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.

What type of dominance is illustrated by the ABO blood group system?

Co-dominance

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

• Co-dominance occurs when both alleles in a heterozygous individual contribute to the phenotype. In the case of the ABO blood group system, the I^A and I^B alleles are co-dominant. This means that if an individual has both I^A and I^B alleles (genotype I^AI^B), both alleles are expressed, and the individual has blood type AB, which has both A and B antigens on the surface of the red blood cells.

To contrast with other forms of dominance:

• Incomplete dominance is when the heterozygous phenotype is a blend of the two homozygous phenotypes. For example, if red and white flowers show incomplete dominance, the heterozygous flowers might be pink.
• Complete dominance is when the dominant allele completely masks the effect of the recessive allele in the heterozygous condition. For example, in Mendel's pea plants, the allele for tall plants (T) is completely dominant over the allele for short plants (t), so both TT and Tt plants are tall.
• Epistasis :The prevention of the expression of one pair of genes by another pair of genes is known as epistasis.