Genes located at the same locus of chromosomes are called

Alleles are

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

$A$ man with blood group $A$ marries a woman with blood group $AB$. Which type of offspring would indicate that the man is heterozygous for blood group $A$?

If both parents have blood group $AB$, which blood group cannot be found in their child?

If a man with blood group $A$ marries a woman with blood group $AB$, and their first child has blood group $B$, what is the genotype of the child?