Flavor chirality is combined with genuine left-right symmetry in a unified SU(5${)}_{\mathrm{L}}$⊗SU(5${)}_{\mathrm{R}}$ manner. An SU(2${)}_{\mathrm{L}}$⊗SU(2${)}_{\mathrm{R}}$-singlet fermionic companion to each quark and lepton is predicted; the associated seesaw mechanism is examined in the light of proton stability. The evolution of ${\sin }^2$${\mathrm{\theta }}_{\mathrm{W}}$ exhibits distinctive features: (i) It starts at (3/16 and rises, and (ii) it is N dependent (N being the total number of generations). N=3,4 dictates an intermediate mass scale ∼${10}^7$ GeV, marking the breakdown of SU(5${)}_{\mathrm{R}}$.

• Received 7 April 1987

DOI:https://doi.org/10.1103/PhysRevLett.58.2623