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?

Answer: 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.