  Starting from a complete set of possible parametrisations of the quark-mass
matrices that have the maximum number of texture zeros at the grand unification
scale, and the Georgi-Jarlskog mass relations, we classify the neutrino spectra
with respect to the unknown structure of the heavy Majorana sector. The results
can be casted into a small number of phenomenologically distinct classes of
neutrino spectra, characterised by universal mass-hierarchy and oscillation
patterns. One finds that the neutrino masses reflect the natural hierarchy
among the three generations and obey the quadratic seesaw, for most GUT models
that contain a rather unsophisticated Majorana sector. A scenario with
$\nu_{\tau}$ as the missing hot dark matter component and $\nu_e\leftrightarrow
\nu_{\mu}$ oscillations accounting for the solar neutrino deficit comes
naturally out of this type of models and is very close to the experimental
limit of confirmation or exclusion. In contrast, in the presence of a strong
hierarchy of heavy scales or/and some extra symmetries in the Majorana mass
matrix, this natural hierarchy gets distorted or even reversed. This fact can
become a link between searches for neutrino oscillations and searches for
discrete symmetries close to the Planck scale.
