the placenta and destroy fetal RBCs. Prevention: inject anti-Rh antibodies (RhoGAM) to the mother after the first delivery.
Common Mistake
Students write that “Type O has no antigens and no antibodies.” Type O has no antigens on RBCs but has both anti-A and anti-B antibodies in plasma. This is why Type O individuals can donate to anyone (no antigens to trigger reactions) but can ONLY receive from Type O (their antibodies would attack A, B, or AB donor blood).