In the English language, black sheep is an idiom used to describe an odd or disreputable member of a group, especially within a family. The term stems from sheep whose fleece is colored black rather than the more common white; these sheep stand out in the flock and their wool was traditionally considered less valuable.
The term has typically been given negative implications, implying waywardness.
In psychology, the black sheep effect refers to the tendency of group members to judge likeable ingroup members more positively and deviant ingroup members more negatively than comparable outgroup members.
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In most sheep, a white fleece is not caused by albinism but by a common dominant gene that switches color production off, thus obscuring any other color that may be present. A black fleece is caused by a recessive gene, so if a white ram and a white ewe are each heterozygous for black, in about 25 percent of cases they will produce a black lamb. In fact in most white sheep breeds, only a few white sheep are heterozygous for black, so black lambs are usually much rarer than this.
The term originated from the occasional black sheep which are born into a flock of white sheep. Black wool was considered commercially undesirable because it could not be dyed. In 18th and 19th century England, the black color of the sheep was seen as the mark of the devil. In modern usage, the expression has lost some of its negative connotations, though the term is usually given to the member of a group who has certain characteristics or lack thereof deemed undesirable by that group. Jessica Mitford described herself as "the red sheep of the family", a communist in a family of aristocratic fascists.
The idiom is also found in other languages, e.g. German, French, Serbian, Bulgarian, Hebrew, Portuguese, Bosnian, Greek, Turkish, Hungarian, Dutch, Afrikaans, Swedish, Danish, Spanish, Catalan, Czech, Slovak, Romanian and Polish. During the Second Spanish Republic a weekly magazine named El Be Negre, meaning 'The Black Sheep', was published in Barcelona.