ABO blood group in humans is an example of:

The correct answer is Option (2) → Codominance

The ABO blood groups are controlled by the gene I. It has three alleles: I^A, I^B and i. I^A and I^B each make a different sugar on the surface of red blood cells, while i makes none. When I^A and I^B are present together, both express their own sugar, and the person has blood group AB. When both alleles of a heterozygote show fully in this way, it is called co-dominance.

(1) Incomplete dominance — Incorrect. In incomplete dominance the heterozygote shows an in-between trait, like the pink flowers of the dog flower (snapdragon). Blood group AB is not in-between A and B; it shows both A and B fully.

(2) Codominance — Correct. In the AB person, I^A and I^B are both fully expressed.

(3) Pleiotropy — Incorrect. Pleiotropy is one gene affecting many traits, as in phenylketonuria. The ABO gene decides only the blood group.

(4) Polygenic inheritance — Incorrect. A polygenic trait, like human skin colour, is controlled by three or more genes. ABO blood group is controlled by a single gene with three alleles.

Note: A similar question was also asked in CUET (UG) 2023 on 28-05-2023 (Slot 1).

NCERT Extract:
Co-dominance –