  The radiative see-saw mechanism of Witten generates the right-handed neutrino
masses in SO(10) with the spinorial 16_H Higgs field. We study here
analytically the 2nd and 3rd generations for the minimal Yukawa structure
containing 10_H and 120_H Higgs representations. In the approximation of small
2nd generation masses and gauge loop domination we find the following results :
(1) b-tau unification, (2) natural coexistence between large theta_l and small
theta_q, (3) degenerate neutrinos.
