We study the flavor sector in models where the three families of matter are unified in a (16, 3) representation of the $SO(10)\times A_4$ group. The necessary ingredients to realize tribimaximal mixing in the lepton sector are identified systematically. The nonrenormalizable operators contributing to the fermion mass matrices play an important role. We also present a mechanism to explain the interfamily mass hierarchy of quarks and charged leptons, which relies on a “universal seesaw” mechanism and is compatible with tribimaximal mixing.

• Received 11 October 2008

DOI:https://doi.org/10.1103/PhysRevD.78.116018