Haematology: Blood Typing and Genetics

Blood Genetics and Inheritance

• The human ABO gene is located specifically on chromosome 9.

• Every individual possesses two copies of chromosome 9, meaning they have two ABO genes.

• One copy of the gene is inherited from the biological mother, and the other copy is inherited from the biological father.

• The blood type gene expresses multiple allelism, meaning there are three versions (called ‐alleles‐) of this gene:

  • A ($I^A$)

  • B ($I^B$)

  • O ($i$)

• A person’s specific blood type is determined by the specific combination of alleles they inherit from their parents.

Phenotype and Genotype

• Genotype: Refers to the genetic makeup of an organism. In the context of blood typing, it refers to the A, B, and O allele combination a person carries.

• Phenotype: Refers to the visible or observable physical properties of an organism. In this context, the phenotype is the individual's actual blood type.

Dominance Patterns and Allelic Combinations

• Dominant Alleles: The $A$ allele is dominant and the $B$ allele is also dominant.

• Co-dominant Alleles: When both $A$ and $B$ alleles are present together ($I^A I^B$), they are co-dominant, meaning both are expressed.

• Recessive Allele: The $O$ allele is recessive.

• Genotypes and Resulting Phenotypes:

  • $I^A I^A$ or $I^A i$ results in Type A blood.

  • $I^A I^B$ results in Type AB blood.

  • $I^B I^B$ or $I^B i$ results in Type B blood.

  • $i i$ results in Type O blood.

Genetics Scenarios and Punnett Square Practice

• Scenario 1: Determining the child's blood type when the mother has alleles $I^A I^A$ and the father has alleles $I^B I^B$.

  • Mother's Alleles: $I^A$, $I^A$

  • Father's Alleles: $I^B$, $I^B$

  • Punnett Square calculation results in all children having the genotype $I^A I^B$.

  • Child's Blood Type: Type AB.

• Scenario 2 (Practice): Mother has blood Type A with genotype $I^A i$; Father has blood Type B with genotype $I^B i$.

  • Punnett Square Set-up:

  • Across the top (Father): $I^B$ and $i$

  • Down the side (Mother): $I^A$ and $i$

  • Resulting Genotypes:

  • $I^A I^B$

  • $I^A i$

  • $I^B i$

  • $i i$

  • Resulting Phenotypes:

  • Type AB

  • Type A

  • Type B

  • Type O

The Biological Basis: Enzymes, Antigens, and Antibodies

• Coding: The alleles do not directly create the blood type; they ‐code‐ for the production of a specific enzyme.

• Enzyme Function: The enzyme created by the alleles is responsible for producing specific antigens on the surface of the Red Blood Cell (RBC).

• Antigens: An antigen is a protein, encoded from the specific enzyme, that ‐sits‐ on the surface of the RBC.

  • There are two distinct blood antigens: A and B.

  • Possession of the A antigen results in Type A blood.

  • Possession of the B antigen results in Type B blood.

  • Antigens on the Surface Per Blood Type:

  • Type A: A antigens only.

  • Type B: B antigens only.

  • Type AB: Both A and B antigens.

  • Type O: Neither A nor B antigens.

• Antibodies: Proteins found within the blood plasma.

  • The body produces a diverse range of antibodies to identify and attack foreign molecules.

  • Crucially, a person's plasma does not contain any antibodies that will bind to molecules that are already part of their own body (self-antigens).

  • Antibody Distribution Per Blood Type:

  • Type A: Contains B antibodies; lacks A antibodies.

  • Type B: Contains A antibodies; lacks B antibodies.

  • Type AB: Contains neither A nor B antibodies.

  • Type O: Contains both A and B antibodies.

Blood Transfusions and Immune Responses

• It is critical to match donor and recipient blood types accurately.

• Immune Response: If a donor's blood cells contain antigens that are different from the recipient's, the antibodies in the recipient's blood will recognize the donor blood as foreign.

• Blood Clotting: This recognition triggers an immune response which results in blood clotting.

• Transfusion Relationships:

  • Type A: Has A antigens and B antibodies. Can donate to A and AB. Can receive from A and O.

  • Type B: Has B antigens and A antibodies. Can donate to B and AB. Can receive from B and O.

  • Type AB: Has A and B antigens and no antibodies. Can donate only to AB. Can receive from A, B, AB, and O (Universal Recipient).

  • Type O: Has no antigens and both A and B antibodies. Can donate to A, B, AB, and O (Universal Donor). Can receive only from O.

Population Distribution and Statistical Data

• Relative Abundance of Blood Types:

  • Type A: $40-42\%$

  • Type B: $10-12\%$

  • Type AB: $3-5\%$

  • Type O: $43-45\%$

• Specific Stats (Likelihood by Rh Factor):

  • O+: 1 in 3 persons

  • O-: 1 in 15 persons

  • A+: 1 in 3 persons

  • A-: 1 in 16 persons

  • B+: 1 in 12 persons

  • B-: 1 in 67 persons

  • AB+: 1 in 29 persons

  • AB-: 1 in 167 persons

The Rhesus (Rh) Factor

• Definition: The Rhesus factor (Rh) is an additional protein found on the surface of red blood cells.

• Origin: It is named as such because it is a protein also found in Rhesus monkeys.

• Expression: Rh is expressed as either positive ($+$) or negative ($-‐$).

• Prevalence: The majority of people (approximately $85\%$) have a positive Rh factor.

Additional Haematological Facts

• Biological Sex Differences: Men generally possess more red blood cells than women.

• Rare Blood Types: Beyond the basic ABO and Rh systems, other rare blood types exist in the human population.