Blood type is a result of the gene carried on both sets of DNA in the chromosome. Since the A and B mean that the red blood cells carry an antigen (A or B) and O mean no antigen, to be type O you would have OO genetics. To be Type A, you would be AA, or AO. Similarly to be Type B, you would be BB or BO. So if the parents were AO and BO, in about 25% of the cases the baby would be OO. If the parents were (AA, BB), (AO, BB), (AA,BO) there would be no possibility of a type O baby.


