Explain the inheritance of ABO blood groups: phenotypes are A, B, AB and O blood groups and alleles are IA, IB and Io

The inheritance of ABO blood groups is a classic example of multiple alleles and codominance in genetics. The ABO blood group system is determined by the presence of specific antigens on the surface of red blood cells, which are controlled by three alleles: IAIA, IBIB, and $i$.

1. Alleles and Their Functions

• IAIA: This allele codes for the A antigen on red blood cells.
• IBIB: This allele codes for the B antigen on red blood cells.
• $i$: This allele does not produce any antigen (recessive).

2. Phenotypes and Genotypes

The combination of these alleles results in four possible phenotypes (blood types) and their corresponding genotypes:

3. Codominance in ABO Blood Groups

In individuals with the genotype IAIBIAIB, both A and B antigens are expressed simultaneously on the surface of red blood cells. This is a clear example of codominance, where neither allele masks the other, resulting in the AB blood type.

4. Inheritance Patterns

When considering inheritance, each parent contributes one allele to the offspring. The possible combinations of alleles can be illustrated using a Punnett square. For example, if a parent with blood type A (IAiIAi) has offspring with a parent with blood type B (IBiIBi), the potential genotypes of their children would be:

Punnett Square Example

Results:

• Offspring Genotypes:
  • IAIBIAIB (AB)
  • IAiIAi (A)
  • IBiIBi (B)
  • $ii$ (O)

5. Phenotypic Ratios

From this cross, the phenotypic ratio can be summarized as follows:

• 1 AB : 1 A : 1 B : 1 O