Question

How many phenotypes are possible of the human ABO blood group ?

Answer 1

Part 1: Introduction to genetics of ABO blood group
The inheritance of the human ABO blood group system is a good example of multiple allelism. The gene responsible for the ABO blood group system is $I$. This gene exists in three allelic forms - $I^A,I^B$ and i.
$I^A$ and $I^B$ are dominant alleles and i is the recessive allele.

Part 2: Possible phenotype and genotype of ABO blood group There are 4 possible phenotypes and 6 possible genotypes in which these three alleles are found.
$\begin{array}{cc}\text{Blood group (Phenotype)}& \text{Genotype}\\ \text{A}& I^A{I}^A\text{or}{I}^{A}i\\ \text{B}& I^B{I}^B\text{or}{I}^{B}i\\ \text{AB}& I^A{I}^B\\ \text{O}& ii\end{array}$