Genetics in Immunohematology – ABO & Rh Systems

Genetics in Immunohematology – ABO & Rh Systems

1. Basic Genetics Definitions

  • Gene: A segment of DNA that codes for a specific protein.
  • Allele: Different forms of a gene (e.g., A, B, O).
  • Genotype: The genetic makeup of an individual (e.g., AO, BB).
  • Phenotype: The observable trait that results from the genotype (e.g., blood type A, B, O).
  • Homozygous: An organism with two identical alleles (e.g., AA, BB).
  • Heterozygous: An organism with two different alleles (e.g., AO, BO).
  • Dominant: An allele that is expressed in the phenotype when present.
  • Recessive: An allele that is only expressed in the phenotype when two copies are present.
  • Codominant: A situation where both alleles are expressed equally (e.g., both A and B).

2. ABO Blood Group Genetics

  • Gene Control: The ABO blood group is controlled by the ABO gene located on chromosome 9.
  • Alleles: There are three main alleles involved: A, B, and O.
  • Dominance Relation: A and B are codominant with each other, while O is recessive.
  • Function of Alleles:
    • A: Adds N-acetylgalactosamine to the H antigen, modifying it.
    • B: Adds galactose to the H antigen, also modifying it.
    • O: Has no functional enzyme so the H antigen remains unchanged.

3. ABO Genotype to Phenotype Mapping

  • Genotype to Phenotype Table:
    • AA or AO: Type A
    • BB or BO: Type B
    • AB: Type AB
    • OO: Type O

4. H Antigen (VERY HIGH YIELD)

  • Definition: The H antigen is a precursor substance required for the expression of A and B antigens.
  • Bombay Phenotype:
    • Individuals with the Bombay phenotype lack the H antigen.
    • Such individuals are unable to form A or B antigens even if the genes for them are present.

5. Rh Blood Group Genetics

  • Gene Control: The Rh blood group is controlled by the RHD and RHCE genes found on chromosome 1.
  • D Antigen: Determines whether a person is Rh positive or negative.
    • Presence of RHD gene: Indicates Rh positive.
    • Absence of RHD gene: Indicates Rh negative.
  • Other Antigens: The Rh system includes other antigens such as C, c, E, and e.

6. Rh Inheritance

  • Inheritance Mode: Rh blood group is inherited as haplotypes (e.g., DCe, dce).
  • Contribution from Parents: One haplotype is received from each parent.
  • Dominance Relation: The D allele is dominant over the absence of it (denoted as d).
  • Genotypes:
    • Rh+ can either be DD or Dd.
    • Rh- genotype is expressed as dd.

7. Example Cross (Rh)

  • Parent Genotypes:
    • Parent 1: Dd (Rh positive)
    • Parent 2: dd (Rh negative)
  • Offspring Genotype Results:
    • 50% Rh positive (Dd)
    • 50% Rh negative (dd)

8. Clinical Importance

  • ABO Incompatibility: Can lead to severe immediate hemolysis.
  • Rh Incompatibility: Can result in Hemolytic Disease of the Fetus and Newborn (HDFN).
  • Predictive Value of Genetics: Understanding genetics is crucial for predicting inheritance patterns.
  • Applications: Important in paternity testing and ensuring transfusion safety.

9. High-Yield Exam Points

  • Codominance and Recessiveness: A and B alleles are codominant while O is recessive.
  • Universal Recipients and Donors:
    • AB blood type is considered the universal recipient.
    • O blood type is regarded as the universal donor (specifically for red blood cells).
  • Importance of H Antigen: Necessary for the expression of A and B blood group antigens.
  • Bombay Phenotype Implications: Individuals with the Bombay phenotype lack the H antigen and are excluded from the standard ABO classification.
  • Significance of Rh D Antigen: The Rh D antigen is the most important out of the Rh antigens when considering blood transfusions.
  • Antibody Formation: Rh antibodies typically only form after exposure to Rh positive blood in Rh negative individuals.