Another Case of Unequal Crossing Over in Drosophila Melanogaster

Females homozygous for the sex-linked dominant, Bar, occasionally give rise to wild-type reversions and to forms with more extreme eye reduction than Bar. This behavior was shown by Sturtevant to result from unequal crossing over. The Bar-reverted type was considered to be a deficiency for the Bar gene; while the extreme form, called Ultra-Bar or Double-Bar, was interpreted as a duplication for that gene. Later, Wright suggested that Bar itself had something additional present which when lost by unequal crossing over would give back a normal chromosome (Bar-reverted).