What will be the parent’s blood group if the blood group of a child is AB ?
a) A and O
b) B and O
c) AB and O
d) A, B and O

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Hint: Like eye or hair color the blood type is inherited from the parents. Each biological parent passes one of two ABO genes to their child. The A and B alleles are dominant and the O gene is recessive.

Complete answer:
Each biological parent gives one of their two ABO alleles to their child. In the ABO blood group system, the classification of human blood is based on the presence or absence of the antigens A and B on the surface of the red cells. An individual may have type A, type B, type O or type AB blood. If a child is having blood group AB ($i^A$ $i^B$), then the parent's blood will be A ($i^A$ $i^A$) and AB ($i^A$ $i^B$) out of the given options. This is because the rest options are having blood group O ($i^O$ $i^O$) which is incorrect as a child is not having an $i^0$ gene.
Hence, the correct answer is D.
The ABO antigens get well developed before birth and stay behind throughout life. Children acquire ABO antibodies inertly from their mother before birth but from the three months itself infants start making their own antibodies. It is thought that the stimulus for such antibody formation is from the contact with ABO like antigenic substances in nature. ABO incompatibility is the term used in which the antigens of a mother and her fetus are different enough to cause an immune reaction. This condition generally occurs in a small number of pregnancies. Infrequently ABO incompatibility may also give rise to erythroblastosis fetalis that is hemolytic disease of the newborn. It is a type of anemia in which the red blood cells of the fetus get destroyed by the maternal immune system. This condition occurs usually when a mother is type O and her fetus is either type A or type B.

Hence, the correct answer is option (D)

Note: ABO blood group system, the classification of human blood based on the inherited properties of red blood cells (erythrocytes) as determined by the presence or absence of the antigens A and B, which are carried on the surface of the red cells. Persons might have type A, B, O, or AB blood. The A, B, and O blood groups were first recognized by Austrian immunologist Karl Landsteiner in 1901.