  We perform a renormalisation group (RG) analysis of neutrino masses and
mixing angles in the see-saw mechanism in the minimal supersymmetric standard
model with three right-handed neutrinos, including the effects of the heavy
neutrino thresholds. We focus on the case that one of the right-handed
neutrinos provides the dominant contribution to the 23 block of the light
Majorana matrix, causing its determinant to approximately vanish and giving an
automatic neutrino mass hierarchy, so called single right-handed neutrino
dominance which may arise from a U(1) family symmetry. In these models
radiative corrections can increase atomospheric and solar neutrino mixing by up
to about 10% and 5%, respectively, and may help to achieve bi-maximal mixing.
Significantly we find that the radiative corrections over the heavy neutrino
threshold region are at least as important as those usually considered from the
lightest right-handed neutrino down to low energies.
