Heavy neutrino threshold effects in low energy phenomenology

Right handed neutrinos with mass of {\cal O} \; (10^{12}-10^{13}) GeV are required to implement the see-saw mechanism and generate neutrino masses capable of playing a role in structure formation. Moreover models of fermion masses often relate the Yukawa couplings involving these neutrinos to the up-quark Yukawa couplings. Here we study the effects of such couplings on the radiative corrections to quark masses. We find that b-\tau equality at M_{GUT} may still give the correct m_b/m_{\tau}-- ratio at low energies, but only if there is large \mu-\tau mixing in the charged leptonic sector. We propose specific mass matrix ``textures'' dictated by a U(1) family symmetry whose structure preserves m_b=m_{\tau} at M_{GUT}. In these schemes, due to the large \nu_{\mu} - \nu_{\tau} mixing, it is possible to give a simultaneous solution to the solar neutrino deficit and the atmospheric neutrino problem.