In this case there are two possibilities -
Possibility I - The genetic makeup of the tall parent can be TTWW.
Possibility II - The genetic makeup of the tall parent can be TtWW.
According to possibility I, if the tall parent has the genotype TTWW, then all the offsprings that are obtained will be tall bearing white flowers.
According to possibility II, if the tall parent has gentotype TtWW, then half the offsprings will be short bearing white flowers.
oSo, we can say that, the genotype of the tall parent would be TtWW.