Blood group 'AB' is an example of:

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

Option (1) Incomplete dominance — Incorrect. In incomplete dominance the heterozygote is intermediate; in group AB both sugars appear fully.

Option (2) Complete dominance — Incorrect. Complete dominance is I^A or I^B over i, not group AB.

Option (3) Co-dominance — Correct. In blood group AB, both alleles I^A and I^B are present and both express their own type of sugar on the red cells. When both alleles of a pair express themselves fully in the heterozygote, it is co-dominance.

Option (4) Test cross — Incorrect. A test cross is a cross with the recessive parent, not a type of inheritance.

Note 1: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.

Note 2: A similar question was also asked in CUET (UG) 2026 on 19-05-2026 (Slot 1).

NCERT Extract:
Co-dominance – “… A good example is different types of red blood cells that determine ABO blood grouping in human beings. ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles I^A, I^B and i. The alleles I^A and I^B produce a slightly different form of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. I^A and I^B are completely dominant over i, in other words when I^A and i are present only I^A expresses (because i does not produce any sugar), and when I^B and i are present I^B expresses. But when I^A and I^B are present together they both express their own types of sugars: this is because of co-dominance.”