  The first stage accounts for the transmission of genes and goes as follows (Figure 1): each offspring received one copy of chromosome II from each parent. The maternally derived chromosomes must have contained the wild-type allele (since both second-chromosomes of every female parent used in the experiment contained the wild-type allele -- this was known on the basis of previous experiments). The paternally derived chromosomes must have contained the purple allele (since both second-chromosomes of every male parent contained the purple allele -- this was inferred from the knowledge that purple is recessive to red eye-color). Hence, all offspring were heterozygous (pr / +). Having explained the genetic makeup of the progeny by tracing the transmission of genes from parents to offspring, we can proceed to the second stage of the explanation: drawing an inference about phenotypic appearances. Since all offspring were heterozygous (pr / +), and since purple is recessive to wild-type, all offspring had red eye-color (the wild-type character). See Figure 1.