What are alleles?

The blood group of Rameshbhai is $B$ and his daughter's blood group is $AB$. What will be the blood group of his wife?

$A$ man with blood group $A$ marries a woman having blood group $B$. The maximum possible blood groups among their progenies are

$A$ child of $O$ blood group has a $B$ blood group father. The genotype of the father will be:

$A$ child has blood group $O$. If the father has blood group $A$ and mother has blood group $B$,work out the genotypes of the parents and the possible genotypes of the other offspring.

If a man with blood group $A$ marries a woman with blood group $AB$,which of the following offspring would indicate that the man is heterozygous for blood group $A$?