  The mass matrices of charged fermions have a simple structure if expressed in
powers of the small parameter sigma=(m_c/m_t)^{1/2}. It is suggested that the
mass matrix of the three heavy neutrinos occuring in grand unified theories can
be expressed in terms of the same parameter. The requirement that these heavy
neutrinos carry different U(1) generation quantum numbers gives rise to an
almost unique form for this matrix. By applying the see-saw mechanism, the mass
splitting of the two lightest neutrinos comes out to be tiny, favoring the
vacuum oscillation solution for solar neutrinos. The mixing matrix is of the
bimaximal type but contains also CP violating phases.
