In  the  presence  of  the  wild-type bw^+ (brown) allele, the scarlet
   pigment drosopterin is synthesized.
   In  the  presence  of  the  wild-type st^+ (scarlet) allele, the brown
   pigment xanthommatin is synthesized.
   Wild type carrying both pigments yield red eyes; homozygous recessives
   lack the pigments and yield white eyes.