(a) Explain the phenomena of multiple allelism and co-dominance taking ABO blood group as an example. (b) What is the phenotype of the following: (i) IAi (ii) ii

(a) When a gene exists in more than two allelic forms, the phenomenon is called multiple allelism. When two alleles of a gene are not dominant over each other and express themselves independent of the presence or absence of others, the phenomenon is co-dominance. Both of these phenomena can be seen in ABO blood grouping. The I gene controls the blood groups in humans which exist in three different allelic forms as 'i', 'IA' and 'IB'. So, it shows multiple allelism. IA and IB code for a glycoprotein which is present on the surface of RBCs while 'i' does not code for anything. Therefore IA and IB are co-dominant over each other while both are dominant over 'i'. When both IA and IB are present, the blood group becomes AB (b) Phenotype of (i) IAi - blood group A (ii) ii - Blood group O