Question

The possible blood groups of the offspring of the parents with blood group O and AB are
(А) O, A, B and AB
(B) A, B and AB
(С) A and B
(D) O and AB

Answer 1

Hint: Mendel also conducted artificial pollination or cross-pollination experiments using several true-breeding pea lines. In this case, the alleles show codominance.

Complete answer: Mendel selected a total of 14 true-breeding pea varieties. He selected 7 characters, some of them include seed shape, seed colour, pod colour, etc. Mendel crossed pea plants, for instance, stem height and crossed with tall and dwarf plants and got a ration 3:1, but some inheritance differs from this ratio as:

Codominance:
ABO blood type is an ideal example of co-dominance. For ABO blood group system of blood groups, allele A produces N-acetylgalactosamyl transferase enzyme which recognises H antigen present in RBC membrane and adds N-acetylgalactosamine to sugar a part of H antigen to make A antigen. The allele IB produces galactosyl transferase enzyme which adds galactose to sugar a part of H-antigen to make B antigen. Alleles 14 and 15 produce a rather different sort of sugar, while allele i don’t produce any sugar or antigen. IA and i are completely dominant over i, in other words when IA and i are known to occur only if IA expresses, as i don’t produce any sugar, and when IB and i are present, only 1B expresses. When both IA and i are present, both enzymes or sugars both antigens A and B are produced, which is often due to codominance. These antigens determine the sort of blood type. Blood type A has antigen A, B has antigen B, AB has both antigens, while blood type O doesn't carry any antigen. So, the child can have blood type A or B which differs from parents.
Hence, the correct answer is option C.

Note: Besides incomplete dominance, certain alleles show codominance. Here in $\mathop F\nolimits_1 $ hybrid, both alleles express themselves equally and there is no mixing of the effect of both alleles, therefore, the hybrid progeny ($\mathop F\nolimits_1 $) resembles both the parents.