  We explore how the seesaw sector in neutrino mass models may be constrained
through symmetries to be completely determined in terms of low-energy mass,
mixing angle and CP-violating phase observables. The key ingredients are
intra-family symmetries to determine the neutrino Dirac mass matrix in terms of
the charged-lepton or quark mass matrices, together with inter-family or flavor
symmetries to determine diagonalization matrices. Implications for leptogenesis
and collider detection of heavy neutral leptons are discussed. We show that
leptogenesis can succeed in small regions of parameter space for the case where
the neutrino Dirac mass matrix equals the up-quark mass matrix. The model where
the neutrino Dirac mass matrix equals the charged-lepton mass matrix can yield
a heavy neutral lepton as light as about 1 TeV, but detecting such a particle
will be difficult.
