Father's blood group is A and mother's blood group is B. One of their offspring is with group AB. What is the percentage of probability of AB blood group offspring? 

25%

The correct answer is Option (3) –25%

A parent with A blood group can have two possible genotypes, I^A I^A and I^A Iⁱ; similarly parent with B blood group can have two possible genotypes, I^B I^B and I^B Iⁱ. Thus, they have three alleles, I^A, I^B and Iⁱ which can make total six genotypes : I^A I^A, I^A Iⁱ, I^B I^B, I^B Iⁱ , I^A I^B and IⁱIⁱ.

Since, I^A and I^B alleles are dominant over Iⁱ allele; the genotypes I^A Iⁱ and I^B Iⁱ gives A and B blood group respectively. Genotypes I^A I^A, I^B I^B, Iⁱ Iⁱ show A, B and O blood groups respectively.

Due to codominance of I^A and I^B alleles, the genotype I^AI^B produce AB blood group respectively. Thus, the six genotypes give total four phenotypes. Hence, parents with A and B blood group can have child with A, B, AB and O blood groups.

Correct option is "3". So the percentage of probability of AB blood group offspring is 25%.