In a certain plant, blue (B) flower color is dominant to white (b). You have a blue-flowered plant and a white-flowered plant. What do you know about the genotype and phenotype of each plant? Now you cross the plants and all the offspring are blue. What have you learned about the genotypes of the original plants?

The blue flower could have the genotype Bb or BB, because dominant alleles mask recessive alleles. The white plant can only have the genotype bb. If we cross the recessive genotype with our unknown dominant phenotype, that is a test cross. We expect to see all blue flowers if the blue plant has BB genotype because all flowers will be Bb(heterozygous). If any flowers from the cross are white, the blue plant is Bb because we can make the combinations: Bb & Bb.