KCET 2018 Biology Solved Paper

In human blood type AB, alleles that determine the A, B, and AB blood types are a good example of co-dominance. Each A and B allele codes for the production of different enzymes, which in turn add specific sugar molecules to lipids present on the surface of red blood cells. These sugars act as recognition markers for the immune system and are called cell surface antigens. Persons with only A alleles have only A antigens on their red blood cells and thus their blood type is A, whereas those with only B alleles have only B antigens on their red blood cells and their blood type B. Both A and B antigens will be expressed in heterozygous individuals, which will give them AB blood type. In addition to being coded by co-dominant alleles, there are ABO blood types which are coded by more than the two alleles — A, B and O. Thus, ABO blood types contain multiple alleles.