In the abo series, the gene for blood type o is , while a and b are genes, and ab is .

Gene for blood type O is recessive

Gene for blood type A and B is dominant

Gene for blood type AB is neither dominant nor recessive but a form of co-dominance

Explanation:

In the ABO blood group,  

A and B are dominant genes, O is recessive genes.  

So the different genotype and its corresponding phenotype is as follows –  

Genotype    Phenotype

AO    A

BO    B

AB    AB

AA    A

BB    B

OO    O

Hence, the two alleles AB occur and express together. Therefore, AB blood group is an example of co-dominance.  

Blood type also varies with the presence of Rh factor. If Rh factor is present, the blood type will represented with a “+” symbol. And if Rh factor is absent, the blood type will be represented by “-“symbol.

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