Maria has type O blood, and her sister has type A B blood. The girls know that both of their maternal grandparents are type A. What are the genotypes of the girls’ parents?

Maria has genotype of AO and Her sister have genotype of AB.

Explanation:

Blood types are types of blood that are classified based on presence or absence of antigen on the surface of the red blood cells. For example type A blood contain one type of antigen which is A, type AB contain two type of antigen which is A and B.

Blood types are determined by genes or alleles(genotype).

Combination of alleles is called genotype.

A person with type A blood has dorminant allele that is he or she received A antigen from the parent.

From the examples Maria has O blood type. O allele is receive i.e it's always hidden in A or B alleles. Since the grandparents has A type therefore Maria have AO alleles .

Her sister is blood type AB be cause she receive both A and B allele from the parent.