Describe the role of multiple alleles in the inheritance of ABO blood group system in humans. (IFS 2023, 15 Marks)

Introduction

The ABO blood group system in humans is determined by the presence of multiple alleles at a single gene locus on chromosome 9. This system is characterized by the presence of three main alleles - A, B, and O, which give rise to four blood types - A, B, AB, and O. 

Role of Multiple Alleles in the Inheritance of ABO Blood Group System in Humans

• Basic Concept of ABO Blood Group System
  
  • The ABO blood group system is based on the presence or absence of antigens on the surface of red blood cells. These antigens are determined by three alleles: Iₐ, Iᵦ, and i.
  • The gene for the ABO blood group is located on chromosome 9 and follows Mendelian inheritance patterns.
• The Three ABO Blood Group Alleles
  
  • Iₐ (A allele): Produces A antigen on the surface of red blood cells.
  • Iᵦ (B allele): Produces B antigen on the surface of red blood cells.
  • i (O allele): Produces no antigen on the surface of red blood cells (recessive allele).
• Inheritance of Multiple Alleles
  
  • Humans inherit two alleles for the ABO blood group, one from each parent. Each parent passes on one of the three alleles (Iₐ, Iᵦ, or i) to their offspring.
  • The presence of multiple alleles allows for greater genetic diversity and a variety of possible blood types.
• Codominance and Inheritance Pattern
  
  • The ABO system exhibits codominance between Iₐ and Iᵦ. This means that both A and B alleles are expressed equally when inherited together (i.e., AB blood type).
  • The i allele is recessive to both Iₐ and Iᵦ, meaning it is only expressed when inherited as ii (blood type O).
• Possible Genotypes and Blood Types
  
  • IₐIₐ or Iₐi → Blood Type A (A antigens).
  • IᵦIᵦ or Iᵦi → Blood Type B (B antigens).
  • IₐIᵦ → Blood Type AB (A and B antigens, codominance).
  • ii → Blood Type O (no antigens).
• Impact of Multiple Alleles on Blood Group Inheritance
  
  • With three alleles, there are six possible genotypes but only four blood types (A, B, AB, and O). The diversity in alleles increases the number of potential blood group combinations in offspring.
  • This system of multiple alleles results in the observed distribution of blood groups in populations.
• Role of Recessive i Allele in Blood Group O
  
  • The i allele, being recessive, must be inherited from both parents for the offspring to have blood type O (ii).
  • This leads to the relatively rare occurrence of blood type O in the population, as both parents must carry the i allele.
• Significance of Multiple Alleles in Blood Transfusion
  
  • Understanding the ABO blood group system and the role of multiple alleles is crucial for blood transfusions.
  • The correct matching of blood types is necessary to prevent immune reactions that could occur if incompatible blood types are mixed (e.g., Type A blood receiving Type B blood).
• Evolutionary and Population Genetics Perspective
  
  • The existence of multiple alleles in the ABO blood group system can be explained by natural selection and genetic drift.
  • Different alleles offer varying levels of resistance to diseases like malaria, influencing the prevalence of blood types in different geographic regions. For example, blood group O has been associated with a lower susceptibility to malaria.

Inheritance Pattern of Multiple Alleles:

• The ABO system follows codominance and simple dominance:
  
  • Codominance: Both IA and IB alleles are expressed when present together (heterozygous). Therefore, individuals with both IA and IB alleles will have blood type AB.
  • Simple Dominance: The allele i is recessive to both IA and IB. If an individual inherits one i allele, they will express the A or B blood type only if they also inherit IA or IB, respectively.
• An individual has two alleles for blood type (one inherited from each parent). The possible combinations of these alleles result in four blood types:
  
  • Type A: IAIA or IAi
  • Type B: IBIB or IBi
  • Type AB: IAIB (both antigens expressed)
  • Type O: ii (no antigens expressed)

Conclusion

The inheritance of ABO blood groups in humans is governed by the presence of multiple alleles at a single gene locus. The interactions between these alleles result in the diverse range of blood types observed in the human population.