  The origin of the observed masses and mixing angles of quarks and leptons is
one of imperative subjects in and beyond the standard model. Toward a deeper
understanding of flavor structure, we investigate in this paper the minimality
of fermion mass (Yukawa) matrices in unified theory. That is, the simplest
matrix form is explored in light of the current experimental data for quarks
and leptons, including the recent measurements of quark CP violation and
neutrino oscillations. Two types of neutrino mass schemes are particularly
analyzed; (i) Majorana masses of left-handed neutrinos with unspecified
mechanism and (ii) Dirac and Majorana masses introducing three right-handed
neutrinos. As a result, new classes of neutrino mass matrices are found to be
consistent to the low-energy experimental data and high-energy unification
hypothesis. For distinctive phenomenological implications of the minimal
fermion mass textures, we discuss flavor-violating decay of charged leptons,
the baryon asymmetry of the universe via thermal leptogenesis, neutrino-less
double beta decay, and low-energy leptonic CP violation.
