(a) In the figure 19-11, the dominance/additive ratio is 0.
In this case, the additive gene action is 0 because dominance is absent.
Even though the B/B has higher values and B/b have intermediate values and b/b small values, their distributions are seen overlapping. The genotype of intermediate individual can be B/B, B/b or b/b.
(b) If B locus had no effect on the trait value the frequency distributions by the alleles will be seen overlapping on each other exactly. The distribution of B/B and B/b would be seen in the same frequency distribution as b/b.
The figure will be like this.
B/B
B/b
b/b
(c) If D/A is 1.0 it means that there is complete dominance of allele B. In this case there will be only two curves. The phenotypes expressed by B/B genotype and B/b genotype would be the same and they would show dominant phenotype. Both of their frequency distributions will be the same. The second curve is for genotype b/b. This will show the recessive phenotype.