Unlike many other animals, white coloration in sheep is a dominant gene, rather than a recessive gene or mutation like albinism and leucism. Therefore, black sheep are fairly rare, occurring in about 25% of cases when two white sheep that carry the recessive black gene produce offspring.
History
We all know the nursery rhyme “Baa, Baa, Black Sheep”, but you may not know that this poem dates back to 1731, and has remained largely unchanged in over 200 years. The term “black sheep” is used to describe a member of a group or family who stands out and is not like the rest. Variations of this phrase can be found in over 20 languages, although in some languages the term “white crow” is used to mean the same thing.