(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) $I^{A}i$ (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', 'I${}^A$' and 'I${}^B$'. So, it shows multiple allelism.
I${}^A$ and I${}^B$ codes for a glycoprotein which is present on the surface of RBCs while 'i' does not code for anything. Therefore I${}^A$ and I${}^B$ are co-dominant over each other while both are dominant over 'i'. When both I${}^A$ and I${}^B$ are present, the blood group becomes AB.

(b) Phenotype of
(i) I${}^A$i - blood group A
(ii) ii - Blood group O