Blood typing plays a crucial role in medical diagnostics,
transfusions, and genetics. Among the most important systems for classifying
blood are the ABO and Rh systems. Let’s explore how they work, their genetic
basis, and why understanding them is vital for healthcare and parentage
analysis.
The ABO Blood Group System
Discovery and Basis
The ABO blood group system was discovered by Karl
Landsteiner in 1901. It is based on the presence or absence of specific
antigens on the surface of red blood cells (RBCs) and antibodies in the plasma.
- Antigens
are substances that trigger immune responses.
- Antibodies
are proteins produced by the immune system in response to foreign
antigens.
Red blood cells carry two types of antigens: A and B.
The blood plasma contains antibodies against the antigens not found on a
person's own RBCs.
Blood Group Types
There are four major ABO blood types, determined by which
antigens are present on the RBCs and which antibodies are in the plasma:
|
Blood Group |
Antigen on RBC |
Antibody in Plasma |
|
A |
A |
Anti-B |
|
B |
B |
Anti-A |
|
AB |
A and B |
None |
|
O |
None |
Anti-A and Anti-B |
When incompatible blood types are mixed, antibodies may
attack the foreign antigens, leading to agglutination (clumping of
RBCs), which can be dangerous or fatal.
Genetic Inheritance of ABO Blood Groups
The Gene Behind ABO Typing
The ABO blood type is controlled by a single gene located on
chromosome 9, known as the I gene (isohaemagglutinogen). There are three
alleles:
- IA:
codes for A antigen
- IB:
codes for B antigen
- i:
does not code for any antigen
The IA and IB alleles are co-dominant, meaning both
can be expressed if present together. The i allele is recessive to both.
Possible Genotypes and Phenotypes
|
Genotype |
Blood Group |
|
IAIA or IAi |
A |
|
IBIB or IBi |
B |
|
IAIB |
AB |
|
ii |
O |
An individual's blood type remains the same throughout life.
Importance of Blood Typing
1. Safe Blood Transfusions
Matching blood types is essential during transfusions. A
mismatch can cause a severe immune response.
- Type
O is a universal donor (no antigens on RBCs).
- Type
AB is a universal recipient (no antibodies in plasma).
2. Determining Paternity
Since blood type is inherited, it can be used to support or
rule out biological relationships between parents and children.
Predicting Children’s Blood Types:
Genetic Crosses
Scenario: Father is Type A (IAi),
Mother is Type B (IBi)
A Punnett square shows the possible combinations:
|
IB |
i |
|
|
IA |
AB |
A |
|
i |
B |
O |
Possible blood groups for the children:
A, B, AB, O
Case Study: Type B Mother and Type O
Child
- Mother
(Type B): Genotype must be IBi (to pass on
the i allele)
- Child
(Type O): Genotype is ii
For a child to have type O blood, both parents must carry
the i allele. Therefore, the father cannot be type A (IAIA) as he would
not carry the i allele. The father must be either:
- Type
B (IBi)
- Type
O (ii)
The Rh Factor: An Additional Layer
What is Rh Factor?
The Rh system, discovered in the 1930s using Rhesus monkeys,
is another important blood group classification. It’s determined by the
presence of the D antigen on red blood cells:
- Rh-positive
(Rh⁺):
D antigen present
- Rh-negative
(Rh⁻):
D antigen absent
Unlike ABO, Rh antibodies are not naturally present. They
are only produced if an Rh-negative person is exposed to Rh-positive blood.
Genetics of the Rh Factor
The D antigen is encoded by the D gene, which has two
alleles:
- D
(dominant): Rh-positive
- d
(recessive): Rh-negative
|
Genotype |
Rh Type |
|
DD or Dd |
Rh⁺ |
|
dd |
Rh⁻ |
Rh Factor and Blood Transfusion
Rh incompatibility in transfusion is dangerous. An
Rh-negative person should not receive Rh-positive blood, as it can lead to a
life-threatening immune reaction.
Rh Incompatibility in Pregnancy
What is Erythroblastosis Fetalis?
If an Rh-negative mother carries an Rh-positive baby, her
immune system may produce anti-Rh antibodies that attack the baby's red blood
cells—especially in subsequent pregnancies.
Complications can include:
- Severe
anemia
- Jaundice
- Brain
damage
- Stillbirth
Prevention
To prevent this, Rh-negative pregnant women are given Rh
immunoglobulin (Rho(D) immune globulin):
- Typically
administered during the second trimester or within 72 hours of delivery
- Neutralizes
fetal Rh-positive cells before the mother's immune system reacts
Practical Examples
Case 1: Rh-negative Woman × Rh-positive
Man (whose father is Rh-negative)
- Woman:
dd
- Man:
Dd (heterozygous, since his father is dd)
Possible child genotypes: Dd (Rh⁺)
or dd (Rh⁻)
Chance of Rh-positive child: 50%
Case 2: Two Heterozygous Rh-positive
Parents Have Three Rh-negative Children
Each parent is Dd. A Punnett square shows:
|
D |
d |
|
|
D |
DD |
Dd |
|
d |
Dd |
dd |
25% chance of Rh-negative (dd) child per
pregnancy.
Though statistically uncommon, it is entirely possible for three children in a
row to be Rh-negative.
Final Thoughts
Understanding the ABO and Rh blood group systems is vital in
medicine and genetics. From ensuring safe blood transfusions to managing
pregnancies, these systems highlight the importance of genetics in everyday
healthcare.
By mastering these concepts, students, medical
professionals, and curious minds alike gain deeper insight into how our bodies
function and how modern medicine applies this knowledge to save lives.
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