  We consider a neutrino mass model where all leptonic mixing is induced by a
heavy Majorana sector through the seesaw type I mechanism, while the Dirac mass
matrices are diagonal. Such a pattern occurs naturally in grand unified
theories. Constraints on the parameters of the models are considered and it is
shown that a normal neutrino mass hierarchy is preferred. The lightest neutrino
mass is typically small, leading to nonobservable rates for neutrinoless double
beta decay in the normal hierarchy case.
