In figure 6-12,
a) The dollar, pound, and yen symbols represent the three mutant varieties of the harebell plants or in simple the null mutations that were expressed in the genes w1 and w2 respectively. The wild type flower of the plant looks blue and the collected three mutant varieties bear white flowers. Though they are phenotypically similar their genotype could be different. Also in order to make sure that, naming them with symbols would add more confusion, which might imply dominance, the collected mutants were designated using dollar ($), pound (£), and yen (¥) symbols.
b) It is found out from the results that for both the mutant varieties $ and £ the defect lies in the same gene of an allele. This gene activity is essential in making sure that the colorless precursor 1 is converted to precursor 2, which would be converted by the enzyme 2 to the blue colored product. Since this gene is a defective one, the conversion of the product is stopped in the intermediate step itself leaving the enzyme 2 to act on no substrate. This is the reason why the heterozygotes in F1 are not able to form blue pigment.