In the given problem, the subject gene is B/b, which determines thorn character. The parents for the cross is a heterozygous, Bb, and a homozygous recessive, bb.
In roses, B=big thorns, and b=little thorns. Cross a heterozygous big thorned rose (Bb) with a homozygous recessive little thorned rose (bb). Show the Punnet square, and provide thephenotypic and genotypic ratios.