In this article the investigation of the cholesteric-nematic transition in thin layers of nematic-cholesteric mixtures is carried out. Nematic-cholesteric mixtures based on a nematic matrix MLC-1 with a low concentration (up to 2%) of an optically active dopant were the object of this investigation. The dependencies of threshold voltages of focal-conic deformation and forward cholesteric-nematic transition for different thicknesses of a liquid-crystal layer are obtained. It was demonstrated that the twist effect in induced cholesterics can be considered as a simple case of a cholesteric-nematic effect in thin liquid-crystal layers.