Extensive Spanish cattle ranching boasts a strong, deeply rooted tradition: the use of completely black sheep, the offspring of white mothers, as magical elements to protect flocks from bad luck. Conditions are very specific, so they are unique, exceptional animals. These characteristics make them special, sacred sheep that are kept in the herd with special care until their death. There is an explanation for this symbolic and magic practice: it has existed to sustain, before Mendel discovered the laws governing genetics, the purity of white wool in sheep by isolating mutations in black ones. In this text we examine the scope of this practice and analyse it from the perspective of the anthropology of symbolism.