The observed pattern of neutrino mass splittings and mixing angles
indicates that their family structure is significantly different from
that of the charged fermions. We investigate the implications of
these data for the fermion mass matrices in grand unified theories
with a type-I seesaw mechanism. We show that, with simple
assumptions, naturalness leads to a strongly hierarchical Majorana
mass matrix for heavy right-handed neutrinos and a partially cascade
form for the Dirac neutrino matrix. We consider various model
building scenarios which could alter this conclusion, and discuss
their consequences for the construction of a natural model. We find
that including partially lopsided matrices can aid us in generating a
satisfying model.