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 of the following statements is true regarding the ABO blood group system?

I^A and I^B are co-dominant, and both express their sugars if present together.

The correct answer is Option (1) –I^A and I^B are co-dominant, and both express their sugars if present together.

The true statement regarding the ABO blood group system is: IA and IB are co-dominant, and both express their sugars if present together.

Here’s an explanation for each statement:

1. I^A and I^B are co-dominant, and both express their sugars if present together.
• True: In the ABO blood group system, the alleles I^A and I^B are co-dominant. This means that when both alleles are present in a genotype (i.e., I^A and I^B), both alleles are expressed equally, resulting in the presence of both A and B antigens on the surface of the red blood cells. This leads to the AB blood type.
2. I^A is recessive to I^B.
• False: I^A is not recessive to I^B; rather, I^A  and  I^B are co-dominant. This means they both contribute equally to the phenotype when present together.
3. i is dominant over I^A and I^B.
• False: The i allele is actually recessive to both I^A and I^B When i is present with I^A (genotype I^Ai), the phenotype is blood type A. When i is present with I^B (genotype I^Bi), the phenotype is blood type B. Only when an individual has two i alleles (genotype ii) does the phenotype express as blood type O, with no A or B antigens.
4. I^B is recessive to I^A.
• False: I^B is not recessive to I^A; rather, I^A and I^B are co-dominant. Both alleles are expressed equally when present together in a genotype.

In summary, the correct understanding of the ABO blood group system is that I^A and I^B are co-dominant alleles, meaning both are equally expressed when present together, leading to the AB blood type.