  Explanations of the transmission of traits relate the presence of alternative genes (genotype) to the presence of alternative observable traits (phenotype). Sometimes this is done in terms of dominant/recessive relations. Purple eye-color, for example, is recessive to the wild-type character (red eye-color). This means that flies with two copies of the purple allele (the mutant form of the gene, which is designated pr) have purple eyes, but heterozygotes, flies with one copy of the purple allele and one copy of the wild-type allele (designated +), have normal wild-type eyes (as do flies with two copies of the wild-type allele). See Table 1.