In wrought magnesium alloys, room temperature plasticity is largely controlled by limited slip systems such as basal slip and tension/compression twins. The insufficient number of active slip systems limits strength and ductility preventing broader structural applicability of Mg-alloys. Hence, we employ first-principle calculations to investigate the effects of Y and Al alloying elements on shearability and dislocation motion on various slip systems through ideal shear resistance and generalized stacking fault energy calculations. Yttrium is seen to lower the ideal shear resistance and dislocation motion energetics on all the slip systems. On the other hand, aluminum increases the ideal shear resistance but decreases the energy barrier for dislocation motion on various slip systems. The profound effects of solute addition result from the charge transfer between the solute atom and surrounding magnesium atoms.