The baby’s blood type can only be O or A. Due to the inheritance law of blood type, O is recessive and A is dominant. The mother’s blood type is O, which can be thought of as 2 O’s; the father’s blood type is A, which can be thought of as O and A2 genetic factors, a total of 4 genetic factors combined separately. The combination of mom’s O and dad’s O and A respectively form OO and OA. 1 other O of mom and dad’s O and A respectively form OO and OA, which will form 2 OO, which is O blood type; 2 OA, which is expressed as A blood type. So the child’s O blood type is 50% possible and A blood type is 50% possible.