  Single and double universal seesaw mechanisms and the hypothesis of universal
strength for Yukawa couplings are applied to formulate a unified theory of
fermion mass spectrum in a model based on an extended Pati-Salam symmetry. Five
kinds of Higgs fields are postulated to mediate scalar interactions among
electroweak doublets of light fermions and electroweak singlets of heavy exotic
fermions with relative Yukawa coupling constants of exponential form. At the
first-order seesaw approximation, quasi-democratic mass matrices with equal
diagonal elements are derived for all charged fermion sectors and a diagonal
mass matrix is obtained for the neutrino sector under an additional ansatz.
Assuming the vacuum neutrino oscillation, the problems of solar and atmospheric
neutrino anomalies are investigated.
