Degenerate neutrino masses are excluded by experiment. The
experimentally measured mass squared differences together with the yet
undetermined absolute neutrino mass scale allow for a quasi-degenerate
mass spectrum. For the lightest neutrino mass larger than roughly
0.1 eV, we analyse the influence of threshold corrections at the
electroweak scale. We show that typical one-loop corrections can
generate the observed neutrino mixing as well as the mass differences
starting from exactly degenerate masses at the tree-level. Those
threshold corrections have to be explicitly flavour violating. Flavour
diagonal, non-universal corrections are not sufficient to simultaneously
generate the correct mixing and the mass differences. We apply the new
insights to an extension of the Minimal Supersymmetric Standard Model
with non-minimal flavour violation in the soft breaking terms and
discuss the low-energy threshold corrections to the light neutrino mass
matrix in that model.