  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.
