Wild-type  (B^+)  flies  possess  one  copy  of  region  16A  of the X
   chromosome.
   This region is duplicated in B flies.
   Unequal  crossing over in a B individual can result in a B^D chromatid
   with  a  triplicated region 16A and a B^+ wild-type, producing a 2:1:1
   ratio in the gametes.