  We study the magnetotransport properties of dual-gated graphene bilayers, in
which the total density and layer density imbalance are independently
controlled. As the bilayer is imbalanced we observe the emergence of a quantum
Hall state (QHS) at filling factor $\nu=0$ evinced by a plateau in the Hall
conductivity, consistent with the opening of a gap between the electron and
hole bands. By varying the layer density imbalance at fixed total density, we
observe a suppression of the QHS at filling factors $\nu=8$ and $\nu=12$ when
the layer densities are balanced, an observation at variance with theoretical
expectations in the absence of electron-electron interaction and disorder.
