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                            BALANCERS

     Balancers are chromosomes that are designed to maintain a
   homologous chromosome or a segment thereof intact in a cross
   or stock.  They share three properties:  (1) they are multiply
   inverted in order to suppress recombination with homologous
   chromosomes; (2) they are usually lethal, sterile in one sex
   or the other, or exhibit low viability in the homozygous con-
   dition in order to avoid displacing their homologues in bal-
   anced stocks; and (3) they are marked is such a way that their
   presence in heterozygotes (and, where they survive, as homozy-
   gotes) is recognizable, usually with a combination of dominant
   and recessive mutant alleles.  The most crucial aspect of
   balancers is their ability to suppress recombination, and
   therefore their rearranged sequence.  In this treatment we
   arrange balancers according to chromosome; balancers with the
   same rearranged sequence are treated together as a single
   entry; different marker constitutions as indicated by identi-
   fying mnemonics or version designations are tabulated within
   such entries.