Cross a homozygous B with an O

Homozygous B genotype: BB Homozygous O genotype : OO

Explanation:

If crossing takes place between BB and OO then according to punnett square method of crossing, four offspring would be produced as per laws of inheritance. The offspring will be heterozygous with genotype of BO. This genotype BO will be same in case of all the four offsprings. If homozygous BB is dominant and homozygous OO is recessive alleles then all the 4 offsprings will be heterozygous BO, because B is the dominant allele. The offspring will show the phenotype of the dominant homozygous parent.