Black sheep

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 the genetic effect in sheep whereby a recessive gene occasionally manifests in the birth of a sheep with black rather than white coloring; 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 member more negatively than comparable outgroup members.

In most sheep, a white fleece is not albinism but a dominant gene that actively switches color production off, thus obscuring any other color that may be present. As a result, a black fleece in most sheep is recessive, so if a white ram and a white ewe are each heterozygous for black, in about 25% 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 due to a genetic process of recessive traits. 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.

...
Wikipedia