ABO blood groupings are controlled by the gene I. This ABO blood grou…

The correct answer is Option (3) → Co-dominance

Option (1) Incomplete Dominance — Incorrect. In incomplete dominance the F₁ shows a blend, in-between the two parents, as in the pink flowers of the dog flower (snapdragon). A person with I^AI^B does not have an in-between blood group.

Option (2) Segregation — Incorrect. Segregation is a law of Mendel: the two alleles of a gene separate during gamete formation. It happens for the gene I too, but it is not the kind of dominance seen in blood groups.

Option (3) Co-dominance — Correct. The gene I has three alleles: I^A, I^B and i. When I^A and I^B are present together, both express their own sugar on the red blood cells, and the person has the AB group.

Option (4) Polyploidy — Incorrect. Polyploidy is a change in chromosome number in which a cell has more than two sets of chromosomes. It follows a failure of cytokinesis after telophase and is common in plants. It has nothing to do with the alleles of one gene.

NCERT Extract:
Co-dominance –