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

Many parents speculate about their baby’s blood type even before birth. Blood type refers to the antigen types present on the surface of blood components—specifically, the ABO blood group system is defined by the presence or absence of specific antigens on red blood cell membranes. So, what blood type will a baby have if one parent has type A blood and the other has type O blood? Below, we address this question.

What blood type can result from a type A and type O parent?

A child born to one parent with type A blood and the other with type O blood can only have type A or type O blood—not any other blood type. This is because the possible blood types for such offspring are exclusively type A or type O. Type A blood is determined by a dominant allele; its genotype may be either AA or AO. Type O blood has the genotype OO. During gamete formation, each parent produces sperm or ova carrying either an A or an O allele. Consequently, the probability that their child will have type A blood is 75%, while the probability of type O blood is 25%.

Additional Knowledge: Principles of Blood Type Inheritance

1. Parental blood types: A + A → Possible offspring blood types: A, O; Impossible: B, AB

2. Parental blood types: B + A → Possible offspring blood types: A, B, AB, O; Impossible: none

3. Parental blood types: B + B → Possible offspring blood types: B, O; Impossible: A, AB

4. Parental blood types: One or both parents AB → Possible offspring blood types: A, B, AB; Impossible: O

5. Parental blood types: A + O → Possible offspring blood types: A, O; Impossible: B, AB

6. Parental blood types: B + O → Possible offspring blood types: B, O; Impossible: A, AB

7. Parental blood types: O + O → Possible offspring blood type: O; Impossible: A, B, AB

The above covers all essential information regarding the possible blood types of children born to type A and type O parents. Such couples can only produce children with type A or type O blood. From a genetic standpoint, human blood type remains constant throughout life—though rare exceptions exist. For instance, blood type may change following hematopoietic stem cell transplantation, or in certain autoimmune or hematologic conditions such as systemic lupus erythematosus or leukemia.