Alleles are

Considering the concept of multiple alleles,one individual can have only . . . . . . alleles for that character.

$ABO$ blood groups are controlled by the gene:

If the mother's blood group is '$A$' and the father's blood group is '$AB$',then the possible blood groups of their children will be:

$ABO$ blood grouping in humans is controlled by the gene $I$. It has three alleles: $I^A, I^B,$ and $i$. Due to these three different alleles,six different genotypes are possible. How many phenotypes can be formed from these?

What are the possible blood groups of children born to parents with blood groups $A$ and $AB$?