Multiple Alleles and Inheritance of Blood Groups 

In genetics, multiple alleles refer to the presence of more than two alternative forms of a gene (alleles) controlling a single trait. Although an individual can carry only two alleles (one on each homologous chromosome), a population can exhibit multiple allelic forms of that gene.
A classic example of multiple alleles in humans is the ABO blood group system, which determines a person’s blood type based on the presence or absence of specific antigens on red blood cells.

1.0What Are Multiple Alleles?

• Definition: Multiple alleles are different forms of a single gene that occupy the same locus on homologous chromosomes but produce different expressions of a trait.
• They arise due to mutations and contribute to genetic diversity within populations.
• Even though multiple alleles exist in a population, any one individual inherits only two alleles — one from each parent.

Example:
The gene I (Isoagglutinin) controls the ABO blood group in humans and has three allelic forms — Iᴬ, Iᴮ, and i.

2.0The ABO Blood Group System

1. Alleles of the ABO Gene

The ABO blood group is determined by the I gene, located on chromosome 9.

• Iᴬ → codes for antigen A on red blood cells.
• Iᴮ → codes for antigen B on red blood cells.
• i → does not produce any antigen.

These alleles interact in a codominant and dominant-recessive manner to form the four major blood groups.

2. Genotypes and Blood Groups

Note:

• Iᴬ and Iᴮ are co-dominant — both express equally when present together (AB group).
• i is recessive — it expresses only when homozygous (ii = O group).

3.0Co-Dominance in ABO Blood Groups

The ABO system illustrates co-dominance, in which both alleles are expressed equally in the phenotype.

• When both Iᴬ and Iᴮ are inherited, both antigens A and B appear on the red blood cells.
• Hence, the person has an AB blood group, demonstrating co-dominant inheritance.

4.0Dominance Relationship Among ABO Alleles

The dominance hierarchy can be represented as:
Iᴬ = Iᴮ > i

This means:

• Iᴬ and Iᴮ are equally dominant and both express when present together.
• The allele i is recessive to both.

5.0Inheritance Pattern of Blood Groups

1. Parental Cross Examples

Example 1:

Parent 1: Blood group A (Iᴬi)
Parent 2: Blood group B (Iᴮi)

Offspring Blood Groups: A, B, AB, and O (in a 1:1:1:1 ratio)

Example 2:

Parent 1: Blood group A (IᴬIᴬ)
Parent 2: Blood group O (ii)

Offspring Blood Group: 100% A

2. Pedigree Analysis of Blood Group Inheritance

The ABO blood group system follows Mendelian principles of inheritance. Pedigree charts show clear transmission of alleles from parents to offspring through generations.

6.0Role of Blood Group in Transfusion Compatibility

Blood group inheritance is not just genetic theory — it’s critical in blood transfusion compatibility, paternity testing, and medical diagnostics.

7.0Key Features of Multiple Alleles

• Found at the same locus on homologous chromosomes.
• Control the same trait, but with different intensities or expressions.
• Exhibit hierarchical dominance (one allele dominant over another).
• Only two alleles can exist in a single organism at a time.

8.0Genetic Significance of Multiple Alleles

• Increase genetic variability within a population.
• Help explain complex inheritance patterns beyond simple dominance.
• Provide a better understanding of gene interactions and co-dominance.