Fermion masses and Bi-large lepton mixing from singular matrices

We discuss conditions under which bi-large mixing in the lepton sector is achieved assuming that all Yukawa matrices and the right-handed neutrino mass matrix have the same singular form in the leading order. Due to obvious quark lepton symmetry, this approach can be embedded into grand unified theories. The right-handed neutrino mass scale can be identified with the GUT scale and, as a consequence of third generation Yukawa coupling unification, the mass of the lightest neutrino is given as (mtop2/MGUT)|Uτ1|2 in the leading order. This relation does not depend on the exact form of mass matrices. (author)