In Drosophila, the genes bw and st are located on chromosomes 2 and 3, respectively. Flies homozygous for bw mutations have brown eyes, flies homozygous for st mutations have scarlet eyes, and flies homozygous for bw and st mutations have white eyes. Doubly heterozygous males were mated individually to homozygous bw; st females. All but one of the matings produced four classes of progeny: wild-type, and brown-, scarlet- and white-eyed. The single exception produced only wildtype and white-eyed progeny. Explain the nature of this exception.