How are ABO blood groups inherited?

The inheritance of ABO blood groups is a classic example of genetic principles involving multiple alleles and codominance. Here’s a detailed explanation of how these blood groups are inherited:

Genetic Basis of ABO Blood Groups

1. Alleles Involved:
   • The ABO blood group system is determined by the ABO gene, located on chromosome 9. This gene has three main alleles:
     • IA (A allele)
     • IB (B allele)
     • i (O allele)
   • The A and B alleles are co-dominant, meaning that if an individual inherits both, both antigens will be expressed. The O allele is recessive, requiring two copies (ii) for the phenotype to manifest as blood type O .
2. Antigen Production:
   • The IA allele encodes an enzyme that adds N-acetylgalactosamine to the H antigen, resulting in type A blood.
   • The IB allele encodes a different enzyme that adds d-galactose, resulting in type B blood.
   • The i allele leads to no modification of the H antigen, resulting in type O blood, which has neither A nor B antigens .

Inheritance Patterns

1. Possible Blood Types:
   • There are four possible blood types based on the combinations of these alleles:
     • Type A: Can be either genotype IAIA or IAi.
     • Type B: Can be either genotype IBIB or IBi.
     • Type AB: Genotype IAIB (both A and B antigens are present).
     • Type O: Genotype ii (no A or B antigens) .
2. Parental Combinations:
   • Each parent contributes one allele to their offspring, leading to six possible combinations of alleles from two parents. For instance:
     • If one parent has type A (IAi) and the other has type B (IBi), their children could potentially have any of the following blood types: A (IAi), B (IBi), AB (IAIB), or O (ii) .
3. Punnett Square Analysis:
   • A Punnett square can be used to visualize the potential genotypes and phenotypes of offspring based on the parental genotypes. For example, crossing IAi with IBi would yield a 1:1:1:1 ratio of the possible blood types among the offspring.

Implications of Codominance and Recessiveness

• In individuals with genotype IAIB, both A and B antigens are expressed equally, demonstrating codominance. This is distinct from typical dominance, where one allele masks the expression of another.
• Individuals with type O blood lack both antigens and produce antibodies against both A and B antigens, which is crucial for compatibility in blood transfusions .