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(1) 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(2) suggested that Bar itself had something additional present which when lost by unequal crossing-over would give back a normal chromosome (Bar-reverted).