Reversion to the ancestral type. Two non-allelic, recessive white mutants in domestic fowl (top) complement each other to generate perfect wild-type male (bottom right) and female (bottom left) progeny. Reversion was of polemical value for Darwin in indicating that highly divergent domesticated stocks had been derived from a single ancestral species, but contributed to the widespread confusion over the laws of inheritance. From  plate 4 between pages 100 and 101.
Howard Journal of Biology 2009 8:15 doi:10.1186/jbiol123