Polygenic inheritance is defined as characters which are determined by two or more gene pairs, and they have additive or cumulative effect. Such genes are called polygenes.

In such inheritance, each polygene contribution = (Maximum height - Minimum height)/No. of polygene $=\left(24-6\right)/6=3$ inches. Thus, 3inches height is contributed by each dominant allele of the polygene.

Contribution of dominant polygene in plant having genotype $\mathrm{A}\mathrm{a}\mathrm{B}\mathrm{B}\mathrm{C}\mathrm{c}=3\times 3=9$

Total height of plant with genotype AaBBCc = Minimum height + Contribution of dominant allele $=\mathrm{ }6+9=15\mathrm{ }\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{h}\mathrm{e}\mathrm{s}$