Read the passage and answer the following questions :
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. Hence red blood cells have both A and B types of sugars.

Which among the following is the phenotype of blood group having genotype I^BI^B ,I^Bi ?

B phenotype .

The correct answer is Option (3) –B phenotype .

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,  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. So in this case I^BI^B ,I^Bi genotype will have B phenotype .