Pablo’s blood type is B+ while his mother’s blood type is O-. Which are the potential blood types of his father?

the father may be BB++ BB +- BO ++ or BO +-  

Explanation:

The blood type is a monogenic inherited trait that has three different alleles: A and B alleles are dominant, while the O allele is recessive. Each parent donates one ABO allele to their offspring. On the other hand, the Rh factor single gene inherited trait that has two alleles: the Rh-negative (-) factor is recessive to the Rh positive (+). Each parent donates one Rh factor allele to their offspring. In this case, the mother has the genotype OO -- (recessive for both genes), while Pablo has a phenotype of B+ and a genotype BO+-  (i.e., Pablo is heterozygous for both genes). In consequence, the father may have the following genotypes: BB++ BB +- BO ++ or BO +-, from these combinations you could get a son with the genotype BO+- .