Blood groups in humans are inherited with the alleles for type a and type b acting in a co-dominant manner, and both a and b are dominant to type o. if two parents with type b blood have a type o child, the genotype of the father must be:

In order for a child to have type O blood then both parents genotypes would have to include the recessive O allele. Since we know the father has type B blood and that his genotype must also include the recessive O in order to have an O type offspring then we can deduct that his genotype is BO.