The father must be a person that has blood type A.
The phenotype is dependent on the genotype. The genotype refers to the genes that are inherited from parents wile the phenotype is the traits that are expressed. We know that genes are responsible for traits that are expressed.
Now, if the father has blood type b and the mother has blood type o, it is not possible for the child to have blood type a. The father must be a person that has blood type A.
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