Blood Typing and Applied Human Genetics: Study Notes

Blood Typing and Applied Human Genetics

Introduction

This study guide covers the principles of human blood typing, focusing on the ABO and Rh blood group systems, and explores the genetics underlying these traits. Understanding blood typing is essential for safe blood transfusions and for studying inheritance patterns in human genetics.

ABO Blood Group System

Overview of ABO Blood Groups

The ABO blood group system classifies human blood into four main types: A, B, AB, and O. These types are determined by the presence or absence of specific antigens on the surface of red blood cells and corresponding antibodies in the plasma.

• Antigen: A molecule present on the surface of red blood cells that can trigger an immune response.

• Antibody: A protein in the plasma that reacts against specific antigens.

• Importance: Correct blood typing is crucial for transfusions to prevent agglutination and hemolysis.

Blood Type Antigens and Antibodies

Each blood type is characterized by specific antigens and antibodies:

Blood Type	Antigens on RBC	Antibodies in Plasma	Compatible Blood Types (Emergency)	Genotype
A	A antigen	Anti-B	A, O	$I^A I^A$ or $I^A i$
B	B antigen	Anti-A	B, O	$I^B I^B$ or $I^B i$
AB	A and B antigens	None	A, B, AB, O (universal recipient)	$I^A I^B$
O	None	Anti-A and Anti-B	O (universal donor)	$i i$

Agglutination and Blood Typing

Agglutination is the clumping of red blood cells that occurs when antibodies bind to their corresponding antigens. This reaction is used in blood typing tests to determine an individual's blood group.

• Process: Blood is mixed with anti-A and anti-B sera. Agglutination indicates the presence of the corresponding antigen.

• Example: If agglutination occurs with anti-A serum, the blood type is A.

Genetics of ABO Blood Groups

Inheritance Patterns

The ABO blood group is inherited through multiple alleles: $I^A$, $I^B$, and $i$. The $I^A$ and $I^B$ alleles are codominant, while $i$ is recessive.

• Possible Genotypes:

  • Type A: $I^A I^A$ or $I^A i$

  • Type B: $I^B I^B$ or $I^B i$

  • Type AB: $I^A I^B$

  • Type O: $i i$

• Example: If both parents are type A ($I^A i$), their children could be type A or type O.

Rh Blood Group System

Overview of Rh Factor

The Rh blood group system is based on the presence (+) or absence (−) of the Rh antigen (also called D antigen) on red blood cells. The Rh factor is important in transfusions and pregnancy.

• Rh Positive (Rh+): Has Rh antigen; genotype is $RR$ or $Rr$.

• Rh Negative (Rh−): Lacks Rh antigen; genotype is $rr$.

• Antibodies: Rh-negative individuals can develop antibodies against Rh antigen if exposed.

Genetics of Rh Factor

The Rh factor is inherited as a simple dominant-recessive trait:

• Dominant allele: $R$ (Rh positive)

• Recessive allele: $r$ (Rh negative)

• Genotypes:

  • Rh positive: $RR$ or $Rr$

  • Rh negative: $rr$

Hemolytic Disease of the Newborn (HDN)

HDN occurs when an Rh-negative mother carries an Rh-positive fetus. If fetal blood mixes with maternal blood, the mother may develop antibodies against Rh antigen, which can attack fetal red blood cells in subsequent pregnancies.

• Cause: Maternal anti-Rh antibodies cross the placenta and destroy fetal Rh-positive red blood cells.

• Prevention: Administration of Rho(D) immune globulin to Rh-negative mothers.

• Example: Rh-negative mother ($rr$) and Rh-positive father ($RR$ or $Rr$) can have Rh-positive children.

Selected Human Traits and Genetics

Other Inherited Traits

In addition to blood groups, other human traits are inherited according to Mendelian genetics. Examples include earlobe attachment, tongue rolling, and camptodactyly (bent finger).

• Trait: Camptodactyly (bent finger) is inherited as a dominant trait.

• Genotype: $C$ (dominant) and $c$ (recessive).

• Example: Individuals with genotype $CC$ or $Cc$ will show the trait.

Review Questions

• Where are antigens located?

• Where are antibodies located?

• How does agglutination occur?

• What type of antigen and antibody does a person with A blood type have?

• What are the possible blood types (genotype and phenotype) of children with parents of various blood types?

• What are the implications of Rh factor in pregnancy?

Summary Table: ABO and Rh Blood Groups

Blood Group	Antigen on RBC	Antibody in Plasma	Genotype	Rh Factor
A	A	Anti-B	$I^A I^A$, $I^A i$	Rh+ ($RR$, $Rr$), Rh− ($rr$)
B	B	Anti-A	$I^B I^B$, $I^B i$	Rh+ ($RR$, $Rr$), Rh− ($rr$)
AB	A and B	None	$I^A I^B$	Rh+ ($RR$, $Rr$), Rh− ($rr$)
O	None	Anti-A and Anti-B	$i i$	Rh+ ($RR$, $Rr$), Rh− ($rr$)

Additional info: The notes have been expanded to include definitions, inheritance patterns, and clinical relevance for exam preparation.