Answer:
In the ABO series, the gene for blood type O is recessive, while A and B are dominant genes, and AB is co-dominant.
Step-by-step explanation:
According to the ABO blood system, the A blood group might have the genotype AA or AO. The B blood group might have genotype BB or BO. The O blood group will have the genotype OO. The Ab blood group will have genotype AB, A, B or O.
The O is the recessive allele but a person with O blood type can transfer blood to all other blood types. The AB is co-dominant and it can accept blood from all other blood types.