What blood type will a child have if one parent has type O blood and the other has type B blood?

Blood type refers to the antigen types present on the surface of blood components—including red blood cells, white blood cells, and platelets. The most commonly known blood group systems are the ABO system (comprising A, B, O, and AB types) and the Rh system. If one parent has type O blood and the other has type B blood, their child may inherit either type B or type O blood—but never type A or type AB. Below, we detail the possible blood types of children born to an O-type and a B-type parent.

What blood type can a child have if one parent is type O and the other is type B?

A child born to an O-type and a B-type parent will have either type O or type B blood. Blood types follow predictable genetic inheritance patterns; thus, knowing the parents’ blood types allows us to determine the possible blood types of their offspring. Specifically, children of an O-type and a B-type parent can only be type O or type B—never type A or type AB. Among these possibilities, type B is more likely, with a probability of 3/4, whereas type O occurs with a probability of 1/4. The genetic reasoning is as follows:

1. If the mother is type O and the father is type B, hemolytic disease of the newborn (HDN) may occur. HDN arises when maternal antibodies—formed against fetal blood group antigens incompatible with the mother’s own blood type—cross the placenta and attack fetal red blood cells, causing alloimmune hemolysis. While in utero, bilirubin generated by hemolysis is largely metabolized by the mother’s liver. After birth, however, the newborn’s immature liver cannot adequately process the large amounts of bilirubin resulting from hemolysis, leading to jaundice—the hallmark clinical manifestation of HDN.

2. If the mother is type B and the father is type O, the child has a 3/4 chance of inheriting type B blood and a 1/4 chance of inheriting type O blood.

3. Regardless of which parent carries type O or type B blood, the child can only be type O or type B—no other blood types are genetically possible.

The above outlines the possible blood types of children born to an O-type and a B-type parent. In standard genetic notation, type O corresponds to genotype “ii,” while type B corresponds to genotype “BB” or “Bi.” Therefore, offspring of an O-type (“ii”) and a B-type (“BB” or “Bi”) parent may have genotype “Bi” (type B) or “ii” (type O). We hope this information is helpful to you.