Identify the correct example of co-dominance.

The correct answer is Option (2) → ABO blood groups

In co-dominance the F₁ shows the traits of both parents, and both alleles express themselves fully. In the ABO system the gene I has three alleles, I^A, I^B and i. When I^A and I^B are present together, each makes its own sugar on the red blood cell, so the person has blood group AB.

(1) Snapdragon — Incorrect. Here the F₁ is pink, in between the red and white parents. This is incomplete dominance, not co-dominance.

(2) ABO blood groups — Correct. Both I^A and I^B show their effect together in group AB.

(3) Skin colour — Incorrect. It is a polygenic trait, controlled by many genes acting together.

(4) Color blindness — Incorrect. It is a sex-linked recessive disorder, carried on the X chromosome.

NCERT Extract:
Co-dominance –