Inheritance of ABO blood group system is an example of

The correct option is multiple allelism.

To understand why the inheritance of the ABO blood group system is an example of multiple allelism, let us break down the genetics behind it step-by-step:

1. Understanding Alleles:
In classical genetics, a gene typically has two alternative forms called alleles (for example, tallness versus dwarfness in pea plants). However, in a population, a single gene can sometimes have more than two alternative alleles. This phenomenon is known as multiple allelism.

2. The ABO Blood Group Gene:
The ABO blood group system in humans is controlled by a single gene, known as the I gene (immunoglobulin gene). This gene determines the type of sugar polymers that protrude from the surface of red blood cells.

3. The Three Alleles of Gene I:
Unlike simple Mendelian traits, the I gene has three common alleles in the human population:
• I^A: Specifies the A-type antigen.
• I^B: Specifies the B-type antigen.
• i: Specifies no antigen (an inactive allele).

4. How Multiple Alleles Combine in Individuals:
Although there are three alleles present in the human population, any single individual can only inherit two of these alleles (one from each parent) because humans are diploid organisms ($2n$). The combination of these alleles determines an individual's blood type:
• I^AI^A or I^Ai results in Blood Group A.
• I^BI^B or I^Bi results in Blood Group B.
• I^AI^B results in Blood Group AB.
• ii results in Blood Group O.

Conclusion:
Since the inheritance of the ABO blood group is governed by three different alleles (I^A, I^B, and i) of a single gene, it serves as a classic textbook example of multiple allelism.