In the figure 6-1,
a. Yellow stars in the genes represent mutations. These stars represent whether the mutation has happened in a single strand or on both the strands of a DNA.
b. Though one gene is mutated the gene present in the opposite strand is still active and could synthesize the product of interest. This wild type gene could produce enough quantum of the product of interest and be functional in a biochemical pathway. Hence, though the quantum of the product has reduced, the reaction has not changed in anyway as a result of the activity of the un-mutated gene. This is the reason why heterozygote functionally behaves the way in which the wild type organism behaves.